Dissertations / Theses on the topic 'Selenium'

El seleni és un micronutrient essencial pels humans. Té diversos rols en la salut i per tant, el seu consum a nivells òptims és altament beneficial. Tot i així, 500-1000 milions de persones al món pateixen dèficit de seleni, degut als baixos nivells de seleni als sòls del camps de conreu. La biofortificació dels cultius amb fertilitzants rics en seleni és la manera més efectiva de contrarestar la deficiència de seleni. Tot i així, la especiació de seleni és fonamental: les plantes són capaces de transformar el seleni inorgànic del sòl, com els ions selenit i selenat, en seleni orgànic, com els selenoamino àcids, que són menys tòxics i més biodisponibles. El blat és el cereal més consumit al món i és capaç de tolerar i acumular més de 100 mg de Se per Kg de pes sec, i per tant, és un candidat adequat per la biofortificació amb seleni per produir un aliment funcional. El seleni en el blat es troba en forma de cinc espècies principals: selenit, selenat, selenometionina, metilselenocisteina i selenocistina. A la present tesis, el contingut i la distribució d’aquestes espècies en el blat s’ha determinat amb el tàndem de cromatografia líquida d’alta precisió (HPLC-ICP-MS) després de una apropiada digestió enzimàtica de la mostra, i per espectroscòpia d’absorció de raig-X (XAS) amb radiació de sincrotró, entre d’altres tècniques. La especiació i la concentració de seleni, les condicions de creixement de la planta, i el temps in que el seleni és aplicat a la planta, defineixen el grau d’absorció, metabolització i distribució de seleni a través dels diferent òrgans de la planta. El selenit es redueix ràpidament a les arrels, i per tant s’acumula en els teixits subterranis. D’altra banda, el selenat és molt mòbil a través del xilem de la planta i la seva translocació és més ràpida que la seva reducció, i per tant s’acumula a la part aèria. La aplicació d’altes concentracions de seleni pot resultar en l’acumulació excessiva als teixits, provocant estrès i toxicitat, fent decréixer la producció de biomassa a la planta i reduint el rendiment del gra. Tan mateix, la fitotoxicitat en el blat pot ser reduïda amb la aplicació del seleni a la florescència, i tot i així aconseguint un enriquiment del gra i una metabolització del seleni similars. El selenit va ser casi completament reduït a espècies orgàniques, principalment a les arrels, on la toxicitat induïda va produir un ambient altament oxidant a la planta, i per tant va resultar en una acumulació de seleni orgànic en forma de selenocistina. En contra, el selenat va mostrar una metabolització més lenta i una acumulació significativa en forma inorgànica a les tiges i fulles, tot i que en el gra el seleni es va trobar com a espècies orgàniques en forma de selenometionina, la qual es pot incorporar de forma no específica a les proteïnes. D’altra banda, la aplicació de els dos anions de forma simultània va contribuir a anivellar l’enriquiment de seleni, degut a les seves vies metabòliques separades. La mescla va causar una distribució més equilibrada de seleni en els teixits vegetals, reduint la seva fitotoxicitat, però resultant en la mateixa concentració total en el gra i uns nivells intermedis de selenometionina i selenocistina. A més, l’anàlisi espacialment resolt de la especiació als grans de blat va mostra una alta acumulació de seleni en l’embrió, el segó i l’àrea vascular pigmentada, i una baixa concentració al endosperma, que correlaciona amb la concentració de proteïnes a les diferents parts del gra. Finalment, s’ha mostrat l’efecte protector del seleni en front de la toxicitat del mercuri, la qual sembla que és deguda a la formació d’un complex proteïna-Se-Hg a les arrels. Aquest complex redueix la translocació del mercuri a les parts aèries i al gra, la mobilitat dels selenat i la reducció del selenit a les arrels, al mateix temps que afavoreix l’acumulació d’amino àcids amb la estructura de C-Se-C com la selenometionina en el gra. D’aquesta forma, el seleni pot contrarestar la fitotoxicitat del mercuri i reduir el risc en conreus exposats a sòls contaminats amb mercuri.
Selenium, as an essential micronutrient for humans, has several roles in health and thus, its intake at optimum levels is highly beneficial. However, 500-1000 million of people worldwide suffer selenium deficiency, due to the low Se levels in soils of agricultural lands. Biofortification of crops with Se-rich fertilizers is the most effective approach to counteract selenium deficiency. However, the selenium speciation is also fundamental: plants are able to transform the soil inorganic selenium, i.e. selenite and selenate ions, into organic selenium, such as selenoamino acids, which are less toxic and more bioavailable. Wheat is the most consumed cereal worldwide and is able to tolerate and accumulate over 100 mg Se per kg of dry weight, thus being a suitable candidate for Se biofortification to produce an enriched functional food. Selenium in wheat is found in the form of five major selenium species: selenite, selenate, selenomethionine, methylselenocysteine and selenocystine. In the present thesis, the content and distribution of these species in wheat was determined by the tamdem of high-performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) after appropriate enzymatic sample digestion, and by X-Ray Absorption Spectroscopy (XAS), using synchrotron radiation, among other techniques. The speciation and concentration of Se, the plant growth conditions and the stage in which selenium is applied to the plant define the degree of selenium uptake, metabolization and distribution through the different plant organs. Selenite is readily reduced in wheat roots, and thus, it accumulates preferentially in underground tissues; on the other hand, selenate is highly mobile through the plant xylem and its translocation is faster than its reduction, therefore accumulating in shoots. The application of high Se concentrations may result in excessive tissue accumulation, and thus, plant stress and Se-induced toxicity, decreased plant biomass production and reduced grain yield. However, wheat phytotoxicity may be reduced by the application of selenium at florescence time, but still achieving similar enrichment of grain and Se metabolization. Selenite was almost completely reduced into organic species, especially in roots, where the induced toxicity effects produced a strong oxidizing environment within the plant, thus producing a high accumulation of organic selenium in grain in the form of selenocystine. Oppositely, selenate showed slower metabolization and a significant accumulation of selenium in inorganic forms in shoots, although in grain selenium was found as organic species in the form of selenomethionine, which can be unspecifically incorporated into proteins. On the other hand, the application of both anions simultaneously contributed to balance the Se enrichment due to their separate metabolic pathways. The mixture caused a more equilibrated distribution of Se in the plant tissues, reducing its phytotoxicity, but resulting in the same total selenium concentration in grain and an intermediate amount of selenomethionine and selenocystine. Furthermore, the spatially resolved speciation analysis of wheat grains, showed high selenium accumulations in the germ, bran and pigment strand, and a low selenium concentration in the endosperm, which correlated positively with the concentration of proteins in the different parts of the grain. Finally, the protective effect of selenium against mercury toxicity was shown and it seems that it was due to the formation of a protein-Se-Hg complex in roots. This complex reduced the translocation of mercury to shoots and grain, the selenate mobility and the selenite reduction in roots, but at the same time it enhanced the accumulation of C-Se-C amino acids, such as selenomethionine, in wheat grain. As a result, selenium counteracted mercury phytotoxicity and reduced the risk in crops exposed to mercury polluted soils.

This thesis describes work carried out on catalytic selenium reagents in a range of organic transformations. Four different areas have been investigated and are reported herein. Chapter 2 reports the unsuccessful development into prochiral ligands, where three different chalcogen atoms are incorporated into either a trisubstituted structure or into a crown ether ring. Then reports how these structures could attach to a metal atom and become chiral. A SSe I SeR Chapter 3 describes a range of selenium-based ligands, which has been used in the palladium allylic substitution reaction to see if there is good co-ordination between selenium and palladium and if good enantioselectivities can be achieved. Chapter 4 describes the use of seleninic acids as catalysts in a range of reactions where the most successful is used in asymmetric Baeyer-Villiger oxidations using a range of ketones with enantiotopic migrating groups. The enantioselectivities were investigated. Chiral Catalyst I I ,0 O H202 O ASe A X R O R R CH2CI2 R ' Chiral Catalyst Chapter 5 describes the successful work on catalytic selenium reagents used to convert /,y-alkenoic acids into their corresponding butenolides. The work describes the optimum conditions investigated, asymmetric version of the reaction and also investigates mechanistic aspects of the catalytic cycle. Cat - (PhSe)2 Oxidant R 0 R C00H " _J Solvent.

Titanium dioxide (TiO2) photocatalysis, which can oxidise or reduce organic and inorganic pollutants, is a developing technology for water and wastewater treatment. The current work investigates the photocatalytic reduction of cadmium and selenium species as the presence of these elements in water are of environmental concern. Although TiO2 has been widely used for the photocatalytic process, its light absorption is limited to the UV region of the solar spectrum. Hence, the current project also explores the possibility to deposit cadmium selenide (CdSe) onto TiO2 to extend the photoresponse to the visible region. This study demonstrated that cadmium (Cd(II)) could be reduced to its metallic form by photocatalysis. The choice of hole scavengers and reaction pH are of importance in determining whether the photocatalytic reduction reaction will occur. It is also essential that both Cd(II) and organic additives are adsorbed on the surface of TiO2. A mechanism for cadmium photoreduction in the presence of formate as the hole scavenger was proposed. The current investigation elucidated the mechanism for the photoreduction of selenite (Se(IV)). Selenite was found to be photoreduced to its elemental form (Se(0)) as films, by direct photoreduction of Se(IV), and as discrete particles, by the reaction between Se(IV) and selenide (Se(2-)) ions. The Se(2-) ions are believed to have been generated from the 6 electron photoreduction of Se(IV) and/or the further photoreduction of the Se(0) deposits. Photocatalytic reduction reactions of Se(IV) and selenate (Se(VI)) using different commercial TiO2 materials was also studied. The current work also successfully deposited CdSe by photocatalysis using Se-TiO2 obtained from the photoreduction of Se(IV) and Se(VI). The mechanism for CdSe deposition was clarified and attributed to the reaction of Cd(II) present in the system and the Se(2-) released from the reduction of Se(0) upon further illumination. The Se??TiO2 photocatalysts obtained from the photoreduction of different selenium precursors (Se(IV) and Se(VI)) resulted in the dominance of different morphologies of the CdSe particles. This suggests a new approach to manipulate the properties of CdSe during its formation, and hence control over electrical and optical properties of this semiconductor.

Prostate cancer is the second most common diagnosed cancer and the third most common cause of death related to cancer among men in developed countries. Several epidemiological studies, prospective cohort studies and animal tumour models state an inverse relationship between selenium status and cancer incidence. Se- methylselenocysteine (SeMSC), present in garlic, onions, leeks and broccoli, has been shown to be the most effective anti-carcinogenic selenium form in animal models. The aim of the work presented in this thesis was to investigate the influence of selenium compounds (Se-methylselenocysteine and selenomethionine) on prostate cancer progression and metastasis using various human cell lines (LNCaP, OU145 and PC3). Standard 20 gel and SILAC (stable isotope labelling with amino acids in cell culture) proteomics were used, in combination with mass spectrometry, to identify selenium- responsive proteins. IMPOH2, GPI, EZR and RGS10 were validated by western blot, while POIA3 and 00X5 showed a selenium response under serum depleted conditions. Some proteins require more scrutinizing (galectin-1, XRCC5, TAGLN2, 00X5 and FLT) as conflicting results were obtained during validation. Preliminary analysis using 20 gel proteomics revealed galectin-1 to be selenium-responsive in PNT1A cells, although this could not be confirmed by Western blot or an in-house ELlSA. Previously, it has been shown that SeMSC decreased the expression of collagen I and increased that of collagen IV and collagen VI. A LNCaP 3D gel suspension model was developed to allow further investigation of extracellular matrix components using fluorescence microscopy. In addition, the effect of selenium exposure on the migration and invasion of PC3 cells was investigated using a transwell kinetic assay and revealed a dose response increase, especially under low baseline selenium concentrations. In order to optimize future selenium in vitro projects the dynamics of several selenium biomarkers were investigated using different conditions, enabling better comparison between cell lines and/or selenium compounds.

The present work describes the selenium-mediated cyclofunctionalisations of alkenes. Three different areas are reported herein. Chapter 2 reports syntheses of several substrates for carbocyclisation reactions and use of selenium and Lewis acids resulting in various dihydronaphthalenes. These dihydronaphthalenes then acted as substrates for second ring forming reactions. This novel tandem double cyclisation comprises a carboannulation, a Friedel-Crafts reaction and a rearrangement. This cascade sequence has been proven to be a useful tool in the selective synthesis of dihydronaphthalenes and benzofluorenes from easily accessible stilbenes and provides fast access to polycyclic ring systems in a single step. Chapter 3 describes electrophilic selenium-mediated reactions which have been used to cyclise a range of /-keto esters to corresponding biaryl compounds under very mild conditions. The products were formed by a carboannulation via addition/elimination sequence and a subsequent rearrangement of range of alkyl and aryl groups. The key starting materials stilbene /-keto esters were readily prepared by Heck coupling and hydrolysis followed by condensation with potassium ethyl malonate. Chapter 4 describes work on catalytic selenium reagents with stoichiometric amount of hypervalent iodine to convert a range of stilbene carboxylic acids into their corresponding isocoumarins. The work also describes the selective synthesis of dihydroisocoumarins using diphenyl disulfide and dimethyl diselenide.

Selenium has been a method to test web applications for over a decade, it is interacting directly with the browser and has gained support from both browsers and the community. With the growing amount of browsers, mobile devices and operating systems which a web application is expected to work with, services providing these systems for testing web applications against has gained interest. These services provide testing as a service (TaaS), and runs Selenium-tests in the cloud. This research tried to compare the different services with each other in regard to flexibility, cost, simplicity and reliability. I have also tried to see differences between running the tests locally and using these services. The results showed that there are some differences between the services, and the one best suited might depend on the web application.

The aqueous chemistry of the reduction of selenious species by sodium dithionite, sodium sulfide, stannous chloride and sulfur dioxide/sulfite was studied. The resultant precipitates were characterized using different techniques, including scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. In the case of sodium dithionite, the research showed that, at initial pH below 1.7 and dithionite stoichiometric excess above three, less than 0.5 ug/L remained in solution less than a minute after the addition of the reagent. However, it was found that the precipitate, mainly composed of red amorphous selenium, was not stable in the presence of the dithionite decomposition by-products and partially redissolved after several hours. In the case of the reaction involving selenious acid and sulfide ions, the results showed that, below a pH of approximately 7.0, the precipitation reaction was complete in less than 10 minutes at a sulfide to selenium ratio above 1.8 with less than 5 ug/L of soluble selenium remaining in solution. The precipitate was deduced to be a Se-S solid solution consisting of ring molecules of the following SenS8−n formula. However, above pH 7 the removal of selenium was incomplete due to formation of a stable colloidal suspension. In the case of industrial solutions, high dissolved sulfur dioxide content (> 5g/L) decreases the effectiveness of the precipitation reaction. Concerning the removal of Se(IV) by tin(II), at an initial pH below 1.3 and molar ratios above two, less than 5 ug/L of selenium(IV) remained in solution after reduction. The reaction products, were determined to be composed of approximately equal amounts of tin selenide and tin dioxide with a small amount of selenium(IV) being adsorbed on the latter. Finally, it was found that Se(IV) can be reduced quantitatively at ambient temperature via a combination of sulfite reduction and ultra-acidification to a colloidal form that can be determined using turbidimetry. The developed technique was shown to be equally effective with real Se(IV)-bearing industrial solutions generated in a zinc concentrate roasting operation hence making the method particularly useful as on-line process monitoring and control tool.
La chimie aqueuse des réactions de réduction entre les ions sélénieux et le dithionite de sodium, le sulfure de sodium et les ions stanneux ont été étudiées. Les précipités résultant de cette réaction ont été caractérisés par différentes techniques comme la microscopie électronique à balayage, la diffraction à rayons X et la spectrométrie de photoélectrons induits par rayons X. Dans le cas du dithionite de sodium, ce projet a démontré que, pour un pH initial inférieur à 1.7 et un ratio stœchiométrique de dithionite supérieur à trois, moins de 0.5 ug/L de sélénium(IV) restait en solution moins d'une minute après l'addition du réactif. Par contre, le précipité, formé en majorité de sélénium rouge amorphe, n'était pas stable en présence des composés issus de la réaction de décomposition du dithionite et était partiellement re-dissous après quelques heures. Dans le cas de la réaction entre l'acide sélénieux et le sulfure de sodium, les résultats indiquaient que pour un ratio molaire sulfur/sélénium plus grand que 1.8 et un pH sous 7.0 la réaction de précipitation était complète et moins de 5 ug/L de sélénium (IV) restait en solution. Aucune précipitation n'a été constatée pour un pH plus élevé que 9.5 et la réaction était incomplète pour un pH entre 7 et 9.5. Le précipité semblait être composé d'une solution solide Se-S formé de molécules circulaires répondant à la formule SenS8−n. Par contre, pour un pH plus élevé que 7, la précipitation du sélénium était incomplète à cause de la formation d'un précipité de sélénium colloïdal. De plus, dans le cas des solutions industrielles, de hautes concentrations de dioxyde de souffre dissous (> 5g/L) diminuait l'efficacité de la réaction de précipitation.En ce qui concerne la réaction entre le sélénium(IV) et l'étain, pour un pH initial sous 1.3 et un ratio molaire étain/sélénium plus grand que deux, moins de 5 ug/L de sélénium restait en solution après la réaction. Les analyses ont révélé que le précipité était composé de parts approximativement égales de séléniure d'étain et de dioxyde d'étain et que ce dernier pouvait adsorber une petite quantité d'ions sélénieux. Finalement, ce projet a démontré que le sélénium(IV) peut être réduit, à la température ambiante, par une solution ultra-acide riche en sulfites. Une fois le sélénium précipité sous forme colloïdale, la concentration originale de celui-ci peut ensuite être déterminée par turbidimétrie. Par ailleurs, cette méthode est aussi efficace sur des solutions industrielles contenant du sélénium(IV) et peut donc être appliquée pour l'analyse en temps réel de cet élément.

Selenium (Se) is an essential nutrient for health. In mammals Se is incorporated into ~25 selenoproteins in the form of the amino-acid selenocysteine encoded by the UGA codon through a complex interacting with selenocysteine insertion sequence (SECIS) in the 3’Unstralated Region. The selenoproteins have functions in antioxidant defence and redox control, thyroid hormone metabolism and mitochondrial metabolism. Previous scientific work has found that Se also affects a group of downstream targets. The aim of my work is to investigate whether expression of selenoproteins or the downstream targets affected by Se is regulated through epigenetic mechanisms involving microRNAs, a small non-coding RNA species that regulates a gene or groups of genes by binding to the mRNA 3’UTR. Gut epithelial Caco-2 cells were grown in either Se deficient or Se-supplemented medium for 72h. RNA extracted and miRNA expression analysed using a custom-designed human genome V2 Agilent 8x15K array. In addition, global mRNA transcriptome expression was analysed using an Illumina HumanRef-8 v3 microarray. Se supply increased the expression of thirteen miRNAs and 53 mRNAs the observed differences were confirmed by real time PCR. miR-185 was selected as a further target of investigation because of its high sensitivity to Se. Bioinformatic analysis of the Se susceptible miRNAs and Se sensitive genes was carried out using miRWalk, MicroCosmo, microRNA.org, miRBase and microRNA.org algorithms and Ingenuity Pathway Analysis. This identified miR-185 recognition elements in the Se-sensitive mRNAs for Glutathione peroxidise 2 (GPX2), Glutathione peroxidise 3 (GPX3), and Selenophosphate Synthetase 2 (SEPHS2). Expression of GPX2 and SEPHS2 was altered by miR-188 with a specific anti-miR and affected differently according to miRNA exposure. The data suggests that these mRNAs are targets of miR-185. In conclusion, the experiment indicates that miRNA expression as regulation of target genes is regulated by Se supply in Caco-2 cells.

Phosphorus-Selenium chemistry has seen a surge in development over the last five to ten years thanks to the optimisation of the synthesis of 2,4-diphenyl-1,3,2,4-diselenadiphosphetane-2,4-diselenide, Woollins' Reagent. This selenium analogue of the well known Lawesson's Reagent has proved itself to be a valuable asset to modern inorganic chemistry, providing a route to novel heterocycles, as well as acting as a selenation reagent with a wide range of functional groups. A series of new ammonium phenylphosphonamidodiselenoate ligands were synthesised via the reaction of Woollins' Reagent with a range of amines. The products were obtained in high yields and could be used as ligands for the synthesis of novel metal complexes. The reaction of diisopropylamine N-isopropyl-P-phenylphosphonamidodiselenoate with nickel(II) acetate produces a dimeric structure, whilst the reaction with copper(II) acetate yields a beautiful cluster of the form Cu₆Se₃L₆. The phenylphosphonamidodiselenoate ligands were further reacted with a range of cis-Pt(PR₃)₂Cl₂ complexes to form a library of 20 novel compounds, which were studied by ³¹P{¹H}, ⁷⁷Se{¹H} and ¹⁹⁵Pt{¹H} NMR spectroscopy. The X-ray crystal structure of one of these compounds was obtained, which confirmed the atom connectivity and spatial arrangement of the complexes and the geometry around the platinum centre. During investigations into the above-mentioned platinum complexes, it was postulated that an increase in steric bulk of the phosphine ligands would aid crystallisation of the products. As such, trimesityl-, dimesitylphenyl- and mesityldiphenyl phosphine were synthesised and reacted with elemental sulfur and selenium and hydrogen peroxide, as well as Pt(cod)Cl₂ and K₂[PtCl₄], yielding nine new structures, all of which were characterised by X-ray crystallography, ³¹P{¹H}, ¹³C{¹H} and ¹H NMR spectroscopy. Finally Woollins' Reagent was reacted directly with a selection of metal complexes, yielding some new insights into its reactivity with inorganic moieties, which has been relatively sparsely reported until now.

El seleni és essencial per a la salut humana Les fonts de Se s’obtenen en gran mesura del consum de vegetals, ja que el cos humà no pot sintetitzar de forma eficaç les espècies de Se orgànic assimilables. El selenit i el selenat són les formes inorgàniques predominants de Se que les plantes poden aborbir i metabolitzar en la seva major part en formes orgàniques assimilables per als animals i la ingesta humana. L’ús de fertilitzants amb Se s’està convertint en una pràctica comú en regions deficients en aquest element per poder obtenir així cultius i aliments enriquits amb Se. Malgrat això, encara hi ha qüestions que s’han d’abordar en relació a la toxicitat induïda pel Se a la planta. En aquest sentit, els bioestimulants vegetals s’utilitzen per millorar la nutrició, la tolerància a l’estrès abiòtic i la qualitat dels cultius. En aquest estudi hem aplicat un bioestimulant basat en un conjunt d’heteropolioxometalats híbrids de molècules amb l’estructura de Keggin barrejades amb àcids húmics. El nostre primer objectiu és avaluar l’efecte d’aquest producte en contrarestar la toxicitat del Se en el desenvolupament normal del blat i, en segon lloc, avaluar la possible modificació de l’especiació del Se en la seva presència. L’estudi de la saba del xilema va demostrar que l’especiació inorgànica de Se té diferents vies metabòliques a la planta. La translocació és un procés clau per controlar l’acumulació d’elements nutritius i de Se a la part aèria. S’ha establert un mètode ràpid i de baix cost, basat en titracions potenciomètriques d’àcid-base, per determinar la resposta de la saba a les espècies de Se. A més, l’estudi de l’aplicació del biostimulant es va realitzar en diferents períodes de creixement del blat; a curt i a llarg termini. Les plantes de l’assaig a curt termini van ser exposades a Se(IV), Se(VI) o a una barreja d’ambdues espècies (Se(MIX)) en presència com en absència del biostimulant, aplicat via foliar (FA) o a través de les arrels (RA). Els nostres resultats mostren que FA no va modificar la biomassa de la planta però que RA va fer augmentar significativament la biomassa de l’arrel en tots els tractaments, així com la biomassa de la tija de plantes exposades a Se(VI) i Se(MIX). El biosestimulant va accelerar la translocació de Se en presència de Se(VI) i Se(MIX). Mitjançant XAS es va identificar el Se orgànic com a l’espècie principal de Se formada en tiges, essent la influència del biostimulant gairebé insignificant en l’especiació Se. Aquests resultats indiquen el potencial d’aquest biostimulant en l’enriquiment dels conreus amb Se evitant al mateix temps l’estrès induït per aquest element. L’assaig a llarg termini ens va centrar en la co-aplicació dels tractaments de Se i l’aplicació foliar del biostimulant (FA) en diferents etapes de creixement (afillament i emergència de la inflorescència) fins a la collita. El biostimulant va tenir un paper fonamental en l’increment tant de la quantitat de gra produït per espiga com de la seva biomassa sense disminuir la quantitat total de Se i mantenint la seva especiació igual que en absència del biostimulant. L’ús d’espectroscòpia de raigs X va mostrar que el Se orgànic és l’espècie principal de Se en el gra de blat i que l’estadi de creixement en el que es realitza l’aplicació del Se afecta a la proporció d’aquest Se orgànic. Aquesta informació serà útil en els programes de biofortificació per minimitzar els costos econòmics y aconseguir un enriquiment de Se més eficient ja que la productivitat dels cultius no es veuria afectada i la quantitat dietètica necessària de formes orgàniques de Se en parts comestibles assoliria un rang adequat.
El selenio es vital para la salud humana. Las fuentes de Se se obtienen en gran medida del consumo de vegetales ya que el cuerpo humano no puede sintetizar eficazmente las especies orgánicas asimilables. El selenito y el selenato son las formas inorgánicas predominantes que las plantas pueden absorber y metabolizar en su mayor parte en formas orgánicas asimilables para los animales y la ingesta humana. El uso de fertilizantes con Se se está convirtiendo en una práctica común en regiones con deficiencias en este elemento para obtener cultivos y alimentos enriquecidos con Se. Pese a ello, todavía hay cuestiones que deben abordarse en relación a la toxicidad inducida por el Se en la planta. En este sentido, los bioestimulantes se utilizan para mejorar la nutrición, la tolerancia a estrés abiótico y la calidad de los cultivos. En este estudio hemos aplicado un bioestimulante basado en un complejo de heteropolioxometalatos híbridos de moléculas con estructura de Keggin mezcladas con ácidos húmicos. Nuestro objetivo es evaluar el efecto de este producto en contrarrestar la toxicidad del Se en el desarrollo normal del trigo y, en segundo lugar, evaluar una posible modificación de la especiación del Se en su presencia. El estudio de savia de xilema demostró que las especies inorgánicas de Se tienen diferentes vías metabólicas en la planta. La translocación es un proceso clave para controlar la acumulación de Se y de elementos nutritivos en la parte aérea. Se ha establecido un método rápido y de bajo costo basado en titriaciones potenciométricas ácido-base para ensayar la respuesta de la savia a las especies de Se. Además, los estudios relativos a la aplicación del bioestimulante se realizaron en distintos períodos de crecimiento del trigo; a corto y a largo plazo. Las plantas del ensayo a corto plazo fueron expuestas a Se(IV), Se(VI) o a una mezcla de ambas especies (Se(MIX)) en presencia y en ausencia del bioestimulante, aplicado por vía foliar (FA) o por vía radicular (RA). Nuestros resultados muestran que FA no modificó la biomasa de la planta, mientras que RA aumentó significativamente la biomasa de las raíces en todos los tratamientos, así como la biomasa de los tallos bajo Se(VI) y Se(MIX). El bioestimulante aceleró la translocación de Se en presencia de Se(VI) y Se(MIX). Mediante XAS se identificó el Se orgánico como la principal especie de Se formada en los tallos, siendo la influencia del bioestimulante casi insignificante en la especiación de Se. Estos resultados indican el potencial de este biostimulant en el enriquecimiento de cultivos con Se evitando al mismo tiempo el estrés inducido por este elemento. El ensayo a largo plazo se centró en la co-aplicación de los tratamientos de Se y de la aplicación foliar del bioestimulante en diferentes etapas de crecimiento del trigo (ahijamiento o emergencia de la inflorescencia) hasta la cosecha. El bioestimulante tuvo un papel clave en el aumento tanto de la cantidad de granos producidos por espiga como de su biomasa sin disminuir la cantidad total de Se y manteniendo la especiación de Se igual como en ausencia de bioestimulante. El uso de espectroscopia de rayos X mostró que el Se orgánico es la principal especie de Se en el grano de trigo y que la etapa de aplicación del Se afecta a la proporción de este Se orgánico. Esta información será útil en los programas de biofortificación para minimizar los costos económicos y lograr una suplementación de Se más efectiva ya que la productividad de los cultivos no se vería afectada y la cantidad dietética necesaria de formas orgánicas de Se en las partes comestibles alcanzaría un rango adecuado.
Selenium is vital to human health. The resources of Se are largely obtained from edible plants since the human body cannot synthesize assimilable organic Se species effectively. Selenite and selenate are the predominant inorganic Se forms that are taken up by plants. Afterwards, they are mostly metabolized to organic forms which are the forms of Se effectively assimilable for animals and human intake. Although crop enrichment with Se-containing fertilizers to obtain Se-biofortified food in Se-deficient regions is becoming a common practice, there are still issues to be addressed regarding the Se-induced toxicity to the plant itself. In this respect, plant biostimulants are used to enhance nutrition efficiency, abiotic stress tolerance and crop quality. In this study we have applied a biostimulant based on a complex of hybrid hetero-polyoxometalates of Keggin structure molecules mixed with humic acids. Our aim is to assess the effect of this product in counteracting the toxicity of Se which hampers the normal development of wheat plants, and second, to evaluate a possible modification on the Se speciation in the presence of the biostimulant. Xylem sap analysis showed that inorganic Se species have different metabolic pathways in the plant tissue. Translocation is a key process to control Se and nutrient elements accumulation in wheat shoots. A quick and low-cost method based on acid-base potentiometric titrations for testing the sap response to the Se species has been established. Furthermore, the studies regarding the application of the biostimulant on wheat plants are performed on short-term and long-term plant growth periods. Short-term wheat plants were exposed to either selenite, selenate or a mixture of both species (Se(MIX)) in presence or absence of the biostimulant either by foliar (FA) or by root application (RA). Our results show that the FA did not modify the plant biomass but RA significantly increased the root biomass in all treatments as well as the shoot biomass under Se(VI) and Se(MIX). The biostimulant accelerated the translocation of Se in the presence of Se(VI) and Se(MIX). XAS allowed to identify organic Se as the main Se species formed in the shoots being the influence of the plant biostimulant almost negligible on the Se speciation. These results indicate the potential of this biostimulant in the Se-enrichment of crops while avoiding the possible stress induced by this element. In a long-term assay we targeted on the co-application of the Se-treatments together with the foliar application of the biostimulant at different growth stages (tillering or heading stage) until harvesting. The biostimulant had a key role in the enhancement of both the amount of grains produced per spike and their biomass without diminishing the total amount of Se and maintaining Se speciation as in the absence of the biostimulant. The use of μ-XAS showed that organic Se is the main Se species in wheat grain and that, Se application stage influence the proportion of organic Se. This information will be useful for biofortification programs to minimize economic costs towards a more effective Se supplementation since the productivity of crops would not be affected and the necessary dietary amount of organic Se forms in edible parts will reach an adequate range.

Thesis (MScAgric)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: In many parts of the world, soil is depleted of selenium (Se), leading to selenium-poor plants, animals and, therefore, humans. It was recognised that a study to examine the functionality of new products on the market to address this problem was required. The purpose of this research were threefold: to compare the effects of sodium selenite (NaSe) and organically bound selenium sources on small ruminant performance, to investigate the bioavailability of these Se sources, and analyse their influence on carcass characteristics, meat quality and antioxidant capabilities. Fourty growing Döhne Merino wethers from the Southern Cape region, a selenium-deficient area, were used for the study. The animals were all fed the same basal diet in the adaptation period and were then allocated to one of four treatment groups: Control (CT), inorganic selenium (IS), organically bound Se A (OSA) or B: (OSB). The period of supplementation was 90 days. This first study assessed the effect of the different Se sources on growth and Se bioavailability in the wethers. The wethers and the feed they consumed were regularly weighed to determine their growth and feed conversion rate (FCR) in the trial period. To gauge their Se level, blood samples were collected via jugular venipuncture at monthly intervals. The wool around the jugular was shorn and samples were collected on day 0 and day 90 for comparative Se level analysis. Liver, skeletal muscle and kidney samples were collected at day 90, directly after slaughter, to determine the Se level in these tissues. No effect could be reported in the growth and FCR of the wethers between the supplementation groups. For whole blood Se levels there was an effect in the early part of the study, with a greater increase in Se levels for the organically bound Se groups, but in the end no effect on whole blood levels could be seen between the different Se treatments. Neither could any difference between the inorganic Se and organic bound Se treatments be found in the liver – however, the total Se concentration of the wool, kidney and meat samples was greater in those animals offered organically bound Se when compared with those receiving a comparable dose of inorganic Se. The second study evaluated the antioxidant capabilities of the different Se supplements in the wethers. Blood samples were taken monthly for plasma collection to test for Glutathione peroxidase (GSH-Px) activity and total antioxidative capacity (TAC) with the oxygen radical absorbance capacity (ORAC) assay. Liver, skeletal muscle and kidney samples were collected at day 90, immediately after slaughter and measured for GSH-Px activity. With TAC, there was a significant effect for the treatment period between day 0 and day 90, however the treatments did not show any significant difference. No significant differences could be established between the different Se treatments for the GSH-Px analysis in any of the tissues. For the mean plasma values of the treatments no significant differences can be reported, but a significant difference was observed at day 30 in the contrast between the organically bound Se and the other treatment groups. The third study was to evaluate the quality and lipid oxidation of muscle from those wethers supplemented with different Se sources. Skeletal muscle samples were collected at day 90, directly after slaughter to determine this. No differences in the meat quality of the wethers could be detected between Se sources after the 90-day supplementation period. Lipid oxidation was measured by determining TBA reactive substances (TBARS) and once again no differences could be detected. Based on the results found in this investigation, it may be inferred that organically bound Se (OSA & OSB) supplementation will hold a number of advantages for small ruminants over inorganic Se supplementation. Animals fed the organically bound Se had reached adequate Se levels sooner on the organically bounded treatments than the inorganically bounded treated animals. The greater bioavailability of organically bounded Se over inorganic Se was proven by the increased Se levels in certain tissues and organs. Additionally, only the organically bounded Se could find a pathway to the wool, confirming that it was carried in an organic form (probably selenomethionine) in the body. Organically bound Se will therefore have a positive impact on small ruminant health and production, which will result in an indirect advantage for consumer health.
AFRIKAANSE OPSOMMING: Die grond in groot dele van die wêreld word selenium-arm en dit lei na selenium-arm plante, diere en mense. Dit is waargeneem dat ‘n studie wat kyk na die funksionaliteit van nuwe produkte op die mark om die probleem aan te spreek nodig is. Die doelwit van die studie was om verskillende selenium (Se) bronne te vergelyk en die uitwerking daarvan op klein herkouer prestasie te evalueer. Daar is gekyk na die biobeskikbaarheid, invloed daarvan op die karkas eienskappe en antioksidant vermoëns van die verskillende Se bronne. Veertig groeiende Dohne Merino-hamels van die Suid-Kaap-streek, 'n Se arm gebied is gebruik vir die studie. Die diere is almal dieselfde basale dieet gevoer in die aanpassing periode en dan toegeken aan een van vier behandelings: kontrole (CT), anorganiese Se (IS), organies gebinde Se A (OSA) of B: (OSB). Die tydperk van die aanvulling was 90 dae. In die eerste studie is gekyk na die effek van die verskillende bronne van Se op die groei en die biobeskikbaarheid daarvan aan die hamels. Die hamels en voer verbruik, is gereeld geweeg sodat hul groei en voer omset verhouding (VOV) in die proef tydperk te bepaal. Bloedmonsters is versamel deur middel van die jugulêre venipuncture vir die Se vlak bepaling daarvan. Lewer, skeletspier en nier monsters is versamel op dag 90, direk na die slagting vir die Se vlak bepaling. Die wol rondom die nekslagaar is geskeer en monsters is versamel op dag 0 en 90 vir Se vlak analise. Geen effek kan gerapporteer word vir die groei en VOV van die hamels tydens die aanvullings periode nie. Vir die bloed Se vlakke was daar 'n uitwerking in die vroeë deel van die studie, met 'n vinniger toename in Se vlakke vir die organies gebinde Se groepe, maar aan die einde kon geen effek gesien word tussen die verskillende Se behandelings nie. Geen verskil tussen die NaSe en organiese gebonde Se behandelings kon gevind word in die lewer nie. Die totale Se konsentrasie van die wol-, nier-en vleis

The current work investigates the photocatalytic reduction of selenium (Se) ions, selenate Se(VI) and selenite Se(IV), from two perspectives: Se ion removal from water and wastewater and the formation of nano-Se compounds. Se ion pollution has become an environmental issue in recent years, and hence there is an urgent need for an efficient removal technique. In addition, there is increasing interest in the formation of nano-size semiconductors for niche applications. Since Se is a semiconductor, its formation onto the semiconductor TiO2 could lead to the discovery of new composite materials. The current study has successfully elucidated the mechanism of Se ions reduction by photocatalysis. Factors such as the simultaneous adsorption of the Se ions (the electron scavenger in this case) and a suitable organic compound (the hole scavenger), and the chemical properties of the hole scavenger were crucial for effective and efficient Se ions photoreduction. Optimum conditions in relation to pH, concentrations and types of hole scavenger were reported and discussed. It was also found that stoichiometric adsorption ratio of formate and selenate resulted to optimum photoreduction rate. A modified Langmuir-Hinshelwood kinetic model that considered the simultaneous adsorption of both solutes was derived. The current investigation has also seen the successful formation Se deposits of different morphologies onto the TiO2 particles. Discrete Se particles of various sizes in the nano-size range as well as a Se film were deposited onto the TiO2 particles under different initial experimental conditions. The Se-TiO2 composite semiconductor was explored for the removal of cadmium Cd2+ ions, which resulted in the formation of CdSe-TiO2 systems. The photoreduction of Se ions using silver-modified TiO2 showed the enhanced reduction of Se ions to Se2- in the form of H2Se gas. It is suggested that the H2Se gas generated from the current photoreduction process could be used as a safer and cheaper technique in the formation of Se-compounds such copper selenide, cadmium selenide and zinc selenide. All these compounds were widely used in optical and semiconducting devices.

The topic of this doctoral thesis is the potential near field immobilization of the radionuclide 79Se after intrusion of groundwater into a spent nuclear fuel canister in a repository. 79Se is a non naturally occurring long lived selenium isotope formed as a result of fission in nuclear fuel. Given the long half life (~3 x 105 y) and that the oxyanions of selenium are expected to be highly mobile and potentially difficult toimmobilize the isotope is of interest for the long term safety assessment of high level waste repositories. In this work the near field has been limited to the study of processes at or near the UO2 surface of (simulated) spent nuclear fuel and to processes occurring at or near the surface of iron (canister material) corroding under anoxic conditions. Selenite (HSeO32-) was found to adsorb onto palladium (simulated noble metal inclusion in spent nuclear fuel). Under hydrogen atmosphere selenite was reduced to elemental selenium with a rate constant of ~2 x 10-9 m s-1 (with respect to the Pd surface, 24 bar H2) forming colloidal particles. The rate constant of selenite reduction was increased by about two orders of magnitude to ~2.5 x 10-7 m s-1 (with respect to the Pd surface, 10 bar H2) for a UO2 surface doped with Pd particles, indicating that UO2 is an efficient co-catalyst to Pd. Selenate (SeO42-) was neither adsorbed nor reduced in the presence of Pd, UO2 and hydrogen. In the iron corrosion studies selenate was found to become reduced to predominantly elemental Se in the presence of a pristine iron surface. Iron covered by a corrosion layer of magnetite did however appear inert with respect to selenate whereas selenite was reduced. The reduction of dissolved uranyl into UO2 by the corroding iron surfaces was found to significantly increase the removal rate of selenite as well as selenate. The uranyl was found to transiently transform the outer iron oxide layers on the iron, forming a reactive mixed Fe(II)/Fe(III) oxyhydroxide (Green rust). Exchanging the solution and increasing the carbonate content (from 2 mM to 20 mM NaHCO3) only resulted in a minor, transient remobilization of uranium. Addition of H2O2 did however result in a significant release of uranium as well as selenium from the iron oxide surfaces. An irradiation experiment was also performed confirming the one electron reduction barrier of selenate as an important factor in systems where selenate reduction would be thermodynamically favorable.
QC 20101208

Methylmercury (MeHg) is a ubiquitous contaminant and potent neurotoxicant with no completely effective therapy, although selenium antagonises MeHg toxicity. Furthermore, nanoparticles are promising as a novel drug delivery system. We researched the potential of selenium nanoparticles (SeNPs) in antagonising MeHg neurotoxicity compared to selenomethionine (SeMet) using primary astrocyte cell cultures and examining outcomes related to oxidative stress. We found that SeNPs were more toxic than SeMet. Increasing SeNPs significantly decreased MeHg cellular uptake and MeHg significantly decreased uptake of SeNPs at the highest concentration. Finally, SeNPs alone produced significantly higher reactive oxidative species and altered the ratio of reduced-to-oxidised glutathione, but MeHg, SeMet, and co-exposures did not. There were no significant effects on glutathione peroxidase or reductase activity. This suggests that SeNPs are more toxic than MeHg in cerebellar astrocytes and that they may not be suitable as a therapy at the doses and formulation used in this research.

14 MeV neutron activation analysis, based on the 82Se(n,2n) Se81m nuclear reaction, has been investigated for the measurement of selenium in environmental waters. The levels of the selenium present in environmental waters and the relatively high limit of detection achievable with fast neutron activation analysis resulted in the need to develop a concentration procedure for the extraction of selenium from large volumes of environmental water before neutron activation.

Selective isolation protocols of selenium (Se) species integrated to Se specific atomic absorption spectroscopy (AAS) detection were developed and optimized for Se speciation in food supplements, including selenized yeasts. By ultrafiltration, 69.18% of Se in the extract was found as a low molecular weight soluble form, the remaining 30.82% was bound to high molecular weight components. After a cation-exchange chromatography of the ultrafiltrate, 3.77% of the Se in the extract was found in the aqueous washings of the column indicating the presence of free inorganic anions of Se; the 65.41% of Se retained on the column corresponded to the free organic Se cations. The limit of detection for the HPLC-THG-AAS system was 1.85 ng of Se. Se was shown to be widely distributed over all the proteins with one sharp peak corresponding to the free forms of Se. Four major peaks were found at MW $>$ 250 000 Da (15.97% of Se recovered), between 102 330 and 117 490 Da (7.06%), between 48 977 and 53 703 Da (12.71%) and close to the dye migration band (17.25%).
Selective isolation and HPLC-AAS protocols were also developed and optimized for the determination of free organic forms e.g. selenomethionine (SeMet), selenocystine (SeCystine) and inorganic forms of selenium in aqueous solutions, and in complex matrices such as nutritional supplements and mixtures of free amino acids. The selenoamino acid in alkaline solution was first derivatized with 1-fluoro-2,4-dinitrobenzene. After removal of excess of reagent by partitioning with diethyl ether, the N-dinitrophenyl (DNP)-derivatized selenoamino acid was acidified and extracted with diethyl ether. Inorganic Se(IV) was extracted from the acidic aqueous phases by complexation with 1,2-phenylenediamine, forming a piazselenol. Se derivatives were determined selectively by HPLC-THG-AAS. A selective chromatographic mechanism based on $ pi$-electron interactions was optimized using a silica stationary phase derivatized with p-nitrophenyl moieties. Co-injections of DNP-SeMet, DNP-SeCystine and piazselenol save retention times of 3.7, 4.0 and 4.9 min, respectively, using a methanolic mobile phase containing 1.5% triethylamine and 0.013M acetic acid. Primary analytical validation parameters including stability, linearity and limits of detection were obtained using purified DNP-SeMet, DNP-SeCystine and piazselenol standards which were characterized by $ sp1$H-, $ sp{13}$C- and $ sp{77}$Se-NMR analysis and/or fast atom bombardment MS techniques. The calibration graphs for sequential dilutions of these Se standards were linear and the limits of detection from the resultant calibration graphs were 17 ng, 0.21 ng and 18.53 ng of Se, respectively. The purified DNP-SeMet and DNP-SeCystine were found to be photosensitive. The recovery of SeMet, SeCystine and inorganic Se from the stock solutions and/or nutritional supplements was virtually quantitative. In the presence of a 500-fold excess of other amino acids, the recovery of SeMet and SeCystine (96.1 $ pm$ 3.9% and 98.08 $ pm$ 4.2%, respec

The C͂¹B₂ ←　X͂¹A₁ electronic transition of SeO₂ has been investigated under high resolution, at a rotational temperature of around 10 K, using the technique of Laser Excitation Spectroscopy. The vibrationally-resolved survey spectrum contained around 100 new bands in addition to the bands which had been reported in a previous study of the same region (G.W. King and P.R. McLean, J. Mol. Spec. 51, 1974). In the light of this new spectrum a number of bands have been reassigned, most significantly the O⁰₀ band, and a number of progressions have been extended. This led to a revised determination of the vibrational constants of the excited state, and a more acceptable estimate of v'₃ than was suggested in the previous work. These reassignments and extensions of existing assignments accounted for only a small fraction of the newly observed bands; those remaining are thought to be due to a different electronic transition which lies in the same region as the C͂¹B₂ ←　X͂¹A₁ transition. The 1³₀, 1²₀ and 1¹₀ bands of the C͂¹B₂ ←　X͂¹A₁ transition were also recorded at rotational resolution and analysed using the method of ground state combination differences. The 1³₀ band was found to be perturbed, which was one of the major factors which prompted the survey study described above. From the analysis of these bands the rotational constants of the excited state were determined and hence the geometry of the SeO₂ molecule in the given vibrational levels of the ¹B₂ excited state was calculated. This in turn enabled the rotational constants and the geometry of the (00) vibrational level of the excited state to be estimated. This work confirms that the symmetry of the excited state is ¹B₂ and the transition studied is C͂¹B₂ ←　X͂¹A₁. An additional band around 31957 cm⁻¹ was also recorded at rotational resolution, which was initially though to be the O⁰₀ band, on the basis of King and McLean's assignments. However in the light of the reassignments the nature of this band is not known, and attempts to assign it as vibrationally cold band of the C͂¹B₂ ←　X͂¹A₁ transition were unsuccessful, implying that it is probably either a hot band of the C͂¹B₂ ←　X͂¹A₁ transition or a band belonging to different electronic transition.

Selenium and arsenic are important metalloids in the food chain from nutritional and toxicological point of view. These two metalloids are potentially enriched through geogenic processes and anthropogenic activities and they could sometimes co-exist in nature and become available to plants thereby entering the food chain. While selenium is known as an essential element to humans, it could also be toxic. Arsenic on the other hand is a potentially toxic element posing serious health risks to livestock and humans. They have been found to neutralise each other’s effects in animals but their interactions in plants are not well understood. Speciation analysis, which is a set of activities leading to identification and quantification of different forms or species of elements present in an entity, is required for a holistic understanding of the mechanisms and interactions involved in the plants’ metabolism of contaminants and essential elements. Many techniques are currently being used for speciation of selenium and arsenic in plants and they sometimes give contradictory outcomes. The hyphenation of HPLC with MS and synchrotron techniques are the two most commonly used state of the art techniques for speciation of these metalloids. This research therefore sought to access, explore and/ or develop analytical methods appropriate for the speciation of selenium and arsenic in plants. Many selenium and arsenic species have been identified and reported in the literature using well established procedures. The presence of elemental selenium in plants has also been widely reported in plants but to our knowledge this presence has never been experimentally proven and fully quantified. Because this species is non toxic, its proven occurrence in plants will represent a potential detoxification mechanism. Therefore in this study, a method was specifically developed for identification and quantification of elemental selenium. In order to investigate the occurrence of elemental selenium in plants, the newly developed method was applied using Thunbergia alata as a model plant. Arsenic is known to activate the synthesis of PC using glutathione and the complexation of the activating arsenic ions with the synthesised PCs is a well established detoxification mechanism for arsenic. However, very little is known about the role of glutathione and PCs in selenium detoxification. In order to be able to gain better insight into the interaction between selenium and arsenic in plants, the role of glutathione and PCs in selenium metabolism was investigated using Arabidopsis thaliana as a model plant. Sensitivity tests and speciation analysis were carried out on Arabidopsis thaliana WT and the mutants’ one of which is deficient in GSH synthesis and the other deficient in PC synthesis using selenite and arsenate as toxicants. The study revealed that selenium induces the synthesis of glutathione but rather use it as reductant and precursor for transformation and incorporation into peptides and neither GSH nor PCs play any role in selenium detoxification. It was also observed that when selenium and arsenic co-exist there could be competition for PCs between the ions of the metalloids with potentials for increasing arsenic toxicity. Human exposure to inorganic arsenic, a group 1 carcinogen, through Oryza sativa (rice), the staple food for about half the population of the world, has raised serious concerns. Most worrisome are the findings that rice grown in arsenic contaminated areas is characterised by reduced essential amino acids and micronutrients including selenium. A study was therefore conducted to to investigate and understand the interactions between selenium and arsenic in rice. The study confirmed that arsenic could limit the amount of selenium that is taken and translocated to the grains. Selenium was also found to reduce the toxicity of arsenic and most importantly, the study showed that at an appropriate selenium concentration, arsenic uptake and translocation can be reduced.

Epidemiological evidence indicates that selenium supplementation may increase risk for ocular hypertension and glaucoma. The purpose of this project was to determine the effects of selenium on the conventional "trabecular" aqueous outflow pathway, a likely site of pathology for glaucoma. Human trabecular meshwork (HTM) cells and human umbilical vein endothelial cells (HUVECs) were treated with selenium (MSeA) at or near physiologically relevant concentrations. Selenium uptake by cells was monitored using mass spectrometry. While detectible changes in intracellular selenium were observed after exposure to 1-10 uM MSeA for 24 hours, the majority remained in the conditioned medium. The high concentrations of extracellular selenium we observed raised the possibility that selenium has an extracellular target.To investigate the role of selenium in extracellular matrix turnover, I examined alterations in protein secretion and intracellular signaling. MSeA treatment (5-10 uM) led to a significant decrease in the secretion of matrix metalloproteinase -2 and its inhibitor after 6-24 hours and to a dose-dependent decrease in kinase signaling. Later, I investigated the possibility that integrins are an extracellular target of selenium by monitoring morphological changes in HTM cells and by treating them with divalent cations. MSeA stimulated morphological changes consistent with a decrease in integrin function. These occurred before (less than 3 hours) alterations in protein secretion and intracellular signaling (3-6 hours). Zinc treatment prevented MSeA-mediated alterations in protein secretion and changes in cell-matrix adhesion.Finally markers of HTM cell homeostasis were examined. MSeA treatment (5 uM) led to a 60% decrease in protein synthesis after 3 hours and a 60% reduction in protein secretion, without causing significant alterations in cell viability and total ATP. To assess the physiological relevance of my results, anterior segments were perfused with MSeA to determine its effects on aqueous outflow facility. Preliminary results suggest that MSeA leads to a decrease in outflow facility.The combination of MSeA-induced decreases in several indicators of HTM cell homeostasis (without adversely effects on cell viability at physiologically relevant doses) and decreases in outflow facility provide a possible mechanism for selenium-associated ocular hypertension.

Aquetsa tesi descriu el treball realitzat amb reactius que contenen enllaços bor-sofre o bor-seleni. Aquest reactius s’han fet servir en reaccions de tioboració i selenoboració de substrats amb insaturacions conjugades a cetones o esters i en reaccions d’inserció amb grups diazo per a sintetitzar nous compostos organosulfurats i organoselenats. La tesi es divideix en quatre capítols. El primer d’ell és una breu introducció al compostos organosulfurats i organoselenats, parlant sobre les seves principals aplicacions i els mètodes per sintetitzar-los més representatius fins al moment. En aquest primer capítol també és parla de la síntesis dels reactius de bor-sofre i bor-seleni i les reaccions en les que s’han fet servir. El segon capítol parla de la reactivitat dels compostos de bor-sofre amb cetones i aldehids α,β-insaturats. S’ha observat que el propi grup carboxil és capaç d’activar el reactiu i fer entrar la unitat de sofre en la posició beta. El tercer capítol parla de la reacció amb triples enllaços conjugat a cetones i esters per sintetitzar vinil sulfats i vinil selenats. Aquesta mateixa reacció en presència d’una fosfina permet obtenir compostos anti-3,4-selenoborats que són precursors de compostos amb el grup seleni en la posició alfa. L’últim capítol parla sobre les reaccions d’inserció de compostos diazo en l’enllaç bor-sofre que permet obtenir molècules molt funcionalitzades amb grups Si, B, S i H que degut a les seves diferents propietats químiques poden continuar sent funcionalitzades per sintetizar una gran varitat de compostos orgànics.
Esta tesis describe el trabajo realizado con reactivos que contienen enlaces boro-azufre o boro-selenio. Estos reactivos se han usado en reacciones de tioboración y selenoboración de sustratos con insaturaciones conjugadas a cetonas o esteres y en reacciones de inserción con grupos diazo para sintetizar nuevos compuestos organosulfurados y organoselenados. La tesis se divide en cuatro capítulos. El primero es una breve introducción a los compuestos organosulfurados y organoselenados, hablando sobre sus principales aplicaciones y los métodos para sintetizarlos más representativos hasta el momento. En este primer capítulo también se presenta el método de síntesis de los reactivos de boro-azufre y boro-selenio y las reacciones en las que se han usado. El segundo capítulo habla de la reactividad de los compuestos de boro-azufre con cetonas y aldehídos α,β-insaturados. Se ha observado que el propio grupo carboxilo es capaz de activar el reactivo y hacer entrar la unidad de azufre en la posición beta. El tercer capítulo habla de la reacción con triples enlaces conjugado a cetonas y esters para sintetizar vinilo sulfatos y vinilo selenatos. Esta misma reacción en presencia de una fosfina permite obtener compuestos anti-3,4-selenoborats que son precursores de compuestos con el grupo selenio en la posición alfa. El último capítulo habla sobre las reacciones de inserción de compuestos diazo en el enlace boro-azufre que permite obtener moléculas muy funcionalizadas con los grupos Si, B, S y H que debido a sus diferentes propiedades químicas pueden continuar siendo funcionalizados.
This thesis describes the work done with reagents that contain links boron-sulphur or boron-selenium. These reagents have used in reactions of thioboration and selenoboration of substrates with unsaturation conjugated to ketones or esters and in reactions of insertion with diazo compounds to synthesize new organosulfides and organoselenides compounds. The thesis is divided in four chapters. The first is a brief introduction to the organosulfides and organoselenides compounds, reporting its main applications and its more representative methods of synthesis. The first chapter also reported the synthesis of the boron-sulphur and boron-selenium reagents and the reactions where they have been used. The second chapter is about the reactivity of the compounds of boron-sulphur with α,β-unsaturated ketones and aldehydes. It has observed that the carboxyl group is able to activate the B-S reagent and deliver the sulphur unit in the beta position. The third chapter shows the reactivity with triple bonds conjugated to ketones and esters to synthesise vinyl sulphates and vinyl selenates. The same reaction in presence of a phosphine allow to obtain anti-3,4-selenoborated compounds that they are precursors of compounds with the selenium moiety in the alpha position. The last chapter is about the insertion reactions of diazo compounds into the boron-sulphur bond that allow to obtain molecules very functionalized with groups Si, B, S and H than due to its different chemical behaviour can be further functionalized.

Selenium (Se) is an element with important health implications that is emitted in significant amounts from volcanoes. Attracted by the fertility of volcanic soils, around 10% of the world population lives within 100 km of an active volcano. Nevertheless, the behaviour of Se in volcanic environments is poorly understood. Therefore, the main aim of this thesis is to investigate the role of soils in the Se cycling in volcanic environments. Prior to the geochemical studies, precise and accurate methods for the determination of Se contents, speciation and isotopic signatures were developed. Afterwards, a combination of field studies and lab controlled experiments were performed with soils from two contrasting European volcanic settings: Mount Etna in Sicily (Italy) and Mount Teide in Tenerife (Spain). The results showed a strong link between Se behaviour and soil development, indicating that Se mobility in volcanic soils is controlled by sorption processes and soil mineralogy.
El selenio (Se) tiene afectas sobre la salud y es emitido por los volcanes. Atraída por la fertilidad de los suelos volcánicos, un 10% de la población mundial vive <100 km de volcanes activos. Sin embargo, el comportamiento geoquímico del Se en ambientes volcánicos es aún poco conocido. El objetivo de esta tesis es contribuir a la comprensión del papel del suelos en el ciclo del Se en medios volcánicos. En una primera etapa se desarrollaron métodos analíticos para determinar el contenido, la especiacíon y las relaciónes isotópicas del Se. Los estudios geoquímicos se realizaron con suelos de dos lugares con características muy distintas: el Monte Etna (Italia) y el volcán Teide (Tenerife). Los estudios de terreno y de laboratorio revelaron una gran interrelación entre el comportamiento del Se y el desarrollo de suelos, y que la movilidad de Se está controlado por los procesos de adsorción y la mineralogía en suelos volcánicos.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Universidade Estadual Paulista (UNESP)
O objetivo do trabalho foi comparar a biodisponibilidade e os efeitos antioxidantes de fontes de selênio na forma orgânica e inorgânica em dietas para cães. Foram utilizados 24 cães machos da raça Beagle distribuídos em três tratamentos: controle (TC-0,11 ppm de Se), inorgânico (TI-0,30 ppm de Se na forma de selenito de sódio) e orgânico (TO-0,30 ppm de Se na forma de seleno levedura). O experimento seguiu um delineamento em blocos casualizados com medidas repetidas no tempo, os animais foram blocados por idade. Os animais foram mantidos por 10 dias em gaiolas metabólicas e submetidos à coleta total de fezes e urina, e mais 70 dias em baias para demais análises. Nas amostras de ração, fezes, urina e pêlos foram feitas análises de selênio. No sangue total foi determinada a atividade da GSH-Px e no 11º dia do experimento foi realizada a curva pós-prandial de absorção de selênio, até doze horas após a ingestão das dietas. O status oxidativo e a capacidade antioxidante foram avaliados no soro sanguíneo através da metodologia de TBARS (substâncias reativas ao ácido tiobarbitúrico) e a capacidade antioxidante total (TAC), respectivamente. Os dados foram submetidos à análise de variância e as médias comparadas pelo teste de Tukey (P≤0,05). Para as varáveis ingestão de Se, excreção de Se nas fezes e retenção de Se o os animais do TC apresentaram menores valores quando comparados aos animais do TI e TO (P≤0,05). Não houve diferença para a excreção urinária e biodisponibilidade. Houve efeito de período para a concentração de selênio no plasma sanguíneo ocorrendo aumento para todos os animais. As concentrações de Se no pêlo dos animais dos grupos TI e TO foram maiores que as do TC ao final do experimento (P≤0,05). A área abaixo da curva do Se plasmático dos animais TO foi maior do que as dos demais, mostrando uma maior absorção...
The aim of this study was to compare the bioavailability of organic and inorganic selenium sources and their antioxidants functions in diets to dogs. Twentyfour male dogs were used and distributed in three treatments: control (TC-0,11 ppm of Se), inorganic (TI-0,30 ppm of sodium selenito) and organic (TO-0,30 ppm of Sel- Plex®). The experimental design was in a randomized blocks, the blocking factor was age. The experimental period was 80 days, the selenium balance trial was conducted at the first 10 days then the animals were transferred to kennels and the experiment lasted more 70 days. Feces, hair, urine, blood plasma and food were analyzed for selenium concentrations. The GSH-Px was determined in whole blood and at the 11th day a curve of selenium plasmatic absorption was conducted, the samples of blood plasma were obtained each 2 hours after feeding per 12 hours. The methodologies to access oxidative stress and antioxidant capacity were TBARS and total antioxidant capacity, respectively. The results were performed by analysis of variance and the means were compared by Tukey test (P≤0.05). For Se intake, fecal excretion of Se and Se retention, dogs from TC group showed lower values then TI and TO animals. There were no difference for urinary excretion of Se and Se bioavailability. For Se concentration in blood plasma the animals showed a higher value within 80 days even for animals from TC group. The concentration of selenium in hair increased within 80 days for animals from the TI and TO group. TO group had the major area under the curve differing statistically from the others groups showing a better absorption. The supplemented groups had the better values for GSH-Px activity. However, there were no significant differences among treatments for TBARS. For TAC there were no differences among the treatments, but during the experimental time the TAC values increased... (Complete abstract click electronic access below)

A study has been made of the xerographic properties of amorphous selenium doped with chlorine at concentrations below 100 parts per million (ppm) by weight. The sample consisted, in each case, of an aluminum plate with a thin deposited r.f. sputtered aluminum oxide film, on which a 50 micrometer layer of amorphous selenium was deposited by evaporation, with a substrate temperature of 50$ sp circ$C. It was found that the chlorine decreased the acceptance voltage following corona charging, increased the dark decay rate and decreased the residual voltage after illumination discharge. The changes were such as to be beneficial xerographically for small additions of chlorine to the selenium in the ppm range. Analysis of the time derivative of the dark decay voltage indicated that depletion discharge was the dominant process in the decrease of dark decay voltage with time and a modified relation was introduced to describe the increase of bulk space charge density with time, arising from thermal excitation of holes from deep discrete centres in the photoreceptor. This analysis indicated a decrease of the release time of the holes with increase of chlorine content, whereas an observed decrease of single cycle and cycled-up residual voltages with increased chlorine indicated an increase of hole capture time from their relevant trapping centres. Capacitance and incremental resistance measurements on the aluminum oxide blocking layers indicated higher resistance in films deposited by r.f. sputtering than by d.c. reactive sputtering on aluminum substrates. However, no difference in xerographic performance could be detected between the two sputtering methods. Oxide layers were also deposited on nickel and gold substrates by r.f. sputtering.

Successful preparations were: (1) bis(pentamethylcyclopentadienyl)selenium, prepared by the reaction of selenium diethydithiocarbamate with lithiated pentamethylcyclopentadiene; (2) Cp*Se_nLi (n = 2,3,4), prepared by the reaction of elemental selenium with lithiated pentamethylcyclopentadiene in THF and its subsequent reaction with iodomethane to produce (3) methyl(pentamethylcyclopentadienyl)selenium or water to produce the polyselenide mixture (4) Cp*₂Se_n (n = 2,3,4). (4) was also produced by the low temperature reaction of selenium chloride with lithiated pentamethylcyclopentadiene. The compounds were characterised by conventional methods including ⁷⁷Se NMR spectroscopy. The x-ray structural determination was obtained for compound (1) revealing the largest C-Se-C bond angle recorded for a simple diorganoselenide. The reactivities of the compounds (1) (3) and (4) were investigated in reactions with unsaturated transition metal fragments. In the reactions of (1) with sources of the tungsten and chromium carbonyl compounds [W(CO)₅THF], [W(CO)₃(CH₃CN)₃], [Cr(CO)₅THF], [Cr(CO)₄nbd] and [Cr(CO)₃(CH₃CN)₃] the first example of reduction of a selenide to form a diselenide within the transition metal coordination sphere was observed. These produced the complexes [M(CO)₂{Se₂Cp*₂}] (M = W, Cr). The x-ray structure determinations are included. These complexes contain the longest Se-C bonds recorded. A 1,2 metal shift was observed for the tungsten complex using the technique of two dimensional exchange NMR spectroscopy. (3) reacts with [W(CO₅THF] or [Cr(CO)₅THF] to produce the complexes [W(CO)₅{Se(Me)Cp*}] and [Cr(CO)₅{Se(Me)Cp*}]. The first recorded Se-W satellites are reported for the former. The compounds (1), (3) and (4) were reacted with diiron nonacarbonyl to produce the complex [Fe₃Se₂(CO)₅]. Compound (1) was reacted with a half molar equivalent of [PdCl₂(PhCN)₂] to produce the complex [Pd₂Cl₄(SeCp*₂)].

This study investigated the interaction between polychlorinated biphenyls (PCBs) and selenium to explain the mechanism involved that could affect selenium metabolism and its anti-cancer property. PCBs congeners and mixtures were previously found to reduce hepatic Se and Se-dependent glutathione peroxidase activity. I hypothesized that certain PCB congeners affect selenium metabolism in the rat liver resulting in diminished antioxidant capacity of selenoproteins, which could alter the ability of Se to protect against PCBs induced tumor promotion. In the first study, the influence of 3,3,4,4-tetrachlorobiphenyl (PCB 77) on hepatic Se and glutathione peroxidase (GPx1) activity as well as cytochrome P450 1A1 induction was examined by employing a time-course study, which showed that PCB 77 significantly reduced the hepatic selenium level and GPx1 activity and that this effect was influenced by gender. The next study explored how PCB 77 could deplete hepatic selenium by determining selenium concentrations in different tissues, feces and urine. This study demonstrated that PCB-77 decreased hepatic Se by increased excretion of Se in urine but not in feces. Unlike glutathione peroxidase, thioredoxin reductase activity was not affected by PCB 77. The third study investigated the effect of selenium supplementation on the tumor promoting activity of PCB 77 and 2,2,4,4,5,5-hexaclorobiphenyl (PCB 153) using a 2-stage carcinogenesis model. Se supplementation did not diminish the induction of altered hepatic foci by coplanar PCB 77 or ortho-substituted PCB 153. Instead of protection, the number of foci per cubic centimeter and per liver among the PCB-77 treated rats was increased as the selenium dietary level increased. PCB 153 did not show the same selenium dose-response effect; nevertheless, selenium supplementation did not confer protection against foci development. On the other hand, supranutritional selenium reduced the mean focal volume. Supranutritional selenium or PCBs did not affect cell proliferation or thioredoxin reductase activity. Lastly, the use of the Zeeman graphite furnace atomic absorption spectrometry (GFAAS) method and closed microwave digestion technique for selenium determination of biological samples was compared with the neutron activation analysis and fluorometry methods. I found that GFAAS was not as reliable as the other methods.

Potassium-selenium (K-Se) batteries are an interesting alternative to lithium-selenium (Li-Se) batteries with some notable advantages, such as a reduced cost of production and an environmentally cleaner way of production. Although the idea of K-Se batteries emerged a couple of years ago, in the last few years they have been taken seriously as a capable energy storage. Despite some disadvantages, such as a cathode volume expansion and the shuttle effect, it is considered as an interesting research field and many studies have been carried out. Herein, a carbon host to encapsulate selenium was made to try to reduce the shuttle effect. The defects in the carbon host will be controlled by H2O2 and the products will be analyzed to understand the correlation between the amount of H2O2 and the defects. After encapsulating selenium, the electrochemical proprieties will be analyzed using cyclic voltammetry, galvanostatic charge/discharge. (Additionally, the influence of the concentration of electrolyte will be studied due to the fact that it can modify the electrochemical properties of the batteries). The present project aims to determine if there is a considerable influence on the battery performance owing to the defects of the carbon hosts and the concentration of the electrolyte, and hence to find the best working condition both for K-Se batteries and for the encapsulation.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
As plantas não possuem mecanismos de defesa específicos para combater a diversidade de estresses abióticos e poluentes do ambiente, e sua sobrevivência depende da flexibilidade e adaptação dos seus próprios mecanismos de defesa naturais. Além disso, a manutenção da homeostase celular depende de vários mecanismos interligados e complexos, enquanto o sistema de defesa celular não segue um padrão específico de ação e pode ainda variar devido a vários fatores tais como a espécie do vegetal, o tempo de exposição ao estresse, o estágio de desenvolvimento da planta e também nos diferentes órgãos e tecidos analisados. Com base nessas considerações, esta dissertação teve como objetivo destacar e investigar o papel do Enxofre (S) e do Selênio (Se) contra diferentes estresses nas plantas, através das respostas enzimáticas, não enzimáticas e também outros mecanismos de defesa relacionados. No primeiro capítulo, o autor caracteriza os mecanismos bioquímicos gerais da defesa celular antioxidante, especificamente a formação das espécies reativas de oxigênio (EROs) e suas singularidades químicas e o estresse oxidativo induzido, o sistema de defesa antioxidante enzimático, especificamente as enzimas Superóxido Dismutase (SOD) e a Catalase (CAT), os mecanismos não-enzimáticas contra o estresse, incluindo o ciclo Aascorbato-Glutationa, a GSH (glutationa reduzida), as fitoquelatinas e também a formação de prolina. O estado nutricional da planta durante o estresse é crucial a fim de manter uma resposta de defesa adequada. Em vista disso, o capítulo dois apresenta uma revisão sobre a participação de Enxofre (S) na defesa contra o estresse. Este nutriente tem um papel importante em processos fundamentais, tais como o transporte de elétrons, estrutura, regulação, produção de oxigênio fotossintético, resistência a estresses abióticos e bióticos e no metabolismo secundário. Além disso, alguns elementos químicos são considerados benéficos para as plantas, no qual o Selênio (Se) é o mais relevante. No capítulo três, o autor descreve o papel do Se na amenização do estresse induzido pela contaminação por metais pesados, suas poderosas características antioxidantes, a melhoria da atividade de enzimas antioxidantes e também dos mecanismos globais de defesa. O capítulo quatro consiste em um projeto científico conduzido pelo autor. O objetivo deste estudo foi investigar se o Selênio, sob a forma de selenito (Na2SeO3), é capaz de evitar a absorção, translocação e concentração de cádmio (CdCl2), em diferentes tecidos no tomate, indicando os possíveis mecanismos para amenizar o estresse, bem como também analisar o estado geral dos frutos através das análises nutricionais, peso seco, pigmentos e concentração de Prolina livre. Os resultados demonstram que efeito atenuante do Se em tomateiro submetido ao Cd poderia estar relacionado com a restrição da absorção e translocação de Cd2+, aumentando a concentração de micronutrientes nos frutos e, finalmente, aumentando a concentração de prolina livre nos frutos.
Plants do not have specific defense mechanisms to counteract the diverse range of abiotic stresses and pollutants into the environment, and its survival depends on the flexibility and adaptability of its own natural defense mechanisms. Furthermore, the maintenance of cellular homeostasis depends on several interlinked and complex mechanisms, while the cellular defense system does not follow a specific pattern of action and may differ due to various factors such as plant species, exposure time to the stress, plant developmental stage, different organs and tissues analyzed. In the light of these considerations, this dissertation aimed to highlight and investigate the role of Sulfur and Selenium against different plant stresses, through the enzymatic and non-enzymatic plant responses and other related defense mechanisms. In the first chapter the author characterize the general biochemical mechanisms of the antioxidant cell defense, specifically the reactive oxygen species (EROs) formation and its chemical singularities and the induced oxidative stress, the enzymatic antioxidant defense system, specifically the superoxide dismutase (SOD) and Catalase (CAT) enzymes, the non-enzymatic mechanisms against the stress, including the Ascorbate-Glutathione cycle, the GSH (reduced glutathione), the phytochelatins and also proline formation. The plant nutritional status during the stress is crucial in order to maintain a proper defense response. In view of this, the chapter two is a published review about the participation of Sulfur (S) on the stress defense. This nutrient has a role in fundamental processes such as electron transport, structure, regulation and it is also associated with photosynthetic oxygen production, abiotic and biotic stress resistance and secondary metabolism. Moreover, few chemical elements are considered benefic to plants, while Selenium (Se) is the most relevant. In the chapter three the author describes the role of Se to detoxify the stress induced by heavy metal contamination, its powerful antioxidant characteristics and the improvement of the antioxidant enzymes activity and overall defense mechanisms. The chapter four consists of a scientific project conducted by the author. The aim of this study was to investigate whether Selenium, under the form of selenite (Na2SeO3), may avoid the uptake, translocation and concentration of Cadmium (CdCl2), in different tomato tissues, indicating possible mechanisms to counteract the stress, as well as to analyze the fruits overall status through the nutritional analyses, dry weight, pigments and proline concentration. The results demonstrate that alleviating effect of Se in tomato under Cd contamination could be related to restriction of Cd2+ uptake and translocation, enhancing micronutrient concentration in fruits and, finally, enhancing fruit proline concentration.
CAPES: 445978/2014-7

Orientador: Priscila Lupino Gratão
Coorientador: André Rodrigues dos Reis
Banca: Tiago Tezotto
Banca: Tiago Santana Balbuena
Abstract: Plants do not have specific defense mechanisms to counteract the diverse range of abiotic stresses and pollutants into the environment, and its survival depends on the flexibility and adaptability of its own natural defense mechanisms. Furthermore, the maintenance of cellular homeostasis depends on several interlinked and complex mechanisms, while the cellular defense system does not follow a specific pattern of action and may differ due to various factors such as plant species, exposure time to the stress, plant developmental stage, different organs and tissues analyzed. In the light of these considerations, this dissertation aimed to highlight and investigate the role of Sulfur and Selenium against different plant stresses, through the enzymatic and non-enzymatic plant responses and other related defense mechanisms. In the first chapter the author characterize the general biochemical mechanisms of the antioxidant cell defense, specifically the reactive oxygen species (EROs) formation and its chemical singularities and the induced oxidative stress, the enzymatic antioxidant defense system, specifically the superoxide dismutase (SOD) and Catalase (CAT) enzymes, the non-enzymatic mechanisms against the stress, including the Ascorbate-Glutathione cycle, the GSH (reduced glutathione), the phytochelatins and also proline formation. The plant nutritional status during the stress is crucial in order to maintain a proper defense response. In view of this, the chapter two is a publis... (Complete abstract click electronic access below)
Resumo: As plantas não possuem mecanismos de defesa específicos para combater a diversidade de estresses abióticos e poluentes do ambiente, e sua sobrevivência depende da flexibilidade e adaptação dos seus próprios mecanismos de defesa naturais. Além disso, a manutenção da homeostase celular depende de vários mecanismos interligados e complexos, enquanto o sistema de defesa celular não segue um padrão específico de ação e pode ainda variar devido a vários fatores tais como a espécie do vegetal, o tempo de exposição ao estresse, o estágio de desenvolvimento da planta e também nos diferentes órgãos e tecidos analisados. Com base nessas considerações, esta dissertação teve como objetivo destacar e investigar o papel do Enxofre (S) e do Selênio (Se) contra diferentes estresses nas plantas, através das respostas enzimáticas, não enzimáticas e também outros mecanismos de defesa relacionados. No primeiro capítulo, o autor caracteriza os mecanismos bioquímicos gerais da defesa celular antioxidante, especificamente a formação das espécies reativas de oxigênio (EROs) e suas singularidades químicas e o estresse oxidativo induzido, o sistema de defesa antioxidante enzimático, especificamente as enzimas Superóxido Dismutase (SOD) e a Catalase (CAT), os mecanismos não-enzimáticas contra o estresse, incluindo o ciclo Aascorbato-Glutationa, a GSH (glutationa reduzida), as fitoquelatinas e também a formação de prolina. O estado nutricional da planta durante o estresse é crucial a fim de manter uma re... (Resumo completo, clicar acesso eletrônico abaixo)
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