Journal articles: 'Tautomerism'

1

Mirkovic, Jelena, Gordana Uscumlic, Aleksandar Marinkovic, and Dusan Mijin. "Azo-hydrazone tautomerism of aryl azo pyridone dyes." Chemical Industry 67, no. 1 (2013): 1–15. http://dx.doi.org/10.2298/hemind120309053m.

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In the last three or four decades disperse dyes derived from pyridones (in particular azo pyridone dyes) have gained in importance, and are widely used in various fields. These compounds have excellent coloration properties, and are suitable for the dyeing of polyester fabrics. Basic features of these dyes are simplicity of their synthesis by diazotation and azo coupling. They generally have high molar extinction coefficient with medium to high light and wet fastness. The absorption maxima of these dyes show their visible absorption wavelength ranging from yellow to orange, which can be attributed to poorly delocalized electrons in the pyridone ring. However, there are several dyes with deep colors such as red or violet. Pyridone dyes with alkyl and aryl groups in ortho position to azo group show 2-pyridone/2-hydroxypyridine tautomerism, while those containing OH and NHR groups conjugated with the azo group show azo-hydrazone tautomerism. Determining azo-hydrazone tautomerism could be therefore interesting, since the tautomers have different physico-chemical properties and most importantly different coloration. The literature on azo-hydrazone tautomerism, determination of equilibrium position, and investigation of substituent and solvent influence on tautomerism has been summarized in the presented review. The general conclusion is that the equilibrium between two tautomers is influenced by the structure of the compounds and by the solvents used. The tautomeric behavior patterns of the arylazo pyridone dyes in the reviewed literature has been studied using various instrumental techniques, including FT-IR, UV-vis, and NMR spectroscopy. The quantum chemical calculations related to the azo-hydrazon tautomerism have also been included. A large number of pyridone dyes exist in hydrazone form in solid state, while in solvents there is a mixture of tautomers. In addition, the X-ray single-crystal diffraction data analysis of some commercial pyridone dyes has been discussed concluding that they all crystallize in the hydrazone form.

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Hansen, Poul Erik. "Tautomerism of β-Diketones and β-Thioxoketones." Encyclopedia 3, no. 1 (January 30, 2023): 182–201. http://dx.doi.org/10.3390/encyclopedia3010013.

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The present overview concentrates on recent developments of tautomerism of β-diketones and β-thioxoketones, both in solution and in the solid state. In particular, the latter has been a matter of debate and unresolved problems. Measurements of 13C, 17O, and 2H chemical shifts have been used. Deuterium isotope effects on chemical shifts are proposed as a tool in the study of this problem. Photoconversion of β-diketones and β-thioxoketones are discussed in detail, and the incorporation of β-diketones into molecules with fluorescent properties is assessed. Finally, docking studies of β-diketones are scrutinized with an emphasis on correct tautomeric structures and knowledge about barriers to interconversion of tautomers.

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Oziminski, Wojciech P., and Agata Wójtowicz. "New theoretical insights on tautomerism of hyperforin—a prenylated phloroglucinol derivative which may be responsible for St. John’s wort ( Hypericum perforatum ) antidepressant activity." Structural Chemistry 31, no. 2 (November 22, 2019): 657–66. http://dx.doi.org/10.1007/s11224-019-01434-6.

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AbstractThe thermodynamic aspects of keto-enol tautomerism of hyperforin were investigated theoretically using density functional theory methods. At the B3LYP/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ level of theory the enol tautomer dominates the tautomeric mixture and the second enol tautomer 1OH-HB has Gibbs free energy higher by 1.2 kcal/mol, despite possessing an intramolecular hydrogen bond. The purely keto tautomer is less stable by 3.3 kcal/mol compared with the 1OH tautomer, which means that the percentage of the keto tautomer in the tautomeric mixture is only about 0.4%. This is a different picture than in the parent compound of hyperforin—the phloroglucinol, where the keto tautomer is more stable than corresponding enol 1OH tautomer by 0.6 kcal/mol. To explain this difference, several in-between model molecules reflecting gradual transformation from phloroglucinol to hyperforin were build, and all the tautomeric forms were optimized for each molecule. It turned out that the addition of an aliphatic three-carbon bridge to phloroglucinol ring is crucial for the reversal of the tautomer stability order to that for hyperforin. The probable reason is the unfavorable strain in the keto tautomer introduced by the carbon bridge, which forces a specific geometric configuration which destabilizes in consequence the keto tautomer. This picture of hyperforin tautomerism underlines the dominance of enol tautomers, which can be important when studying the antidepressant activity of hyperforin—its interactions with neurotransmitters receptors.

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4

Fabian, Walter M. F. "Tautomerism in Five-membered Nitrogen Heterocycles. A Test of the Reliability of Semiempirical (AMI, PM3, MNDO) Quantum Chemical Methods." Zeitschrift für Naturforschung A 45, no. 11-12 (December 1, 1990): 1328–34. http://dx.doi.org/10.1515/zna-1990-11-1216.

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AbstractThe reliability of three popular semiempirical quantum chemical methods (AM1, PM3, MNDO) for the treatment of tautomeric equilibria is tested in a series of five-membered nitrogen heterocycles. The known flaw of MNDO to overestimate the stability of compounds with two or more adjacent pyridine-like lone pairs is also present in AM1 and to a somewhat lesser extent in PM3. Tautomeric species differing in the number of adjacent pyridine-like lone pairs, thus, cannot be adequately treated by these semiempirical methods. Both AM1 as well as PM3, however, represent major improvements over MNDO in the case of lactam-lactim tautomerism. The stability of N-oxides as compared to N-hydroxy tautomers seems to be overestimated by the PM3 method. All three semiempirical methods yield quite reliable ionization potentials and dipole moments.

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5

Allegretti, P. E., C. B. Milazzo, and J. J. P. Furlong. "Mass Spectrometry as a Tool for the Determination of Heats of Tautomerization of Thioamides in the Gas Phase." European Journal of Mass Spectrometry 11, no. 1 (February 2005): 53–63. http://dx.doi.org/10.1255/ejms.691.

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Tautomerism of several thioamides was studied by mass spectrometry. The analysis of the corresponding mass spectra allowed some fragmentations to be assigned to specific tautomers and heats of tautomerization to be determined through temperature effects and electron energy studies. Experimental determinations were strongly supported by theoretical calculations. AM1 semi-empirical results indicate that the thioamide–thioimidol equilibrium can be studied by mass spectrometry. Also, additional evidence was obtained regarding tautomerism occurrence between neutral species.

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6

Stoyanov, Stefan, Ivan Petkov, Liudmil Antonov, Tatyana Stoyanova, Petros Karagiannidis, and Paraskevas Aslanidis. "Thione–thiol tautomerism and stability of 2- and 4-mercaptopyridines and 2-mercaptopyrimidines." Canadian Journal of Chemistry 68, no. 9 (September 1, 1990): 1482–89. http://dx.doi.org/10.1139/v90-227.

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The possible thione–thiol tautomerism of 2- and 4-mercaptopyridines, 2-mercaptopyrimidine, and 4,6-dimethyl-2-mercaptopyrimidine in solution is studied by means of absorption (UV–VIS) spectroscopy. In accordance with earlier observations, polar solvents and self-association shift the apparent tautomeric equilibrium significantly towards the thione form. In dilute solutions of nonpolar solvents the thiol form predominates. On standing, significant changes are observed in the absorption spectra of these tautomeric compounds in ethanol, dioxane, and water. The time course of the tautomerization, followed spectrophotometrically, reveals quantitative transformation of the thiol form to the corresponding symmetrical disulfides. The influence of concentration, temperature, and irradiation with indirect sunlight are discussed. This thione–disulfide process is reversible in water, starting either from the tautomeric thione or from its symmetrical disulfide, implying a possible importance in biological systems. Keywords: thione–thiol tautomerism, absorption spectra, thiol–disulfide oxidation, mercaptopyridines and pyrimidines.

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7

Sun, Chun-Lin, Xiao-E. Luo, Huan Xu, Qi-Wei Song, Zhi-Ping Fan, Xiao-Zhen Wang, Jing-Jing Cao, Zi-Fa Shi, and Hao-Li Zhang. "Aromaticity and tautomerism of a 4n π electron dihydrohexaazapentacene." Organic Chemistry Frontiers 7, no. 2 (2020): 405–13. http://dx.doi.org/10.1039/c9qo01285k.

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We report herein the intriguing tautomerism behaviors of new dihydrohexaazapentacene derivatives. The dihydrohexaazapentacene molecules with 4n π electrons exist as two stable tautomers in solution and exhibits globe aromaticity.

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8

Avendaño, Carmen, María Teresa Ramos, José Elguero, María Luisa Jimeno, Juana Bellanato, and Feliciana Florencio. "Tautomerism of bis(2-benzothiazolyl)methanes." Canadian Journal of Chemistry 66, no. 6 (June 1, 1988): 1467–73. http://dx.doi.org/10.1139/v88-236.

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Tautomerism of dibenzothiazolylmethane (1) and its C-methyl derivative (2) has been studied by 1H nuclear magnetic resonance, ultraviolet, and infrared spectroscopy using C,C-dimethyl (3) and N-methyl (4) derivatives as model compounds of the "CH" and "NH" forms, respectively. X-ray diffraction analysis of the "fixed" N-methyl derivative 4 shows that it corresponds to the Z-sE isomer 4b2. The CH tautomers are unstable in solution and they slowly isomerize into a mixture of NH tautomers that depends on the solvent and on the C-substituent (H or CH3).

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9

Štoček, Jakub Radek, and Martin Dračínský. "Tautomerism of Guanine Analogues." Biomolecules 10, no. 2 (January 22, 2020): 170. http://dx.doi.org/10.3390/biom10020170.

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Tautomerism of nucleic acid (NA) bases is a crucial factor for the maintenance and translation of genetic information in organisms. Only canonical tautomers of NA bases can form hydrogen-bonded complexes with their natural counterparts. On the other hand, rare tautomers of nucleobases have been proposed to be involved in processes catalysed by NA enzymes. Isocytosine, which can be considered as a structural fragment of guanine, is known to have two stable tautomers both in solution and solid states. The tautomer equilibrium of isocytosine contrasts with the remarkable stability of the canonical tautomer of guanine. This paper investigates the factors contributing to the stability of the canonical tautomer of guanine by a combination of NMR experiments and theoretical calculations. The electronic effects of substituents on the stability of the rare tautomers of isocytosine and guanine derivatives are studied by density functional theory (DFT) calculations. Selected derivatives are studied by variable-temperature NMR spectroscopy. Rare tautomers can be stabilised in solution by intermolecular hydrogen-bonding interactions with suitable partners. These intermolecular interactions give rise to characteristic signals in proton NMR spectra, which make it possible to undoubtedly confirm the presence of a rare tautomer.

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10

Abdeldjebar, Hasnia, Yamina Belmiloud, Wassila Djitli, Sofien Achour, Meziane Brahimi, and Bahoueddine Tangour. "Proton transfer in the benzimidazolone and benzimidazolthione tautomerism process catalyzed by polar protic solvents." Progress in Reaction Kinetics and Mechanism 44, no. 2 (May 2019): 143–56. http://dx.doi.org/10.1177/1468678319825740.

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The tautomeric equilibrium of benzimidazolone and benzimidazolthione have been studied by the density functional theory method using the CAM-B3LYP functional together with the 6-311G(d,p) basis set. Two separate mechanisms have been investigated: a direct intramolecular transfer using the polarizable continuum model and an indirect proton transfer assisted by a molecule of solvent (C6H12, H2O, CH3OH, and H2O2). In both cases, the results obtained indicate that ketone and thione are the most stable forms. However, the enhanced height of the activation barrier for the four-center mechanisms describing the tautomerism reaction as a direct intramolecular transfer implicates a relatively disadvantaged process. The participation of a polar protic solvent molecule allows the lowering of the activation energy barrier. Potential energy profiles of keto-enol and thio-enol tautomerism assisted by methanol and water are very different. The former one describes a concerted mechanism but the latter does not because it is associated with asynchronous processes that take place during the thio-enol tautomerism.

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11

Slota, Michael, Marian Blankenhorn, Eric Heintze, Minh Vu, Ralph Hübner, and Lapo Bogani. "Photoswitchable stable charge-distributed states in a new cobalt complex exhibiting photo-induced valence tautomerism." Faraday Discussions 185 (2015): 347–59. http://dx.doi.org/10.1039/c5fd00088b.

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We report the synthesis and magnetic and photomagnetic behaviour of a novel valence tautomeric cobalt complex, [Co(3,5-dbbq)₂(μ-bpym)] (1) (3,5-dbbq = 3,5-di-tert-butyl-1,2-benzoquinone and μ-bpym = 2,2′-bipyrimidine). The synthesis is performed by reacting Co₂(CO)₈ and μ-bpym in the presence of the ligand 3,5-dbbq in a mixed solvent under inert atmosphere. The magnetic behavior clearly shows the presence of electron transfer from the catecholate ligand to the cobalt center, producing valence tautomers of [Co^II(SQ)₂] with a transition temperature (T_1/2) of 215 K. Photomagnetic studies, performed via both SQUID magnetometry and X-band electron paramagnetic resonance, show the clear presence of photoinduced valence tautomerism, at temperatures considerably higher than previous systems. A metastable charge distribution is observed, strengthening previous investigations on the character of mixed valence ligands. Entropy-driven valence tautomeric interconversion is observed, and drives the transition to the most stable charge distribution. The complex has the ability to coordinate and can be used as a photoswitchable building block, with the photomagnetic characterisation evidencing a metastable state lifetime of the photo-induced valence tautomeric process of ca. 2.9 × 10⁴ s below 20 K. The observed yields are higher than ones in similar systems, showing that tiny changes in the molecular structures may have a huge impact.

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12

Pis-Diez, Reinaldo, Gustavo A. Echeverría, Oscar E. Piro, Jorge L. Jios, and Beatriz S. Parajón-Costa. "A structural, spectroscopic and theoretical study of an o-vanillin Schiff base derivative involved in enol-imine and keto-amine tautomerism." New Journal of Chemistry 40, no. 3 (2016): 2730–40. http://dx.doi.org/10.1039/c5nj01039j.

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The potassium salt of the Schiff base derived from o-vanillin and taurine is involved in enol-imine (I) and keto-amine (II) tautomerism. Both tautomeric forms coexist in the crystal and they are stabilized by strong O–H⋯N and O⋯H–N intramolecular hydrogen bonds.

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13

Colasurdo, Diego D., Matías N. Pila, Dacio A. Iglesias, Sergio L. Laurella, and Danila L. Ruiz. "Tautomerism of uracil and related compounds: A mass spectrometry study." European Journal of Mass Spectrometry 24, no. 2 (June 20, 2017): 214–24. http://dx.doi.org/10.1177/1469066717712461.

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It has been demonstrated that uracil has a preponderant tautomeric form, but it is also known that different tautomers co-exist in this equilibrium. In this work, mass spectrometry is used as a helpful tool to analyse the equilibria, using derivative compounds to forbid the presence of some tautomers and ion trap mass spectrometry to follow relevant fragmentation pathways. Theoretical calculations were performed to confirm tautomers abundance by energy minimization in gas phase. Analysis of mass spectra of uracil, three methyl-substituted uracils, 2-thiouracil and three benzouracils suggest that uracil exists mainly as three tautomers in gas phase: one major structure that corresponds to the classical structure of uracil (pyrimidine-2,4(1H,3H)-dione) bearing two carbonyls and two NH moieties, and two minor enolic forms (4-hydroxypyrimidin-2(1H)-one and 2-hydroxypyrimidin-4(1H)-one). Such tautomeric distribution is supported by theoretical calculations, which show that they are the three most stable tautomers.

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14

Chermahini, Alireza Najafi, Hossein A. Dabbagh, and Abbas Teimouri. "Theoretical studies on tautomerism of dihydropyrimidine tautomers." Journal of Molecular Structure: THEOCHEM 857, no. 1-3 (May 2008): 105–10. http://dx.doi.org/10.1016/j.theochem.2008.02.012.

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15

Kwiatkowski, Adam, Erkki Kolehmainen, and Borys Ośmiałowski. "Conformational and Tautomeric Control by Supramolecular Approach in Ureido-N-iso-propyl,N’-4-(3-pyridin-2-one)pyrimidine." Molecules 24, no. 13 (July 8, 2019): 2491. http://dx.doi.org/10.3390/molecules24132491.

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Ureido-N-iso-propyl,N’-4-(3-pyridin-2-one)pyrimidine (1) and its 2-methoxy pyridine derivative (1Me) has been designed and prepared. The conformational equilibrium in urea moiety and tautomerism in the pyrimidine part have been investigated by variable temperature and 1H NMR titrations as well as DFT quantum chemical calculations. The studied compounds readily associate by triple hydrogen bonding with 2-aminonaphthyridine (A) and/or 2,6-bis(acetylamino)pyridine (B). In 1, the proton is forced to 1,3-tautomeric shift upon stimuli and keeps it position, even when one of the partners in the complex was replaced by another molecule. The observed tautomerism controlled by conformational state (kinetic trapping effect) opens new possibilities in molecular sensing that are based on the fact that reverse reaction is not preferred.

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16

Milata, Viktor. "Nitro and aminobenzimidazoles." Acta Chimica Slovaca 11, no. 2 (October 1, 2018): 182–88. http://dx.doi.org/10.2478/acs-2018-0026.

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AbstractA summary of the preparation methods of 2 tautomeric and 4N-methylated benzimidazoles with a nitro group on the benzene ring (1–6) and with an amino group in the same positions (7–12) were summarized. Annular tautomerism of the title compounds1–12has been studied using1H,13C and15N NMR spectra in liquid and solid state (CPMAS), UV spectra and quantum chemical calculations.

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17

Mchedlov-Petrossyan, Nikolay O., and Natalya A. Vodolazkaya. "Protolytic Equilibria in Organized Solutions: Ionization and Tautomerism of Fluorescein Dyes and Related Indicators in Cetyltrimethylammonium Chloride Micellar Solutions at High Ionic Strength of the Bulk Phase." Liquids 1, no. 1 (February 20, 2021): 1–24. http://dx.doi.org/10.3390/liquids1010001.

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Ionic equilibrium of 22 hydroxyxanthenes, including halogen and nitro derivatives of fluorescein, and their thio- and aza analogues, were studied spectrophotometrically in micellar solutions of cetyltrimethylammonium chloride at ionic strength of the bulk phase 4.0 M KCl. This micellar pseudophase is characterized by the electrostatic surface potential of +(15–16) mV and the ETN value of 0.623. In the case of dyes bearing the COOH group, colorless lactone is the predominant tautomer of the molecular form H2R. A new classification of fluoresceins is developed. The dyes were divided into four groups based on the nature of tautomerism of the anions. In the case of the fluorescein type, the monoanions HR− exist predominantly as “carboxylate” tautomers, with ionized carboxylic and non–ionized hydroxylic group. For the dyes of the eosin type, the situation is opposite, while for the intervening type of compounds, the concentrations of the two tautomers are comparable. Dyes capable of forming lactone anions HR− were classified as the fourth type. For some of them, even the dianion R2− exists as a lactone. The relationship between the stepwise ionization constants, Ka1/Ka2, varies from 1.3 to 1.07 × 105 and is determined by the state of tautomeric equilibrium of molecules and ions.

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18

López, Concepción, Rosa María Claramunt, Ibon Alkorta, and José Elguero. "Solution and solid state (CPMAS) NMR studies of the tautomerism of six-membered heterocyclic compounds related to 2-pyridones." Spectroscopy 14, no. 3 (2000): 121–26. http://dx.doi.org/10.1155/2000/421654.

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Several13C and15N chemical shifts of 2-pyridone(1), 4(3H)-pyrimidone(2), uracil(3)and cytosine(4)have been measured in solution and in the solid state. These data have been discussed in relation with the tautomerism of the four heterocycles. GIAOab initiocalculations of absolute shieldings have been carried out to identify the predominant tautomers in the case of compounds(1)and(2).

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19

Saraví Cisneros, Hebe, Sergio Laurella, Danila L. Ruiz, Agustín Ponzinibbio, Patricia E. Allegretti, and Jorge J. P. Furlong. "Spectrometric Study of the Nitrile-Ketenimine Tautomerism." International Journal of Spectroscopy 2009 (September 9, 2009): 1–18. http://dx.doi.org/10.1155/2009/408345.

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Mass spectrometry is used to evaluate the occurrence of the nitrile-ketenimine tautomerism. Mass spectra of two differently substituted nitriles, ethyl-4,4-dicyano-3-methyl-3-butenoate and diethyl-2-cyano-3-methyl-2-pentenodiate are examined looking for common mass spectral behaviors. Ion fragmentation assignments for specific tautomers allow to predict the presence of the corresponding structures. Additionally, the mass spectrum and nuclear magnetic resonance spectra of ethyl-4,4-dicyano-2,2-diethyl-3-methyl-3-butenoate and that of the corresponding amination product support the occurrence of the ketenimine tautomer in the equilibrium.

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20

Traven, Valery F., Vadim V. Negrebetsky, Larisa I. Vorobjeva, and Edward Andrew Carberry. "Keto–enol tautomerism, NMR spectra, and H–D exchange of 4-hydroxycoumarins." Canadian Journal of Chemistry 75, no. 4 (April 1, 1997): 377–83. http://dx.doi.org/10.1139/v97-043.

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4-Hydroxycoumarin 1, 4,5-dihydroxycoumarin 2, and 4,7-dihydroxycoumarin 3 undergo H–D exchange at the C(3) atom of the lactone ring. Although only the 4-hydroxy-2-chromenone tautomeric forms are seen in the 1H and 13C NMR spectra of compounds 1–3, the equilibrium between the 4-hydroxy-2-chromenone and 2,4-chromandione forms is suggested to be the key step in the H–D exchange reaction. 4,5-Dihydroxycoumarin shows the highest rate of the reaction, since H-bonding between 5-hydroxyl and 4-keto functional groups can provide relative stability to the 5-hydroxy-2,4-chromandione tautomeric form, a probable intermediate of the exchange. NMR spectra and tautomeric transformations of 3-(4-methoxyphenylazo)-4-hydroxycoumarin 4 and 3-acetyl-4-hydroxycoumarin 5 are also discussed. The stabilities of different tautomeric forms of compounds 1–5 have been evaluated by MNDO calculations. Keywords: 4-hydroxycoumarin derivatives, keto-enol tautomerism, H–D-exchange.

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21

Türker, Lemi. "Tautomerism in 11-Hydroxyaklavinone: A DFT Study." Scientific World Journal 2012 (2012): 1–7. http://dx.doi.org/10.1100/2012/526289.

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The antharquinone-based chromophore of 11-hydroxyaklavinone is present in the structure of an anticancer agent, daunomycin. On the other hand, aklavinone is the parent aglycone of certain anthracycline antibiotics that possess anti-cancer activity too. The structures of aklavinone and its 11-hydroxy derivative have many –OH groups, and two keto groups which may take place in certain tautomeric equilibria. Of these tautomeric forms, presently the one involving the anthraquinone based tautomers of 11-hydroxyaklavinone has been investigated quantum chemically in the framework of the density functional theory at the levels of RB3LYP/6-31G(d) and RB3LYP/6-31G(d,p).

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Borbulevych, Oleg, Roger I. Martin, Ian J. Tickle, and Lance M. Westerhoff. "XModeScore: a novel method for accurate protonation/tautomer-state determination using quantum-mechanically driven macromolecular X-ray crystallographic refinement." Acta Crystallographica Section D Structural Biology 72, no. 4 (March 30, 2016): 586–98. http://dx.doi.org/10.1107/s2059798316002837.

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Gaining an understanding of the protein–ligand complex structure along with the proper protonation and explicit solvent effects can be important in obtaining meaningful results in structure-guided drug discovery and structure-based drug discovery. Unfortunately, protonation and tautomerism are difficult to establish with conventional methods because of difficulties in the experimental detection of H atoms owing to the well known limitations of X-ray crystallography. In the present work, it is demonstrated that semiempirical, quantum-mechanics-based macromolecular crystallographic refinement is sensitive to the choice of a protonation-state/tautomer form of ligands and residues, and can therefore be used to explore potential states. A novel scoring method, calledXModeScore, is described which enumerates the possible protomeric/tautomeric modes, refines each mode against X-ray diffraction data with the semiempirical quantum-mechanics (PM6) Hamiltonian and scores each mode using a combination of energetic strain (or ligand strain) and rigorous statistical analysis of the difference electron-density distribution. It is shown that usingXModeScoreit is possible to consistently distinguish the correct bound protomeric/tautomeric modes based on routine X-ray data, even at lower resolutions of around 3 Å. These X-ray results are compared with the results obtained from much more expensive and laborious neutron diffraction studies for three different examples: tautomerism in the acetazolamide ligand of human carbonic anhydrase II (PDB entries 3hs4 and 4k0s), tautomerism in the 8HX ligand of urate oxidase (PDB entries 4n9s and 4n9m) and the protonation states of the catalytic aspartic acid found within the active site of an aspartic protease (PDB entry 2jjj). In each case,XModeScoreapplied to the X-ray diffraction data is able to determine the correct protonation state as defined by the neutron diffraction data. The impact of QM-based refinementversusconventional refinement onXModeScoreis also discussed.

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Iglesias, Dacio Adhemar, Danila Luján Ruiz, and Patricia Ercilia Allegretti. "2-Cyanobenzoic Acids: Tautomeric Equilibria Study." Australian Journal of Chemistry 68, no. 3 (2015): 461. http://dx.doi.org/10.1071/ch14273.

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Evidence of ring–chain tautomerism in the gas phase from mass spectrometry and in solution determined by means of 1H NMR and 13C NMR is reported for 2-cyanobenzoic acids. The analysis of the corresponding spectra has allowed specific assignment of fragment ions to tautomers. The predictive value of this methodology is supported by the influence of the substitution pattern of these compounds on this equilibrium.

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Antonov, Liudmil. "Tautomerism in Azo and Azomethyne Dyes: When and If Theory Meets Experiment." Molecules 24, no. 12 (June 17, 2019): 2252. http://dx.doi.org/10.3390/molecules24122252.

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The performance of 26 hybrid density functionals was tested against a tautomeric dataset (TautData), containing experimental information for the keto-enol tautomeric equilibrium in 16 tautomeric azodyes and Schiff bases in cyclohexane, carbon tetrachloride and acetonitrile. The results have shown that MN12-SX, BHandH and M06-2X can be used to describe the tautomeric state of the core structures in the frame of ~0.5 kcal/mol error and correctly predict the tautomeric state in respect of dominating tautomeric form. Among them MN12-SX is the best performer, although it fails to describe the nonplanarity of some of the enol tautomers. The same experimental dataset was used to develop and test a special DFT functional (TautLYP) aimed at describing the tautomeric state in azo- and azomethyne compounds in solution when nonspecific solvents are used.

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Anisimova, N. A., and D. A. Melkova. "Acyl derivatives of 2,3,6,7-tetrahydro-1,4-diazepines: synthesis, structure, and tautomeric transformations." Журнал общей химии 93, no. 11 (December 15, 2023): 1650–63. http://dx.doi.org/10.31857/s0044460x23110021.

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The reaction of 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene with carboxylic acid chlorides afforded a series of N -and C -acylsubstituted 2,3,6,7-tetrahydro-1,4-diazepines, which in solutions of deuterated solvents (CD3OD, DMCO) undergo keto-enol and imino-enamine tautomerism. Structure of the obtained N - and C -acyldiazepines and their tautomeric transformations were characterized by 1Н and 13С NMR spectroscopy using two-dimensional experiments 1Н-13С HMQC, HMBC, 1Н-1Н COSY.

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Deneva, Vera, Georgi Dobrikov, Aurelien Crochet, Daniela Nedeltcheva, Katharina M. Fromm, and Liudmil Antonov. "Tautomerism as primary signaling mechanism in metal sensing: the case of amide group." Beilstein Journal of Organic Chemistry 15 (August 8, 2019): 1898–906. http://dx.doi.org/10.3762/bjoc.15.185.

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The concept for sensing systems using the tautomerism as elementary signaling process has been further developed by synthesizing a ligand containing 4-(phenyldiazenyl)naphthalene-1-ol as a tautomeric block and an amide group as metal capturing antenna. Although it has been expected that the intramolecular hydrogen bonding (between the tautomeric hydroxy group and the nitrogen atom from the amide group) could stabilize the pure enol form in some solvents, the keto tautomer is also observed. This is a result from the formation of intramolecular associates in some solvents. Strong bathochromic and hyperchromic effects in the visible spectra accompany the 1:1 formation of complexes with some alkaline earth metal ions.

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27

Obukhova, Olena M., Nikolay O. Mchedlov-Petrossyan, Natalya A. Vodolazkaya, Leonid D. Patsenker, and Andrey O. Doroshenko. "Stability of Rhodamine Lactone Cycle in Solutions: Chain–Ring Tautomerism, Acid–Base Equilibria, Interaction with Lewis Acids, and Fluorescence." Colorants 1, no. 1 (February 24, 2022): 58–90. http://dx.doi.org/10.3390/colorants1010006.

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The equilibrium between different tautomers that can be colored or colorless is an important feature for rhodamine dyes. Presently, this phenomenon is mostly discussed for rhodamine B. Herein, we studied the tautomerism and acid–base dissociation (HR+ ⇄ R + H+) of a set of rhodamines in organic media. Form R is an equilibrium mixture of the colored zwitterion R± and colorless lactone R0. Absorption spectra in 90 mass% aqueous acetone reflects the correlation between the dyes structure and the equilibrium constant, KT = [R0]/[R±]. Increase in the pKa value on transferring from water to organic solvents confirms the highly polar character of the R± tautomer. To reveal the role of the solvent nature, the tautomerism of an asymmetrical rhodamine, 2-(12-(diethyliminio)-2,3,5,6,7,12-hexahydro-1H-chromeno[2,3-f]pyrido[3,2,1-ij]quinolin-9-yl)benzoate, was examined in 14 media. This chain–ring tautomerism is an intramolecular acid–base reaction; the central carbon atom acts as a Lewis acid. The interaction with other Lewis acids, Li+, Ca2+, Mg2+, and La3+, results in rupture of lactone cycle. In polar solvents, lactones undergo photocleavage resulting in formation of highly fluorescent R±, whereas the blue fluorescence and abnormally high Stokes shift in low-polar media may be explained either by another photoreaction or by spiroconjugation and charge transfer in the exited state.

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28

Kazeri-Shandiz, Shima, Ali Beyramabadi, and Ali Morsali. "Geometry, tautomerism and non-covalent interactions of the drug halofuginone with the carbon-nanotube and γ-Fe2O3 nanoparticles: A DFT study." Journal of the Serbian Chemical Society 83, no. 3 (2018): 305–15. http://dx.doi.org/10.2298/jsc170608100k.

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Halofuginone is a potential anti-malarial drug, which could exist as three possible tautomers. Herein, using density functional theory (DFT), and handling the solvent effects with the PCM model, the tautomerism of halofuginone was investigated. Intramolecular H-bonds play an important role in the stability of the tautomers. The conformer H1a is the most stable. Noncovalent interactions of the H1a conformer with the armchair (5,5) single-wall carbon nanotubes and ?-Fe2O3 nanoparticles were explored in several manners. The most stable form of them was determined. The intermolecular H-bonds play a substantial role in the energy behavior of the interaction between ?-Fe2O3 nanoparticles and halofuginone.

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29

Garifzianova, Guzel G. "Theoretical study of the isomerization of 1-amino-4-phenylamino-9,10-anthraquinone." Butlerov Communications 60, no. 12 (December 31, 2019): 37–42. http://dx.doi.org/10.37952/roi-jbc-01/19-60-12-37.

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This paper presents the results of computer simulation of tautomeric transformations of the molecule 1-amino-4-phenylamino-9,10-anthraquinone. It is known from the literature that the presence of substituents in the 1,4-position of anthraquinone-9,10 leads to various tautomeric transformations, with a shift in the absorption maximum and the appearance of absorption bands in the red wave region in electronic spectra. Both the 1-amino-4-hydroxyanthraquinone described in the literature and the 1-amino-4-phenylamino-9,10-anthraquinone are characterized by two types of prototropic tautomerism – keto-enol and amino-imine. Quantum-chemical modeling contributes to the calculation of the relative energies of tautomers and isomers, the barriers of their interconversions, as well as finding their structural parameters. The aim of this study was to study the mechanism of the formation of tautomers during hydrogen transfer in the molecule of 1-amino-4-phenylamino-9,10-anthraquinone, as well as the formation of isomers during migration of the OH group. The calculations were performed using the Gaussian09 program. To study of various tautomers of 1-amino-4-phenylamino-9,10-anthraquinone, the B3LYP method with the def2TZV basis was used. A search was conducted for transition states during hydrogen transfer and OH group migration. The descent along the reaction path was calculated to confirm that the transition state is in the path of the desired reaction. The minima corresponding to the starting material and product were localized. The activation enthalpies of the studied reactions were calculated. Migration of the OH group in the 1-amino-4-phenylamino-9,10-anthraquinone molecule leads to the formation of 4-phenylamino-9-amino-1,10-anthraquinone. As the calculation shows, the keto-form of 1-amino-4-phenylamino-9,10-anthraquinone is energetically more profitable than all the isomers studied in this work, including the enol form. The smallest difference in total potential energies is 23.7 kJ/mol between the initial ketone form of 1-amino-4-phenylamino-9,10-anthraquinone and the last transformation structure – the 4-phenylamino-9-amino-1,10-anthraquinone molecule.

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30

Tivendale, Nathan D., Noel W. Davies, James Horne, John J. Ross, and Jason A. Smith. "Analysis of the Enol–Keto Tautomers of Indole-3-pyruvic Acid." Australian Journal of Chemistry 68, no. 2 (2015): 345. http://dx.doi.org/10.1071/ch14343.

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Indole-3-pyruvic acid (IPyA) is an important naturally occurring biosynthetic intermediate whose quantitation by standard analytic techniques can be complicated as it can exist as either the keto or enol tautomer. Here, we present a detailed analysis of the tautomerism of IPyA and provide further evidence that the two tautomers of IPyA may be readily separated by ultra high-performance liquid chromatography and that the relative proportions of each form may be controlled using temperature and pH.

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31

Nieto, Carla I., Pilar Cabildo, M. Ángeles García, Rosa M. Claramunt, Ibon Alkorta, and José Elguero. "An experimental and theoretical NMR study of NH-benzimidazoles in solution and in the solid state: proton transfer and tautomerism." Beilstein Journal of Organic Chemistry 10 (July 16, 2014): 1620–29. http://dx.doi.org/10.3762/bjoc.10.168.

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This paper reports the 1H, 13C and 15N NMR experimental study of five benzimidazoles in solution and in the solid state (13C and 15N CPMAS NMR) as well as the theoretically calculated (GIAO/DFT) chemical shifts. We have assigned unambiguously the "tautomeric positions" (C3a/C7a, C4/C7 and C5/C6) of NH-benzimidazoles that, in some solvents and in the solid state, appear different (blocked tautomerism). In the case of 1H-benzimidazole itself we have measured the prototropic rate in HMPA-d 18.

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32

Hansen, Poul Erik. "Structural Studies of β-Diketones and Their Implications on Biological Effects." Pharmaceuticals 14, no. 11 (November 20, 2021): 1189. http://dx.doi.org/10.3390/ph14111189.

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The paper briefly summarizes methods to determine the structure of β-diketones with emphasis on NMR methods. Density functional calculations are also briefly treated. Emphasis is on the tautomeric equilibria of β-diketones in relation to biological effects. Relevant physical parameters such as acidity and solubility are treated. A series of biologically active molecules are treated with respect to structure (tautomerism). Characteristic molecules or groups of molecules are usnic acids, tetramic and tetronic acids, o-hydroxydibenzoylmethanes, curcumines, lupulones, and hyperforines.

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33

Lopez, Concepcion, Rosa Ma Claramunt, Swiatoslaw Trofimenko, and José Elguero. "A 13C NMR spectroscopy study of the structure of N-H pyrazoles and indazoles." Canadian Journal of Chemistry 71, no. 5 (May 1, 1993): 678–84. http://dx.doi.org/10.1139/v93-092.

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The 13C chemical shifts and some 1H–13C coupling constants of 23 pyrazoles and indazoles are reported and discussed. The spectra were recorded in CDCl3 and in DMSO-d6 solutions and, in several cases, also in the solid state (CP/MAS technique). These data allow us to determine the tautomerism of these compounds in the solid state (usually only one tautomer) and in solution. The position of the tautomeric equilibrium is related to the Hammett σm value of the 3(5)-substituent.

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34

Masuda, Seizo, Tahei Tomida, Masami Tanaka, and Yutaka Asahi. "Polymerizable tautomers, 11. Tautomerism and polymerization of p-vinylbenzoylacetone." Macromolecular Chemistry and Physics 196, no. 2 (February 1995): 621–28. http://dx.doi.org/10.1002/macp.1995.021960217.

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35

Solea, Atena B., Ivan Cornu, Vera Deneva, Aurelien Crochet, Katharina M. Fromm, Liudmil Antonov, Christophe Allemann, and Olimpia Mamula. "Tautomerism and Self-Association in the Solution of New Pinene-Bipyridine and Pinene-Phenanthroline Derivatives." Molecules 25, no. 2 (January 11, 2020): 298. http://dx.doi.org/10.3390/molecules25020298.

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Two novel pinene-type ligands have been synthesized and their tautomeric and self-associating behavior studied in solution and in the solid state. The first ligand, an acetylated derivative of 5,6-pinene-bipyridine, displays keto-enol tautomerism in solution. This tautomeric equilibrium was studied by NMR and UV-Vis spectroscopy in various solvents, and the results were compared with the ones obtained through DFT calculations. The second ligand was obtained by an unusual oxidation of the phenanthroline unit of a pinene-phenanthroline derivative. This compound exists as a single tautomer in solution and aggregates both in solution (as observed by NMR) and in the solid state through H-bonding as observed by X-ray structure determination and confirmed by DFT studies.

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36

Anderson, John C., and A. Douglas Broadbent. "The influence of pH in the tautomerism of 9,10-anthracenediols and 1,3-diketones." Canadian Journal of Chemistry 64, no. 1 (January 1, 1986): 127–32. http://dx.doi.org/10.1139/v86-022.

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Equations have been derived to describe how the ratio of the total concentration of all keto forms to that of all enolic forms (K′) varies with changing pH, for a tautomeric system in which the enol and ketone are polyprotic acids. The influence of pH on the tautomerism of 2-amino-9,10-anthracenediol, 2,9,10-anthracenetriol, 2,4-pentanedione, and 1-(3-pyridyl)-1,3-butanedione was examined. The equilibrium constants for all the acid–base equilibria involved were calculated from the dependence of K′ on pH.

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37

Albayrak Kaştaş, Çiğdem, and Gökhan Kaştaş. "Dependence of tautomerism on substituent type in o-hydroxy Schiff bases." Macedonian Journal of Chemistry and Chemical Engineering 38, no. 1 (May 16, 2019): 85. http://dx.doi.org/10.20450/mjcce.2019.1531.

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Quantum computational methods were used to elucidate the structures of the o-hydroxy Schiff bases with different substituents. It is possible for a Schiff base to have different tautomeric structures depending on intramolecular proton transfer from the phenolic oxygen atom to the nitrogen atom. Proton transfer results in two tautomeric structures known as the phenol-imine and keto-amine forms. To explain the substituent effect on the proton transfer process in five o-hydroxy-Schiff bases, possible geometric structures in gas phase were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. To describe tautomerism including intramolecular proton transfer, potential energy surface (PES) scans were performed starting from the optimized geometry of the phenol-imine form. HOMA indices were calculated in order to estimate p-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.

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38

Nikitina, Polina A., Tatiana Yu Koldaeva, Marina A. Zakharko, and Valery P. Perevalov. "Synthesis and Study of Prototropic Tautomerism of 2-(2-Furyl)-1-hydroxyimidazoles." Australian Journal of Chemistry 73, no. 11 (2020): 1098. http://dx.doi.org/10.1071/ch20044.

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Novel 2-(2-furyl)imidazole derivatives were synthesised. 2-(2-Furyl)-1-methoxyimidazoles and 2-(2-furyl)-1-methylimidazole 3-oxides were used as model compounds in the study of the prototropic tautomerism of 2-(2-furyl)-1-hydroxyimidazoles by means of 1H, 13C NMR and UV/vis spectroscopies. It was demonstrated that the interaction of the π-excessive furyl moiety with an electron-withdrawing carbonyl group in position 5 of imidazole stabilised the N-hydroxy tautomeric form in both deuterated chloroform and d6-DMSO. In ethanol the N-oxide tautomer is also present along with the prevailing N-hydroxyimidazole.

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39

Babiker, Musa E. Mohamed, Ahmed A. Alzharani, and Ayyob M. Bakry. "Computational Study of the Keto-Enol Tautomerism of 3-Phenyl-2,4-Pentanedione in the Gaseous Phase and Solvents Using DFT Methods." Oriental Journal Of Chemistry 39, no. 1 (February 28, 2023): 40–46. http://dx.doi.org/10.13005/ojc/390105.

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The study of tautomerics equilibria is of vital importance as tautomeric compounds reactivity highly depends on the proportion of each tautomer. Herein, the tautomeric equilibrium of the 3-phenyl-2,4-pentanedione was studied theoretically by the b3lyp/6-31+g(d)methods. The effect of four solvents was considered (water, methanol, carbon tetrachloride and Cyclohexane).The tautomeric equilibrium takes place through four-membered ring transition state. The barrier heights energies of the tautomerics equilibria reaction of the transition state with reference to Keto were found to be 31.26, 31.23, 30.84, 30.82 and 30.61 kcal mol-1 in water, methanol, carbon tetrachloride, Cyclohexane and the gas-phase, respectively. Furthermore, the electronics energies differences between the Keto-form and Enol-form were found to be -16.50,-16.55, -17.27, -17.34 and -17.89 kcal mol-1 in the same previous solvents respectively. The DFT calculations revealed that the Keto-form is more stable one in all investigations.

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40

Bihdan, O. A., and N. A. Alk Khalaf. "DFT-analysis of protolytic equivalents of 5-(aryl)-4-(methyl,amino)-1,2,4-triazole-3(2H)-thione." Current issues in pharmacy and medicine: science and practice 15, no. 2 (August 1, 2022): 133–39. http://dx.doi.org/10.14739/2409-2932.2022.2.254474.

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The use of modern computer methods in aspects of quantum chemistry and systematic analysis of their results give an idea of the reactivity of organic compounds, as well as to understand the essence of known experimental data, correct predictions, and quantitative estimates. Undoubtedly, theoretical calculations are useful in solving such an urgent problem of modern chemistry as prototropic equilibria and properties of substances in the gas phase, solutions, and solid-state. The aim of the work – until recently assigned to a theoretical vivification in the infusion of solvents on tautomeric equilibrium and acid-base powers і know more broadly practical stasis in the pharmaceutical industry. Materials and methods. The effect of solvation effects on tautomerism and antitropic properties of 1,2,4-triazole derivatives was studied on the example of model compounds. All calculations were performed using the Gauss-View 6.0.1 molecular link visualization program and Gaussian 98, Gaussian 03 software packages and the use of default convergence criteria. After optimizing the geometry, frequency calculations followed. Thus, the stationary structures are confirmed by checking that all ground states have only real frequencies, and all transition states have only one imaginary frequency. The same method and established basis were used to optimize the geometry. Solvation calculations were performed in the framework of continuous models (D-PCM, C-PCM, IEF-PCM, IPCM, SCIPCM) of discrete and combined models using the Hartree–Fock constraint method, the method of density functional theory B3LYP with basic sets 6-31G (d), 6-31G (d,p), 6-31G++ (d,p), cc-pVDZ, as well as semi-empirical methods in the MOPAC6 package. Results. For the first time, various quantum chemical calculations of solvated model compounds using different approaches and models, variation of the basis in non-empirical calculations, identification of the role of electronic correlation effects, method of geometry optimization, etc. were carried out within the theory of self-consistent reaction field. The main stage of this study was to compare trends in the equilibrium change in the relative stability of tautomeric forms of thione-thiol tautomerism of 1,2,4-triazole-2(3H)-thions in the gas phase and different prototropic solvents due to the possibility of using different models and calculation methods for quality predictions of the effect of solvation on the position of tautomeric equilibrium in compounds of this class. It was found that the selected various solvents according to all used quantum chemical methods and models (D-PCM, C-PCM. IEF-PCM, IPCM, SCIPCM) reduce the difference in the stability of tautomeric forms of the investigated compounds in comparison with the gas phase, while the greatest stabilizing effect is observed in the solvation of NH-tautomers derived from 1,2,4-triazole-2(3H)-thiones. Using all energy parameters (∆Etot, ∆E0, ∆H298, ∆G298) allowed to determine the effect of complexation on the relative stability of tautomeric forms of the studied compounds. The difference in the values of the energy levels of HOMO and LUMO – orbitals indicate the reactivity of the molecule and its activation energy, which indicates the chemical reactivity of the molecule to electronic transport and the manifestation of biological activity with intramolecular charge transfer. Conclusions. For the first time, complex quantum chemical calculations of thione-thiol tautomers of 5-(aryl)-4-(methyl, amino)-1,2,4-triazole-3(2H)-thiones were performed and it was found that prototropic solvents reduce the difference in all models. In the stability of tautomeric forms of the investigated compounds in comparison with the gas phase. The calculated values of electronic correlation models on the hydrogen atom make a significant contribution to the relative stability of tautomeric forms, while the use of polarization functions of quantum chemical methods on hydrogen atoms has practically no effect on the tautomeric equilibrium. From the obtained data it becomes clearer that in the gas phase and aprotic solvents the thione tautomer with the center of NH-acidity is the most stable, and the thiol tautomer of 1,2,4-triazole-3(2H)-thione predominates in the transition to polar proton-donor solvents. The obtained data indicate the possibility of conducting an electrophilic substitution reaction (eg, alkylation) in the form of an anion. The partially negative charge of the Nitrogen atoms of the 1,2,4-triazole ring promotes electrophilic addition reactions. In the thionic form, on the contrary, electrophilic substitution reactions are possible.

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41

Szczepaniak, K., M. Szczesniak, W. Szajda, W. B. Person, and J. Leszczynski. "Infrared spectra of tautomers and rotamers of 9-methylguanine. An experimental and theoretical study." Canadian Journal of Chemistry 69, no. 11 (November 1, 1991): 1705–20. http://dx.doi.org/10.1139/v91-251.

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Both amino-oxo and amino-hydroxy tautomeric forms of 9-methylguanine have been identified in approximately equal abundance in. infrared studies of these molecules isolated in the hydrophobic environment of an argon matrix at 12 K. The amino-hydroxy tautomer occurs in two different rotamers correlated with the rotation of the OH group. The ratio of concentrations of the two rotamers is sensitive to UV irradiation, and this ratio then relaxes to an equilibrium value after irradiation is stopped. This sensitivity allows us to separate the experimental spectra related to the oxo tautomer and to each of the rotamers of the hydroxy tautomer. The relative concentrations of the amino-oxo and amino-hydroxy tautomers ([a-o]/[a-h] = K(o-h) = 1.0 ± 0.3) and of the two rotamers (K(h1-h2) = 0.31 ± 0.10 in an argon matrix at 12 K and about 30 ± 15 in the vapor at 470 K) are estimated from the observed relative infrared absorbances. From these relative concentrations the differences between the free energies of the tautomers (ΔG470 (o-h) = 0 ± 0.5 kJ mol−1) and of the two rotamers (ΔG(h1-h2) = 0.12 ± 0.03 kJ mol−1 in the argon matrix at 12 K and ΔG470 between +2 and −13 kJ mol−1 in the vapor at 470 K) have been estimated. The electronic absorption spectrum of 9-methylguanine isolated in the argon matrix at 12 K and the effect of brief ultraviolet irradiation on it have also been studied. In an effort to interpret the experimental results, ab initio calculations of the infrared spectra have been made for 9-methylguanine at the 3-21G//3-21G level. Comparison with the experimental spectra is of some help with the assignment of the infrared spectra for the different tautomers. Key words: 9-methylguanine, tautomerism, infrared and ultraviolet spectra, matrix isolation, ab initio calculation.

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42

Hansen, Poul Erik. "NMR of Natural Products as Potential Drugs." Molecules 26, no. 12 (June 21, 2021): 3763. http://dx.doi.org/10.3390/molecules26123763.

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This review outlines methods to investigate the structure of natural products with emphasis on intramolecular hydrogen bonding, tautomerism and ionic structures using NMR techniques. The focus is on 1H chemical shifts, isotope effects on chemical shifts and diffusion ordered spectroscopy. In addition, density functional theory calculations are performed to support NMR results. The review demonstrates how hydrogen bonding may lead to specific structures and how chemical equilibria, as well as tautomeric equilibria and ionic structures, can be detected. All these features are important for biological activity and a prerequisite for correct docking experiments and future use as drugs.

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43

Jawhar, Zanko S., Hiwa O. Ahmed, Asmaa A. Haydar, Halala A. Abdullah, and Sawza A. Mahamad. "One-Pot Synthesis, Pharmacological Evaluation, Docking Study, and DFT Calculations for Selected Imidazolidine-2,4-Diones." Science Journal of University of Zakho 6, no. 4 (December 30, 2018): 150–54. http://dx.doi.org/10.25271/sjuoz.2018.6.4.541.

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The title compounds with different 5-substituted imidazolidine-2,4-dione were synthesized through a solvent-free reaction. Imidazolidine-2,4-dione derivatives are found to be an active pharmacophore for design and development of various bioactive lead compounds. Positive values of energy obtained for compound 1and 3, while a negative value for compound 2 was calculated by DFT in Gaussian. keto-enol tautomerism was supported by energy values and indicated the most stable tautomeric form. The biological evaluation has been supported by docking studies using molecular operating environment program to show binding with androgen receptor. Supplementary Materials: https://sjuoz.uoz.edu.krd/suppma

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44

Jafari Rad, Azadeh, Maryam Abbasi, and Bahareh Zohrevand. "Iron Chelation by thiocytosine: Investigating electronic and structural features for describing tautomerism and metal chelation processes." Main Group Chemistry 21, no. 1 (April 8, 2022): 177–84. http://dx.doi.org/10.3233/mgc-210110.

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This work was performed regarding the importance of iron (Fe) chelation for biological systems. This goal was investigated by assistance of a model of thiocytosine (TC) for participating in Fe-chelation processes. First, formations of tautomeric conformations were investigated to explore existence of possible structures of TC. Next, Fe-chelation processes were examined for all four obtained tautomers of TC. The results indicated that thiol tautomers could be seen at higher stability than thio tautomers, in which one of such thiol tautomers yielded the strongest Fe-chelation process to build FeTC3 model. As a consequence, parallel to the results of original TC tautomers, Fe-chelated models were found to be achievable for meaningful chelation processes or sensing the existence of Fe in media. Examining molecular orbital features could help for sensing purposes. The results of this work were obtained by performing density functional theory (DFT) calculations proposing TC compounds suitable for Fe-chelation purposes.

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45

Zaharieva, Lidia, Ivan Angelov, and Liudmil Antonov. "Stationary External Electric Field—Mimicking the Solvent Effect on the Ground-State Tautomerism and Excited-State Proton Transfer in 8-(Benzo[d]thiazol-2-yl)quinolin-7-ol." Molecules 29, no. 15 (July 26, 2024): 3506. http://dx.doi.org/10.3390/molecules29153506.

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The effect of the external electric field on the ground-state tautomerism in 8-(benzo[d]thiazol-2-yl)quinolin-7-ol has been studied by using density functional theory. The compound exists as an enol tautomer (off state) and under the influence of the external electric field a long-range intramolecular proton transfer can occur, placing the tautomeric proton at the quinolyl nitrogen atom (on state). This is a result of the much higher dipole moment of the end keto tautomer and indicates that the external electric field can be used to mimic the implicit solvent effect in tautomeric systems. In the excited state, the further stabilization of the most polar on state leads to a situation when the excited-state intramolecular proton transfer becomes impossible, limiting the intramolecular rotation to the conical intersection region.

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46

Bunting, John W., James P. Kanter, Raymond Nelander, and Zhennan Wu. "The acidity and tautomerism of β-diketones in aqueous solution." Canadian Journal of Chemistry 73, no. 8 (August 1, 1995): 1305–11. http://dx.doi.org/10.1139/v95-161.

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The acidity and keto–enol tautomerism of a series of symmetrical β-diketones (RCOCH2COR (1): R = methyl (a), phenyl (b), 3-pyridinyl (c), 4-pyridinyl (d), 3-(N-methyl)pyridinio (e), and 4-(N-methyl)pyridinio (f)) and two series of unsymmetrical β-diketones (RCOCH2COCH3 (7a–7f) and RCOCH2COC6H5 (8a–8f)) have been investigated in aqueous solution at 25 °C and ionic strength 0.1. Values of [Formula: see text] were measured spectrophotometrically, and the acidities of the enols [Formula: see text] were obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolateconjugate bases. These data in turn allowed the calculation of the acidities of the keto tautomers [Formula: see text] and the equilibrium constants for enolization (KE = [Enol]/[Keto]). In general, KE is greater for the symmetrical ketones(1) than for the corresponding R-substituted unsymmetrical ketones (7 and 8). KE is much more sensitive to the nature of the R substituent in these three series of β-diketones than in the corresponding series of β-keto esters and amides. Correlations between [Formula: see text] for 8 and 7 combined with the known acidities of the corresponding β-keto esters and amides provide the first accurate estimates of the acidities of dimethyl malonate (pKa = 13.0)and malonamide (pKa = 12.5) in aqueous solution. Keywords: acidity, tautomerism, β-diketones.

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47

MACIEJEWSKA, Agnieszka M., Katarzyna D. LICHOTA, and Jarosław T. KUŚMIEREK. "Neighbouring bases in template influence base-pairing of isoguanine." Biochemical Journal 369, no. 3 (February 1, 2003): 611–18. http://dx.doi.org/10.1042/bj20020922.

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Assuming that the efficiency of the incorporation of 5-methyl-2′-deoxyisocytosine-5′ triphosphate (dMiCTP) and dTTP opposite isoguanine (iG) is a measure of the proportion of the keto and enol tautomers of iG in the Thermus aquaticus DNA polymerase active centre, we studied the influence of temperature and iG-neighbouring bases in the template on base-pairing of iG. On the basis of experiments with four sequences (3′-TXT-5′, 3′-GXG-5′, 3′-CXC-5′, 3′-CXT-5′, where X = iG) at 37, 50, 65 and 80°C, we found that 3′-neighbours decrease the fraction of the keto tautomer in the order C>GT, whereas temperature apparently does not influence the tautomeric equilibrium of iG. The variability of the ratio of incorporation of dMiCTP versus dTTP (5—20) primarily reflects the variability of Km values, since Vmax values are roughly similar, which indicates that the iG·MiC and iG·T pairs fit the polymerase active centre equally well. The altering of the base-pairing of iG by sequence context is discussed in relation to tautomerism and miscoding of this oxidized adenine derivative. A key derivative for preparation oligodeoxynucleotides, O2-diphenylcarbamoyl-N6-[(dimethylamino)ethylidene]-2′-deoxyisoguanosine, is extremely labile (t1/2 = 3.5min) in 3% trichloroacetic acid/dichloromethane, i.e. under the conditions of automated DNA synthesis, which results in low yield and length heterogeneity of templates.

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48

Pasca, Roxana-Diana, Atena Moldovan, Ossi Horovitz, and Alexandru Lupan. "On the thermodynamics of triazines. A DFT and topological study." International Journal of Computational Physics Series 1, no. 1 (February 27, 2018): 12. http://dx.doi.org/10.29167/a1i1p12.

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Abstract:

Tautomerism is the relation between isomeric organic compounds, which readily interconvert, usually by relocation of a proton. It plays an important role in the biological activity of compounds. In the series of tautomeric triazines (monoxime and dioxime derivatives) energy computations have been performed. The results have shown that for the monoxime, the amino keto tautomer is favored by 10.9 kJ/mol vs. the enol form; for the dioxime tautomer, the imino keto form is 17.6 kJ/mol lower in energy in comparison with the most stable amino keto tautomer. Topological aspects have been considered. Computations have been done at DFT M06-2X/6-311G(d,p) level of theory.

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Fain, V. Ya, B. E. Zaitsev, and M. A. Ryabov. "Keto-oxy tautomerism." Russian Journal of General Chemistry 80, no. 3 (March 2010): 550. http://dx.doi.org/10.1134/s1070363210030345.

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50

Hernandez-Folgado, Laura, José María Cumella, Paula Morales, Ibon Alkorta, José Elguero, and Nadine Jagerovic. "Tautomerism of hydroxychromenopyrazoles." Journal of Molecular Structure 1015 (May 2012): 162–65. http://dx.doi.org/10.1016/j.molstruc.2011.07.031.

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Journal articles on the topic 'Tautomerism'

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