Letteratura scientifica selezionata sul tema "Small-Angle X-Ray and neutron scattering (SAXS/SANS)"

Small Angle Neutron Scattering (SANS) and Small Angle X-ray Scattering (SAXS), anong other available techniques, are the nost sought after techniques for studying the sizes and shapes of nanoparticles. The contrast between particle and its surrounding is different for X-rays and neutrons. Thus a combined SANS and SAXS study, at times, provides information about the core and the shell structure of nanoparticles. This paper gives an introduction to the techniques of SANS and SAXS and shows results of a study of core-shell structure for a micelle (nanaoparticle of organic material).

Carbonaceous nanomaterials have become important materials with widespread applications in battery systems and supercapacitors. The application of these materials requires precise knowledge of their nanostructure. In particular, the porosity of the materials together with the shape of the pores and the total internal surface must be known accurately. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) present the methods of choice for this purpose. Here we review our recent investigations using SAXS and SANS. We first describe the theoretical basis of the analysis of carbonaceous material by small-angle scattering. The evaluation of the small-angle data relies on the powerful concept of the chord length distribution (CLD) which we explain in detail. As an example of such an evaluation, we use recent analysis by SAXS of carbide-derived carbons. Moreover, we present our SAXS analysis on commercially produced activated carbons (ACN, RP-20) and provide a comparison with small-angle neutron scattering data. This comparison demonstrates the wealth of additional information that would not be obtained by the application of either method alone. SANS allows us to change the contrast, and we summarize the main results using different contrast matching agents. The pores of the carbon nanomaterials can be filled gradually by deuterated p-xylene, which leads to a precise analysis of the pore size distribution. The X-ray scattering length density of carbon can be matched by the scattering length density of sulfur, which allows us to see the gradual filling of the nanopores by sulfur in a melt-impregnation procedure. This process is important for the application of carbonaceous materials as cathodes in lithium/sulfur batteries. All studies summarized in this review underscore the great power and precision with which carbon nanomaterials can be analyzed by SAXS and SANS.

The certification of a new standard reference material for small-angle scattering [NIST Standard Reference Material (SRM) 3600: Absolute Intensity Calibration Standard for Small-Angle X-ray Scattering (SAXS)], based on glassy carbon, is presented. Creation of this SRM relies on the intrinsic primary calibration capabilities of the ultra-small-angle X-ray scattering technique. This article describes how the intensity calibration has been achieved and validated in the certifiedQrange,Q= 0.008–0.25 Å−1, together with the purpose, use and availability of the SRM. The intensity calibration afforded by this robust and stable SRM should be applicable universally to all SAXS instruments that employ a transmission measurement geometry, working with a wide range of X-ray energies or wavelengths. The validation of the SRM SAXS intensity calibration using small-angle neutron scattering (SANS) is discussed, together with the prospects for including SANS in a future renewal certification.

Vesicle shape and bilayer parameters are studied by small-angle X-ray (SAXS) and small-angle neutron (SANS) scattering in the presence of the saponin aescin. Bilayer dynamics is studied by neutron spin-echo (NSE) spectroscopy.

Innovations in small-angle X-ray and neutron scattering (SAXS and SANS) at major X-ray and neutron facilities offer new characterization tools for researching materials phenomena relevant to advanced applications. For SAXS, the new generation of diffraction-limited storage rings, incorporating multi-bend achromat concepts, dramatically decrease electron beam emittance and significantly increase X-ray brilliance over previous third-generation sources. This results in intense X-ray incident beams that are more compact in the horizontal plane, allowing significantly improved spatial resolution, better time resolution, and a new era for coherent-beam SAXS methods such as X-ray photon correlation spectroscopy. Elsewhere, X-ray free-electron laser sources provide extremely bright, fully coherent, X-ray pulses of <100 fs and can support SAXS studies of material processes where entire SAXS data sets are collected in a single pulse train. Meanwhile, SANS at both steady-state reactor and pulsed spallation neutron sources has significantly evolved. Developments in neutron optics and multiple detector carriages now enable data collection in a few minutes for materials characterization over nanometre-to-micrometre scale ranges, opening up real-time studies of multi-scale materials phenomena. SANS at pulsed neutron sources is becoming more integrated with neutron diffraction methods for simultaneous structure characterization of complex materials. In this paper, selected developments are highlighted and some recent state-of-the-art studies discussed, relevant to hard matter applications in advanced manufacturing, energy and climate change.

The structural changes in nanospheres with a crystalline core and an amorphous diffuse shell were investigated by small-angle neutron scattering (SANS), small-, medium-, and wide-angle X-ray scattering (SAXS, MAXS and WAXS), and differential scanning calorimetry (DSC).

Abstract The metallic glasses Cu50Ti50, Ni30Ti60Si10, Ni32Ti52Si16 , Ni16Ti68Si16 and Ti84Si16 were produced by melt spinning. The alloys in the blank state as well as after loading with hydrogen or deuterium were investigated by small angle neutron (SANS) and X-ray (SAXS) scattering. The scattering of the different amorphous alloys exhibited common features. SANS follows a power-law with exponent of the scattering vector between -3 and -4. The melt-spun glasses contain extended structural inhomogeneities which are associated rather with the local composition than with the local density. SAXS measurements did not show effects above the background level. Loading the alloys with hydrogen or deuterium causes strong effects in the SANS behaviour. From the results it is concluded that the amorphous alloys contain inner surfaces where the hydrogen atoms segregate.

Exploiting small-angle X-ray and neutron scattering (SAXS/SANS) on the same sample volume at the same time provides complementary nanoscale structural information in two different contrast situations. Unlike an independent experimental approach, the truly combined SAXS/SANS experimental approach ensures the exactness of the probed samples, particularly for in situ studies. Here, an advanced portable SAXS system that is dimensionally suitable for installation in the D22 zone of ILL is introduced. The SAXS apparatus is based on a Rigaku switchable copper/molybdenum microfocus rotating-anode X-ray generator and a DECTRIS detector with a changeable sample-to-detector distance of up to 1.6 m in a vacuum chamber. A case study is presented to demonstrate the uniqueness of the newly established method. Temporal structural rearrangements of both the organic stabilizing agent and organically capped gold colloidal particles during gold nanoparticle growth are simultaneously probed, enabling the immediate acquisition of correlated structural information. The new nano-analytical method will open the way for real-time investigations of a wide range of innovative nanomaterials and will enable comprehensive in situ studies on biological systems. The potential development of a fully automated SAXS/SANS system with a common control environment and additional sample environments, permitting a continual and efficient operation of the system by ILL users, is also introduced.

Small-angle neutron scattering (SANS) has increasingly been used by the structural biology community in recent years to obtain low-resolution information on solubilized biomacromolecular complexes in solution. In combination with deuterium labelling and solvent-contrast variation (H2O/D2O exchange), SANS provides unique information on individual components in large heterogeneous complexes that is perfectly complementary to the structural restraints provided by crystallography, nuclear magnetic resonance and electron microscopy. Typical systems studied include multi-protein or protein–DNA/RNA complexes and solubilized membrane proteins. The internal features of these systems are less accessible to the more broadly used small-angle X-ray scattering (SAXS) technique owing to a limited range of intra-complex and solvent electron-density variation. Here, the progress and developments of biological applications of SANS in the past decade are reviewed. The review covers scientific results from selected biological systems, including protein–protein complexes, protein–RNA/DNA complexes and membrane proteins. Moreover, an overview of recent developments in instruments, sample environment, deuterium labelling and software is presented. Finally, the perspectives for biological SANS in the context of integrated structural biology approaches are discussed.

BAMLET and related compounds are a family of protein-oleic acid complexes that are cytotoxic towards a range of cancer cells and some bacteria, and for which no evidence of similar toxicity towards healthy tissue has yet been presented. This thesis determines the structure of BAMLET-like complexes, revealing that they belong to a new type of lipid-binding protein structure family consisting of the distinctive features of protein located on the periphery encapsulating a drop of oleic acid in the centre. This work is the first to reveal these distinctive structural features of what is now called the liprotide family. The oleic acid component of the BAMLET family member is shown to form a 36 Å diameter spheroid with the partially unfolded bovine α-lactalbumin polypeptide component expanded to a diameter of 79 Å. Small angle X-ray and neutron scattering were the principal techniques used and comprehensively elucidate the structure. The anti-cancer mechanism of BAMLET compounds appears to be novel and the novel structure revealed in this thesis provides part of the explanation for how they achieve broad spectrum activity. In an aqueous environment, the BAMLET complex is stable and its oleic acid cargo is thus solubilised, until a cell membrane is encountered, for which the oleic acid could have higher affinity resulting in activity against the cell. This thesis also demonstrates a possible application to the treatment of mesothelioma for BAMLET-like compounds, that negates the disadvantage that blood components disable BAMLET’s anti-cancer activity. Tissue culture and cell death assay techniques were used to show that BAMLET is equally cytotoxic towards mesothelioma cells that have developed increased resistance to the cisplatin or pemetrexed chemotherapy drugs, as towards mesothelioma cells that do not have heightened resistance. These results position BAMLET as a potentially valuable treatment option for this ultimately treatment-resistant cancer.

TSPO est une petite protéine membranaire translocatrice, ubiquitaire, composée de cinq hélices-α transmembranaires. Chez les mammifères, elle est principalement localisée dans la membrane externe des mitochondries, où elle jouerait un rôle dans le transport du cholestérol et la voie de synthèse des stéroïdes. Cette protéine présente un intérêt pharmacologique majeur en raison de sa forte affinité pour de nombreux ligands utilisés comme marqueurs de l'inflammation en neuro-imagerie. La seule structure atomique connue des TSPOs de mammifères est la structure RMN (2MGY.PDB) de la TSPO de souris (mTSPO). Cependant, cette structure est controversée, car obtenue en repliant la protéine dans une concentration forte de détergent DPC et en présence du ligand (R)-PK11195, qui la rigidifie fortement. En l'absence de ligand, la structure de mTSPO est trop flexible pour être résolue par RMN. De plus, à ce jour, aucune condition amphiphile n'a permis de cristalliser les TSPOs de mammifères, avec ou sans ligand, contrairement aux TSPOs bactériennes.L'objectif de cette étude est de déterminer, par une approche structure/fonction, l'effet de différents environnements amphiphiles sur la structure de mTSPO en condition “apo” (i.e. sans ligand). Nous avons sondé sa structure en solution à différentes échelles par des techniques de diffusion de rayonnement et de spectroscopie optique. En particulier, la diffusion de rayons X et de neutrons aux petits angles (SAXS et SANS), combinée à la chromatographie d'exclusion stérique (SEC), la variation de contraste en SANS et la modélisation ab initio, a permis d'obtenir la conformation du complexe entier mTSPO/amphiphiles et de sonder spécifiquement celles de la protéine et de la bouée d'amphiphiles. La quantité de molécules d'amphiphiles associés à mTSPO, mesurée par MALLS, a permis de valider les modèles proposés. L'effet de l'environnement sur l'affinité du ligand a été mesuré par thermophorèse à micro-échelle (MST).L'apo-mTSPO, produite par voie recombinante dans les corps d'inclusion de la bactérie E. coli, est partiellement dépliée suite à son extraction par le SDS. Nous montrons que la protéine se replie en DPC à la fois au niveau local (augmentation significative de la quantité et des interactions des hélices-α et de la fluorescence des tryptophanes) et au niveau tridimensionnel avec une conformation “apo” plus étendue que la structure RMN 2MGY.PDB. En ajoutant des phospholipides DMPC, pour obtenir un environnement de bicelles mixtes DMPC:DPC partiellement biomimétique, l'apo-mTSPO se structure davantage : la quantité d'hélices-α et leurs interactions augmentent significativement, ainsi que la fluorescence des tryptophanes. Ce repliement est associé à une augmentation significative de l'affinité de la protéine pour le ligand (R)-PK11195 (0.9 µM) comparée à celle en DPC (70 µM) et SDS (absence d'affinité). Nous démontrons ainsi la pertinence de l'utilisation de bicelles DMPC:DPC pour l'étude en solution des protéines membranaires et confirmons le rôle crucial des lipides dans la structure et la fonction de mTSPO. Enfin, nous montrons que cet environnement est favorable à la cristallisation de l'apo-mTSPO et à la reconstitution de la protéine dans des nanodisques de DMPC.Pour comparer ces résultats avec une protéine exprimée en condition native, nous avons développé un nouveau protocole de production de mTSPO en levures. Nous avons réussi à purifier la protéine en condition “apo” en DDM, un détergent connu pour solubiliser les protéines correctement repliées tout en conservant des lipides membranaires associés. Ces travaux de thèse (i) contribuent à une meilleure compréhension de la structure/fonction de mTSPO dans différents environnements amphiphiles, pour déterminer les conditions optimales à des études structurales à plus haute résolution, et (ii) constituent un apport méthodologique significatif pour l'étude des protéines membranaires en solution
TSPO is a small, ubiquitous, translocator membrane protein composed of five transmembrane α-helices. In mammals, it is primarily located in the outer mitochondrial membrane, where it is believed to play a role in cholesterol transport and steroid synthesis pathways. This protein has significant pharmacological interest due to its affinity for various ligands used as markers of inflammation in neuroimaging. The only known atomic structure of mammalian TSPOs is the NMR structure (2MGY.PDB) of mouse TSPO (mTSPO). However, this structure is controversial as it was obtained by refolding the protein using a high concentration of DPC and in the presence of the ligand (R)-PK11195, that stiffens it significantly. In the absence of ligand, the structure of mTSPO is too flexible to be resolved by NMR. Furthermore, to date, no amphiphilic condition has allowed the crystallization of mammalian TSPOs, with and without ligand, unlike bacterial TSPOs.The aim of the present study is to determine, using a structure/function approach, the effect of different amphiphilic environments on the structure of apo-mTSPO (i.e. without the ligand). We investigated mTSPO's structure in solution at different scales using radiation scattering and optical spectroscopy techniques. Small-angle X-ray and neutron scattering (SAXS, SANS), combined with size-exclusion chromatography (SEC), contrast variation in SANS, and ab initio modeling, allowed us to obtain the conformation of the entire mTSPO/amphiphile complex and to specifically probe those of the protein and the amphiphilic belt within the complex. The quantity of amphiphile molecules associated with mTSPO, measured by MALS, allowed the validation of the proposed models. The effect of the environment on ligand affinity was measured by microscale thermophoresis (MST).The apo-mTSPO, produced by a recombinant way in E.coli bacteria inclusion bodies, is partially unfolded following its extraction by SDS. We show that the protein refolds in DPC, both locally (significant increase of content and interactions of α-helices and in tryptophan fluorescence) and three-dimensionally, with a more extended "apo" conformation than the NMR structure 2MGY.PDB. Adding DMPC phospholipids to create a partially biomimetic environment of mixed DMPC:DPC bicelles further structures apo-mTSPO: the quantity and interactions of α-helices, as well as tryptophan fluorescence, increase significantly. This refolding is associated with a significant increase in the protein's affinity for the ligand (R)-PK11195 (0.9 μM) compared to that in DPC (70 μM) and SDS (no affinity). Thus, we demonstrate the relevance of using DMPC:DPC bicelles for studying membrane proteins in solution and confirm the crucial role of lipids in the structure and function of mTSPO. Finally, we show that this environment is favorable for the crystallization of apo-mTSPO and for reconstituting the protein in DMPC nanodiscs.To compare these results with a protein expressed under native conditions, we developed a new protocol for producing mTSPO in yeast cells. We managed to purify the protein in “apo” condition in DDM, a detergent known for solubilizing properly folded proteins, while retaining associated membrane lipids.This thesis work (i) contributes to a better understanding of the structure/function of mTSPO in different amphiphilic environments to determine optimal conditions for higher-resolution structural studies, and (ii) provides a significant methodological contribution to the study of membrane proteins in solution

L’utilisation de la mousse en récupération assistée du pétrole (Enhanced Oil Recovery, EOR) présente un avantage indéniable par rapport à l’injection du gaz seul pour pallier les problèmes de ségrégation gravitaire et de digitations visqueuses. Son utilisation systématique en ingénierie du réservoir nécessite des connaissances plus approfondies sur son comportement en milieu poreux. La littérature montre deux types d’approches expérimentales basées soit sur des études pétrophysiques effectuées sur des systèmes poreux 3D et basées sur des mesures de pressions intégrées sur l’ensemble du milieu poreux, soit sur des études micro-fluidiques qui permettent une visualisation directe de l’écoulement mais qui sont limitées à des systèmes modèles dans des géométries à 1 ou 2 dimensions. L’objectif de cette thèse est de faire le pont entre ces deux approches. La stratégie proposée consiste à caractériser in situ l’écoulement de la mousse dans des milieux poreux 3D à différentes échelles, en utilisant des techniques complémentaires permettant d’accéder à une large gamme de résolutions spatiale et temporelle. Un environnement instrumenté donnant accès aux mesures pétro-physiques classiques a été développé puis couplé à différentes cellules d’observation conçues spécifiquement pour chaque instrument de caractérisation. Dans un premier temps, un scanner X a été utilisé pour décrire et visualiser les écoulements de la mousse à l’échelle de la carotte. La rhéologie de la mousse à cette échelle a pu être étudiée en fonction des conditions d’injections comme la vitesse interstitielle du gaz et la qualité de mousse. Dans un deuxième temps, la technique de diffusion des neutrons aux petits angles (SANS) a permis de sonder la texture de la mousse en écoulement sur trois ordres de grandeurs en taille. Des informations in situ sur la texture de la mousse en écoulement (taille et densité des bulles et des lamelles) ont pu être mesurées pour différentes qualités de mousse puis en fonction de la distance au point d’injection. Une comparaison avec les caractéristiques géométriques du milieu poreux a également été effectuée. Dans un troisième temps, la micro-tomographie X rapide haute résolution sur Synchrotron a été utilisée pour visualiser la mousse en écoulement à l’échelle du pore. Cette technique a permis de confirmer de visu certaines caractéristiques de la mousse mesurées par SANS et de décrire en sus les effets d’intermittence du piégeage de la mousse. Cette étude constitue une étape importante de la caractérisation multi-échelle de l’écoulement des mousses en milieux poreux 3D et apporte des éléments de réponse à certaines hypothèses admises
Foam has long been used as a mobility control agent in Enhanced Oil Recovery (EOR) processes to enhance sweep efficiency and overcome gravity segregation, viscous fingering and gas channeling, which are gas-related problems when the latter is injected alone in the reservoir. However, the systematic use of foam in reservoir engineering requires more in-depth knowledge of its dynamics in porous media. The literature shows two types of experimental approaches based either on petrophysical studies carried out on 3D porous systems and based on pressure measurements, or on microfluidic studies that allow direct visualization of foam flow but are limited to 1D or 2D model systems. The research investigated in this thesis aims to bridge the gap between these two approaches. The proposed strategy is to characterize in situ the foam flow in 3D porous media with techniques providing a wide range of temporal and spatial resolutions. A coreflood setup giving access to classical petro-physical measurements was developed and then coupled to different observation cells designed specifically for each characterization instrument. First, an X-ray CT scanner was used to describe and visualize the foam flow at the core scale. The rheological behavior of foam on this scale was studied as a function of the injection conditions such as gas velocity and foam quality. Secondly, Small Angle Neutron Scattering (SANS) was used to probe the foam structure in situ during the flow, on a wide length scale, up to three orders of magnitude in size. In situ foam texture (size and density of bubbles and lamellae) was measured for different foam qualities and at different propagation distances from the injection point. A comparison to the geometric characteristics of the porous medium was also realized. Thirdly, High Resolution Fast X-ray Micro-tomography on a Synchrotron was used to visualize the foam flow at the pore scale. This allowed to confirm visually some foam characteristics measured with SANS and to investigate on local intermittent gas trapping and mobilization. This study is an important step in the multi-scale characterization of foam flow in 3D porous media and provides some answers to certain generally accepted assumptions

L’utilisation de la mousse en récupération assistée du pétrole (Enhanced Oil Recovery, EOR) présente un avantage indéniable par rapport à l’injection du gaz seul pour pallier les problèmes de ségrégation gravitaire et de digitations visqueuses. Son utilisation systématique en ingénierie du réservoir nécessite des connaissances plus approfondies sur son comportement en milieu poreux. La littérature montre deux types d’approches expérimentales basées soit sur des études pétrophysiques effectuées sur des systèmes poreux 3D et basées sur des mesures de pressions intégrées sur l’ensemble du milieu poreux, soit sur des études micro-fluidiques qui permettent une visualisation directe de l’écoulement mais qui sont limitées à des systèmes modèles dans des géométries à 1 ou 2 dimensions. L’objectif de cette thèse est de faire le pont entre ces deux approches. La stratégie proposée consiste à caractériser in situ l’écoulement de la mousse dans des milieux poreux 3D à différentes échelles, en utilisant des techniques complémentaires permettant d’accéder à une large gamme de résolutions spatiale et temporelle. Un environnement instrumenté donnant accès aux mesures pétro-physiques classiques a été développé puis couplé à différentes cellules d’observation conçues spécifiquement pour chaque instrument de caractérisation. Dans un premier temps, un scanner X a été utilisé pour décrire et visualiser les écoulements de la mousse à l’échelle de la carotte. La rhéologie de la mousse à cette échelle a pu être étudiée en fonction des conditions d’injections comme la vitesse interstitielle du gaz et la qualité de mousse. Dans un deuxième temps, la technique de diffusion des neutrons aux petits angles (SANS) a permis de sonder la texture de la mousse en écoulement sur trois ordres de grandeurs en taille. Des informations in situ sur la texture de la mousse en écoulement (taille et densité des bulles et des lamelles) ont pu être mesurées pour différentes qualités de mousse puis en fonction de la distance au point d’injection. Une comparaison avec les caractéristiques géométriques du milieu poreux a également été effectuée. Dans un troisième temps, la micro-tomographie X rapide haute résolution sur Synchrotron a été utilisée pour visualiser la mousse en écoulement à l’échelle du pore. Cette technique a permis de confirmer de visu certaines caractéristiques de la mousse mesurées par SANS et de décrire en sus les effets d’intermittence du piégeage de la mousse. Cette étude constitue une étape importante de la caractérisation multi-échelle de l’écoulement des mousses en milieux poreux 3D et apporte des éléments de réponse à certaines hypothèses admises
Foam has long been used as a mobility control agent in Enhanced Oil Recovery (EOR) processes to enhance sweep efficiency and overcome gravity segregation, viscous fingering and gas channeling, which are gas-related problems when the latter is injected alone in the reservoir. However, the systematic use of foam in reservoir engineering requires more in-depth knowledge of its dynamics in porous media. The literature shows two types of experimental approaches based either on petrophysical studies carried out on 3D porous systems and based on pressure measurements, or on microfluidic studies that allow direct visualization of foam flow but are limited to 1D or 2D model systems. The research investigated in this thesis aims to bridge the gap between these two approaches. The proposed strategy is to characterize in situ the foam flow in 3D porous media with techniques providing a wide range of temporal and spatial resolutions. A coreflood setup giving access to classical petro-physical measurements was developed and then coupled to different observation cells designed specifically for each characterization instrument. First, an X-ray CT scanner was used to describe and visualize the foam flow at the core scale. The rheological behavior of foam on this scale was studied as a function of the injection conditions such as gas velocity and foam quality. Secondly, Small Angle Neutron Scattering (SANS) was used to probe the foam structure in situ during the flow, on a wide length scale, up to three orders of magnitude in size. In situ foam texture (size and density of bubbles and lamellae) was measured for different foam qualities and at different propagation distances from the injection point. A comparison to the geometric characteristics of the porous medium was also realized. Thirdly, High Resolution Fast X-ray Micro-tomography on a Synchrotron was used to visualize the foam flow at the pore scale. This allowed to confirm visually some foam characteristics measured with SANS and to investigate on local intermittent gas trapping and mobilization. This study is an important step in the multi-scale characterization of foam flow in 3D porous media and provides some answers to certain generally accepted assumptions

Title: Self-assembly of polymers with amphiphilic compounds (surfactants) in aqueous solutions Abstract: This PhD Thesis is devoted to the co-assembly in systems containing electrically charged polymers (polyelectrolytes and block copolymers containing polyelectrolyte sequences). I studied the interactions between block copolymers and oppositely charged surfactants in aqueous solutions, and the structure and properties of co-assembled nanoparticles by a combination of several experimental methods. I found that the spontaneous formation, solubility and stability of complex nanoparticles depend not only on the electrostatic attractive forces but also on the hydrophobic effects. In a major part of my Thesis, I studied the interaction of polyelectrolytes with oppositely charged gemini surfactants (containing two charged head-groups interconnected by a short linker and two hydrophobic tails) which is a relatively new topic - much less studied than the co-assembly with conventional single tail surfactants. Better understanding of the formation and properties of complexes containing gemini surfactants and polymers provides knowledge that should lead to novel tailor-made nanoparticles with desired properties for applications in medicine and new technologies (including nano-technologies). We have shown that the...

Abstract The theory and practice of small-angle X-ray scattering (SAXS) dates back to the 1930s and is well described in the classic book of Guinier and Fournet (1955). Small-angle neutron scattering (SANS) came much later. It was not until the early 1970s, when position-sensitive detectors on cold neutron guides became available, that systematic studies began using small-angle neutron scattering, but after this relatively late start the technique rapidly became one of the most popular and productive of all neutron scattering methods. SANS has been applied to a very wide range of problems, including the study of polymer conformations and morphology, the study of biological structures, the characterization of voids and precipitates in alloys, and the study of flux-line lattices in superconducting materials. The technique of ‘contrast variation’ is largely responsible for the success of SANS, especially in the study of soft condensed matter and biological systems. There are over 30 SANS instruments in operation worldwide at both reactor and spallation sources. In spite of an apparent wealth of instruments, the demand for beam time considerably outstrips the time available.

Abstract This chapter on the application of Small-angle X-ray scattering (SAXS) to biological macromolecules can be read independently from other contributions in the series of volumes of Hercules lectures but it does not ignore them. The general considerations on the phenomenon of light scattering by matter are kept to a minimum and focused on the case of particles in solution. The reader is referred to the contributions by Geissler (1994) and Williams (1993) for further developments presented from a different standpoint. The section on data analysis and the formalism of spherical harmonics can be complemented by reading the contribution by Stuhrmann (1994). The question of interactions between particles is barely touched upon since it has been dealt with byTardieu (1994). Finally this chapter is obviously not the place for an exhaustive presentation of the theory of SAXS. Beyond the other contributions already mentioned, the interested reader is referred to the textbooks listed in the bibliography.

Unlike the crystallization of small inorganic molecules, the problem of protein crystallization was first approached by trial and error methods without any theoretical background. A physico-chemical approach was chosen because crystallographers and biochemists needed criteria to rationally select crystallization conditions. In fact, the problem of the production of homogeneous and structurally perfect protein crystals is set the same as the production of high-quality crystals for opto-electronic applications, because, in both cases, the crystal growth mechanisms are the same. Biological macromolecules and small organic molecules follow the same rules concerning crystallization even if each material exhibits specific characteristics. This chapter introduces the fundamentals of crystallization: supersaturation, nucleation, and crystal growth mechanisms. Phase diagrams are presented in Chapter 10. Special attention will be paid to the behaviour of the macromolecules in solution and to the techniques used for their analysis: light scattering (LS), small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and osmotic pressure (OP). Before obtaining any nucleation or growth, it is necessary to dissolve the biological macromolecules under consideration in some good solvent. However, it may immediately be asked whether a good solvent is a solvent in which the material is highly soluble, or in which nucleation is easily controlled, or in which growth is fast, or solvent in which the crystals exhibit the appropriate morphology. In practice, the choice of the solvent often depends on the nature of the material to be dissolved, taking into account the well known rule which says that ‘like dissolves like’. This means that, for dissolution to occur, it is necessary that the solute and the solvent exchange bonds: between an ion and a dipole, a dipole and another dipole, hydrogen bonds, and/or Van der Waals bonds. Therefore, the nature of the bonds depends on both the nature of the solute and the solvent which can be dipolar protic, dipolar aprotic, or completely apolar. Once the material has dissolved, the solution must be supersaturated in order to observe nucleation or growth. The solution is supersaturated when the solute concentration exceeds its solubility. There are several ways to achieve supersaturation.

Block copolymer consists of two or more long blocks with dissimilar chemical structures which are chemically connected. There are different architectures of block copolymers, namely, AB-type diblock, ABA-type triblock, ABC-type triblock, and AmBn radial or star-shaped block copolymers, as shown schematically in Figure 8.1. The majority of block copolymers has long been synthesized by sequential anionic polymerization, which gives rise to narrow molecular weight distribution, although other synthesis methods (e.g., cationic polymerization, atom transfer radical polymerization) have also been developed in the more recent past. Owing to immiscibility between the constituent blocks, block copolymers above a certain threshold molecular weight form microdomains (10–50 nm in size), the structure of which depends primarily on block composition (or block length ratio). The presence of microdomains confers unique mechanical properties to block copolymers. There are many papers that have dealt with the synthesis and physical/mechanical properties of block copolymers, too many to cite them all here. There are monographs describing the synthesis and physical properties of block copolymers (Aggarwal 1970; Burke and Weiss 1973; Hamley 1998; Holden et al. 1996; Hsieh and Quirk 1996; Noshay and McGrath 1977). Figure 8.2 shows schematically four types of equilibrium microdomain structures observed in block copolymers. Referring to Figure 8.2, it is well established (Helfand and Wasserman 1982; Leibler 1980) that in microphase-separated block copolymers, spherical microdomains are observed when the volume fraction f of one of the blocks is less than approximately 0.15, hexagonally packed cylindrical microdomains are observed when the value of f is between approximately 0.15 and 0.44, and lamellar microdomains are observed when the value of f is between approximately 0.44 and 0.50. Some investigators have observed ordered bicontinuous double-diamonds (OBDD) (Thomas et al. 1986; Hasegawa et al. 1987) or bicontinuous gyroids (Hajduk et al. 1994) at a very narrow range of f (say, between approximately 0.35 and 0.40) for certain block copolymers. Figure 8.2 shows only one half of the symmetricity about f = 0.5. Transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) have long been used to investigate the types of microdomain structures in block copolymers.

Small-angle neutron scattering (SANS) experiments from networks were initiated by Benoit and collaborators in the mid-1970s. Currently, SANS is an important major technique used in studying network structure and behavior. Its importance lies in its being a direct method with which observations may be made at the molecular-length scale without the need for a theoretical model for interpreting the data. This feature makes neutron scattering a valuable tool for testing various molecular theories on which current understanding of elastomeric networks is based. The general features of the technique are explained in section 14.1, followed in section 14.2 by a review of relevant experimental work. Section 14.3 then describes different theories of neutron scattering from networks, and compares them with experimental results. The technique of neutron scattering and its application to polymers in the dilute and bulk states, to blends, and to networks are described in several review articles and a book. The reader is referred to this literature for a more comprehensive understanding of the technique and the underlying theory. The neutrons incident on a sample during a typical experiment are from a nuclear reactor. Neutrons leaving the source are first collimated so that they arrive at the sample in the form of plane waves. Figure 14.1 shows such an incident neutron wave on two scattering centers i and j. After interacting with the scattering centers, the neutrons move in various directions. In a neutron scattering experiment, the intensity of the scattered neutron wave is measured as a function of the angle θ shown in the figure, in which the vectors k0 and k are the wave propagation vectors for incident and scattered neutron rays, respectively. In general, the magnitudes of k0 and k differ if there is energy change upon scattering, and in this case the scattering is called inelastic. Inelastic scattering experiments are particularly useful in studying the dynamics of a system, such as relaxation or diffusion.

Phospholipids, which provide valuable model systems for lipid membranes, display a variety of polymorphic phases, depending on their molecular structure and on environmental conditions. High hydrostatic pressure has been used as a physical parameter to study the thermodynamic properties and phase behavior of these systems. High pressure is also a characteristic feature of certain natural membrane environments. In the first part of this article, we review our recent work on the temperature- and pressure-dependent phase behavior of phospholipid systems differing in lipid conformation and headgroup structure. In the second part, we report on the determination of the (T, x, p) phase diagrams of binary phospholipid mixtures. An additional section deals with effects of incorporating ions, small amphiphilic molecules, and steroids into the bilayer on the experimental temperature- and pressure-dependent phase behavior of lipid systems. Finally, we discuss lamellar to nonlamellar thermotropic and barotropic phase transformations, which occur for a number of lipids, such as phosphatidylethanolamines, monoacylglycerides, and lipid mixtures. It has been suggested that nonlamellar lipid structures might play an important role as transient and local intermediates in a number of biochemical processes. High-pressure smallangle x-ray (SAXS) and neutron (SANS) scattering, differential scanning calorimetry (DSC), high-pressure differential thermal analysis (DTA), and p, V, T measurements have been used as experimental methods for the investigation of these systems. Lipid bilayer dispersions, in particular the phosphatidylcholines and phosphatidylethanolamines, are the workhorses for the investigation of biophysical properties of membrane lipids because they constitute the basic structural component of biological membranes. They exhibit a rich lyotropic and thermotropic phase behavior (Cevc & Marsh, 1987; Marsh, 1991; Yeagle, 1992). Most fully hydrated saturated phospholipid bilayers exhibit two principal thermotropic lamellar phase transitions, corresponding to a gel to gel (Lβ′–Pβ′) transition and a gel to liquid-crystalline (Pβ′–Lα) main transition at a temperature Tm. In the fluid-like La phase, the hydrocarbon chains of the lipid bilayers are conformationally disordered, whereas in the gel phases the hydrocarbon chains are more extended and relatively ordered.

Laser light scattering (LLS), small angle x-ray scattering (SAXS) and small angle neutron scattering (SANS) are complementary techniques.1 Together they become unmatched among the physical methods which can investigate the structure and dynamics of polymeric materials over a large range of length and time scales. The unique features of LLS are its ability to determine not only molecular weight, size and internal motions of polymers in solution or of colloids in suspension, but also the size distribution. The applications of LLS to polymer physics and colloid science have been extensive and noteworthy, especially in particle size analysis. In this lecture, three unique examples on (1) Teflon solution characterization,2 (2) coil-to-globule transition3, and (3) supramolecular formation of block copolymers in selective solvents4 are presented.

The structural changes in fibre polymers and dispersion of water in the polymer have been studied at length scales less than 400 Å with contrast variation small angle neutron scattering (SANS) and solid state nuclear magnetic resonance (NMR). The SANS of hydrating paper samples is discussed in different angular regions in terms of a scattering wavenumber vector, q (q = 4π/λ . sin θ/2 where λ is the wavelength of the neutrons and θ is the scattering angle). At low q close to the neutron beam, the Guinier region, voids in the structure are found to disappear as the microfibrils swell with water. The lateral dimensions of the cellulose crystallite are calculated from x-ray diffraction data and there is a good qualitative correlation with relative size of the crystallites and the appearance of short range of order in the SANS in the mid-range of the q studied. The range of the length scale of the SANS feature is slightly larger than the elementary crystallite which is consistent with layers of swollen cellulose and water around the crystallite. In the high q region, the angular region furthermost from the beam, the scattering is discussed in terms of deviation from Porod scattering. According to this interpretation the interface between cellulose and water is not clearly defined and there is an increase in the amount of surface area for water to bind to. These results are consistent with water disrupting the hydrogen bonding in fibre polymers. The NMR spin diffusion experiment monitors the exchange of magnetisation between water and polymer protons. A simplistic model of this transfer process is justified and indicates that water is not uniformly dispersed in the polymer as a function of moisture content.