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PubMed2004

Thermodynamics of temperature-sensitive polyether-modified poly(acrylic acid) microgels.

The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92-PAA-EGDMAmicrogels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127-PAA-EGDMA microgels.

Acrylic Resins↗

PubMed2001

Sugar-Ester Nonionic Microemulsion: Structural Characterization.

Surfactants containing sugar components and fatty acids satisfy the quality standards for food application. The food grade sugar ester in this study is a commercial sucrose monoester of stearic acid (abbreviated SES), the oil phase consists of a 1:1 mixture of n-tetradecane and l-butanol. The originally planned food grade oil, a medium chain triglyceride, is substituted by tetradecane because tetradecane is available as a fully deuterated product, which is necessary for some structural investigations. The investigated system is solid at room temperature, but liquefies and structures into a homogeneous microemulsion when heated to above 37 degrees C. The structural characterization of such microemulsions is the aim of this work. The established methods for this purpose are scattering methods, such as small-angle scattering of X-rays and neutrons and dynamic light scattering. These scattering techniques can be used to obtain valuable information on the size, shape, and internal structure of colloids and complex fluids. We started our investigation with the pseudobinary system SES, tetradecane and l-butanol, varying the SES content. The scattering results show that the sugar ester form inverse globular micelles in the oil phase. The size of these micelles is about 6 nm. While the size is nearly constant in a wide SES concentration regime (5 up to 40% surfactant), the volume or aggregation number increases significantly with SES. This is explained by an increasing replacement of l-butanol molecules by sugar-ester molecules in the micelles formed. Moreover, it can be shown that these micelles strongly overlap. Their center-to-center distance is about 3.8 nm at 40% SES at a micellar diameter of 6 nm. The micellar overlap leads to a highly reduced diffusion of the micelles as was found with dynamic light scattering. When incorporating water in the micellar core, the micelles swell up to about 10 nm and the shape of the aggregates becomes more and more elongated with higher water content. Copyright 2001 Academic Press.

Journal Article↗

PubMedDate not supplied

Quantitative measurement of small-angle gamma ray scattering from water, nylon, and Lucite.

The small-angle (2 degrees-10 degrees) total-differential-scattering cross sections of water, methyl methacrylate, and nylon 6 have been measured at a photon energy of 59.54 keV. An annular target geometry has been used with an acceptance of +/- 0.5 degrees and an experimental uncertainty of approximately 4% achieved. The experimental methods and data corrections necessary to obtain this accuracy are discussed in detail. The results demonstrate clearly the contribution of intermolecular effects to the scattering process and allow critical comparison with calculations using molecular form factors for water obtained from interpolated x-ray diffractometer data; agreement is obtained within the experimental uncertainties. They also support earlier Monte Carlo predictions of scattering in water at angles of the order of 2 degrees which are in disagreement with certain published experimental data.

Caprolactam↗

PubMed2006

Upper critical solution temperature phase behavior, composition fluctuations, and complex formation in poly (vinyl methyl ether)/D2O solutions: small-angle neutron-scattering experiments and wertheim lattice thermodynamic perturbation theory predictions.

Small-angle neutron-scattering measurements are presented for homogeneous mixtures of poly (methyl vinyl ether) (PVME) and deuterium oxide (D(2)O) at high polymer concentrations and for temperatures lower than the equilibrium melting point of the solvent. The experimental data are analyzed to give values for the second-order compositional derivative of the Gibbs energy and the Ornstein-Zernike correlation length. The experimental data together with earlier SANS data determined at higher temperatures cannot be represented with an extended Flory-Huggins (F-H) interaction function depending on composition and temperatures. The experimental data confirm the existence of a narrow upper critical solution temperature (UCST) miscibility gap at high concentrations in agreement with theoretical predictions of the Wertheim lattice thermodynamic perturbation theory (LTPT). The Wertheim LTPT incorporates the influence of hydrogen bonding and predicts not only the existence of bimodal lower critical solution temperature (LCST) phase behavior but also the occurrence of highly unconventional two narrow adjacent UCST miscibility gaps. Finally, the experimental data do not support the existence of a stable molecular complex at the investigated temperatures and compositions. Even at the lowest investigated temperature, the energy required to induce typical Ornstein-Zernike-like concentration fluctuations is smaller than the thermal energy. Also, in this case, the Wertheim LTPT provides a theoretical basis to understand the formation of polymer solvent associations in PVME/water.

Journal Article↗

PubMed2006

Spontaneous formation of vesicles.

his review highlights the relevant issues of spontaneous formation of vesicles. Both the common characteristics and the differences between liposomes and vesicles are given. The basic concept of the molecular packing parameter as a precondition of vesicles formation is discussed in terms of geometrical factors, including the volume and critical length of the amphiphile hydrocarbon chain. According to theoretical considerations, the formation of vesicles occurs in the systems with packing parameters between 1/2 and 1. Using common as well as new methods of vesicle preparation, a variety of structures is described, and their nomenclature is given. With respect to sizes, shapes and inner structures, vesicles structures can be formed as a result of self-organisation of curved bilayers into unilamellar and multilamellar closed soft particles. Small, large and giant uni-, oligo-, or multilamellar vesicles can be distinguished. Techniques for determination of the structure and properties of vesicles are described as visual observations by optical and electron microscopy as well as the scattering techniques, notably dynamic light scattering, small angle X-ray and neutron scattering. Some theoretical aspects are described in short, viz., the scattering and the inverse scattering problem, angular and time dependence of the scattering intensity, the principles of indirect Fourier transformation, and the determination of electron density of the system by deconvolution of p(r) function. Spontaneous formation of vesicles was mainly investigated in catanionic mixtures. A number of references are given in the review.

Journal Article↗

PubMed2005

Strategies for controlling intra- and intermicellar packing in block copolymer solutions: illustrating the flexibility of the self-assembly toolbox.

Block copolymers constitute a class of self-assembling macromolecules that offer remarkable flexibility for controlling nanostructure, both in discrete objects and in bulk materials. Block copolymer micelles may be formed with multiple compartments by judicious choice of ingredients in an ABC triblock copolymer. For example, we have shown that a poly(ethylene oxide-b-styrene-b-fluorinated butadiene) triblock assembles in dilute aqueous solution into large, flat core/shell/corona disks, with the fluorine containing block forming the core. In contrast, the unfluorinated precursor generates large spherical micelles. A numerical analysis suggests that the disk-like motif is characteristic of the so-called superstrong segregation regime, whereby the interfacial tension becomes so large as to overwhelm the conformational entropy of the core blocks. For ABC miktoarm stars comprising polyethylene oxide, polyethylethylene, and polyhexafluoropropylene oxide arms, a much richer variety of micellar structures are observed. Prominent amongst these is a "segmented worm", in which alternating layers (5-7 nm thick) of hydrocarbon and fluorocarbon blocks form disks (6-10 nm in radius) that stack into cylindrical aggregates. The disk radii suggest almost fully stretched blocks, again consistent with the superstrong segregation regime. These structures are rationalized in terms of the constraints imposed by the star architecture, combined with the extremely strong interfacial tensions. In contrast, for lipids, surfactants, and aqueous diblock copolymers, increasing the interfacial tension can induce a transition from spheres to cylinders to flat bilayers; the disk-like motif is not usually seen. Interestingly, in aqueous diblocks both worm-like micelles and vesicles have been well-documented, whereas in "simple" organic systems they have not. We have shown that by suitable choice of block composition and solvent selectivity, the curvature sequence sphere/cylinder/vesicle can also be observed in poly(styrene-b-isoprene) diblocks in dialkyl phthalates. In more concentrated solutions such spherical micelles assemble onto body-centered or face-centered cubic lattices. In some cases a thermoreversible fcc/bcc transition has been noted. We have recently demonstrated that this transition corresponds to a particular aggregation number; higher aggregation numbers, found at lower temperatures, favor the "hard-sphere-like" fcc packing. All of these results are based on a combination of dynamic light scattering, small angle X-ray and neutron scattering, and cryogenic transmission electron microscopy.

Lecture↗

PubMed2000

Structure analysis of regenerated cellulose hydrogels by small-angle and ultra-small-angle x-ray scattering.

Absolute intensities Deltai(q) of small-angle x-ray scattering (SAXS) and ultra-small-angle x-ray scattering (USAXS) were measured in a wide range of scattering vector q from 2x10(-4) to 0.5 A(-1) for transparent (VI-P) and translucent (VI-L) cellulose hydrogels prepared by coagulation and regeneration of viscose in acid solutions with and without acetone, respectively. We obtained the scattering intensities at very small q conveniently by desmearing the combined data measured by SAXS and USAXS. The plot of Deltai(q)q(2) versus log(10) q showed a peak at -2.5<log(10) q<-1.0. By assuming a two-phase model with the high-density phase (phase 1) composed of only cellulose and with the low-density phase (phase 2) composed of cellulose dispersed in water, volume fractions of phase 1 in VI-P and VI-L were determined to be 0.18 and 0.09, respectively, from the mean-square fluctuation of electron density determined as integral of (Deltai(q)q(2)dq). By fitting the observed scattering profile with the theoretical particle scattering functions of spheres, the average diameter of the high-density region including crystallite was determined to be 120 A for VI-P and 80 A for VI-L. Similar analyses were applied also to freeze-dried VI-P. These results were consistent with those obtained by the wide-angle x-ray-diffraction measurement and also with the observation by scanning probe microscopy.

Cellulose↗

PubMed1988

Small-angle intraocular light scatter: a hypothesis concerning its source.

The crystalline lens makes a major contribution to intraocular light scatter, yet the responsible structural features have not been identified. A hypothesis is presented that implicates the fiber architecture of the ocular lens in small-angle scatter, this being the only part contributing to the associated reduction in vision. The main evidence supporting this hypothesis is a calculation of light scatter by a model lattice of fibers simulating the fiber lattice of the ocular lens. The results agree well with the angular dependence of measured intraocular scatter and, for reasonable parametric values, agree also with the measured magnitude.

Cataract↗

PubMed1996

Structural model of the 50S subunit of E. coli ribosomes from solution scattering.

The application of new methods of small-angle scattering data interpretation to a contrast variation study of the 50S ribosomal subunit of Escherichia coli in solution is described. The X-ray data from contrast variation with sucrose are analyzed in terms of the basic scattering curves from the volume inaccessible to sucrose and from the regions inside this volume occupied mainly by RNA and by proteins. From these curves models of the shape of the 50S and its RNA-rich core are evaluated and positioned so that their difference produces a scattering curve which is in good agreement with the scattering from the protein moiety. Based on this preliminary model, the X-ray and neutron contrast variation data of the 50S subunit in aqueous solutions are interpreted in the frame of the advanced two-phase model described by the shapes of the 50S subunit and its RNA-rich core taking into account density fluctuations inside the RNA and the protein moiety. The shape of the envelope of the 50S subunit and of the RNA-rich core are evaluated with a resolution of about 40 A. The shape of the envelope is in good agreement with the models of the 50S subunit obtained from electron microscopy on isolated particles. The shape of the RNA-rich core correlates well with the model of the entire particle determined by the image reconstruction from ordered sheets indicating that the latter model which is based on the subjective contouring of density maps is heavily biased towards the RNA.

Escherichia coli↗

PubMedDate not supplied

Investigation of the nanofibrillar morphology in silk fibers by small angle X-ray scattering and atomic force microscopy.

Small angle X-ray scattering (SAXS) and atomic force microscopy (AFM) measurements have been shown to be consistent with the presence of nanofibrils in the cocoon silk of Bombyx mori and the dragline silk of Nephila clavipes. The transverse dimensions and correlation lengths range from >> 59 to 220 nm and in the axial direction from >> 80 to 230 nm. Also, the two-dimensional Fourier transforms of the height profiles of AFM topographic images of interior surfaces of B. mori follow a power law approximately the same as that for the Porod region of the SAXS data. In this manner, the AFM can be used to help remove ambiguity about the scatterers responsible for SAXS patterns.

Animals↗

PubMed1999

Compactness of the denatured state of a fast-folding protein measured by submillisecond small-angle x-ray scattering.

Time-resolved small-angle x-ray scattering was used to measure the radius of gyration of cytochrome c after initiation of folding by a pH jump. Submillisecond time resolution was obtained with a microfabricated diffusional mixer and synchrotron radiation. The results show that the protein first collapses to compact denatured structures before folding very fast to the native state.

Crystallography, X-Ray↗

PubMedDate not supplied

[A study of the complex-formation of phenylalanyl-tRNA-synthetase from Escherichia coli using the tRNA-Phe method of small-angle x-ray scattering].

The method of small-angle X-ray scattering was employed to analyse the equilibrium enzyme-substrate complexes in solution. A new approach of analysis of the experimental data was developed. This type of analysis provides the determination of dissociation constants and structural parameters of enzyme-substrate complexes. The radius of gyration (Rg) and dimensions of half-axis of the equivalent prolonged ellipsoid (a, b) of E. coliphenylalanyl-tRNA synthetase and its complexes with one or two tRNA(Phe) molecules have been determined. The values of these parameters speak in favour of structural rearrangements due to the interaction of the enzyme with tRNA(Phe). The thermodynamic characteristics of phenylalanyl-tRNA synthetase complexes with tRNA(Phe) testify to the negative cooperativity in binding of two tRNA molecules with the enzyme.

Amino Acyl-tRNA Synthetases↗

PubMed1984

Chromatin superstructure: synchrotron radiation X-ray scattering study on solutions and gels.

X-ray small angle scattering patterns of solutions and gels of native chicken erythrocyte chromatin and chromatin depleted of H5 histones have been measured under several ionic conditions using synchrotron radiation. Features of the patterns are interpreted as resulting from a superstructure with an outer diameter of about 300A which is already present in uncondensed nucleofilaments. This superstructure which is shown to be maintained by the H5 histones also explains the rapid condensation of the nucleofilaments in higher ionic strengths.

Animals↗

PubMed2006

Supramolecular aggregates of amphiphilic gadolinium complexes as blood pool MRI/MRA contrast agents: physicochemical characterization.

In this paper, we present the development of a new potential blood pool contrast agent for magnetic resonance imaging applications (MRA/MRI) based on gadolinium complexes containing amphiphilic supramolecular aggregates. A novel amphiphilic unimer, containing the DTPAGlu chelating agent covalently bound to two C18 alkylic chains, has been synthesized. DTPAGlu is a well-known chelating agent for a wide number of ions such as the paramagnetic metal ion Gd3+ used as contrast agent in MRA/MRI. The wide aggregation behavior of this surfactant, as free base or as gadolinium complex, has been studied and compared by means of dynamic light scattering, small-angle neutron scattering and cryogenic transmission electron microscopy techniques. Near neutral pH in both cases, the dominant aggregates are micelles. The high negative actual charge of the surfactant headgroup causes a strong headgroups repulsion, promoting the formation of large and high curvature aggregates. By decreasing pH and less markedly increasing the ionic strength, we observe a micelle-to-vesicle transition driven by a decreased electrostatic repulsion. A straightforward switch between different aggregation states can be particularly useful in the development of pH-responsive MRA/MRI contrast agents.

Chemical Phenomena↗

PubMed1983

Nonspecific binding of lac repressor to DNA. II. A small-angle neutron-scattering study.

Complexes between lac repressor and DNA fragments from mononucleosomes have been studied by small-angle neutron scattering. Both the radius of gyration and the molecular weight of the complexes were measured, and the experimental results were interpreted according to a model with two types of complex (M. Charlier and J.-C. Maurizot, Biophys. Chem. 18 (1983) 303), and a statistical distribution of repressor on the DNA fragments. Good agreement between the model calculations and the experimental results was obtained. We concluded that there was an absence of strong cooperativity and of network formation between the complexes. The second type of binding, which does not induce any spectroscopic change, is marked by an increase in molecular weight of the complexes. Kinetic measurements were also made, which allowed the determination of the lifetime of the nonspecific DNA-repressor complexes.

DNA↗

PubMed2006

Structural analysis of flexible proteins in solution by small angle X-ray scattering combined with crystallography.

In the last few years, SAXS of biological materials has been rapidly evolving and promises to move structural analysis to a new level. Recent innovations in SAXS data analysis allow ab initio shape predictions of proteins in solution. Furthermore, experimental scattering data can be compared to calculated scattering curves from the growing data base of solved structures and also identify aggregation and unfolded proteins. Combining SAXS results with atomic resolution structures enables detailed characterizations in solution of mass, radius, conformations, assembly, and shape changes associated with protein folding and functions. SAXS can efficiently reveal the spatial organization of protein domains, including domains missing from or disordered in known crystal structures, and establish cofactor or substrate-induced conformational changes. For flexible domains or unstructured regions that are not amenable for study by many other structural techniques, SAXS provides a unique technology. Here, we present SAXS shape predictions for PCNA that accurately predict a trimeric ring assembly and for a full-length DNA repair glycosylase with a large unstructured region. These new results in combination with illustrative published data show how SAXS combined with high resolution crystal structures efficiently establishes architectures, assemblies, conformations, and unstructured regions for proteins and protein complexes in solution.

Crystallography, X-Ray↗

PubMed2002

Structural modifications in the swelling of inhomogeneous microgels by light and neutron scattering.

Small-angle neutron scattering and dynamic light scattering have been used to study the thermodynamics of swelling and the associated structure modifications of highly cross-linked temperature-sensitive poly (N-isopropylacrylamide) [poly(NIPAM)] microgels in D2O. A particle core-shell model is proposed, with the core containing most of the cross-linker molecules. The Flory-Rehner theory, with the inclusion of a concentration dependent Flory solvency parameter, successfully describes the experimental swelling, despite the inhomogeneous character of the particles. Interestingly, the shell evolution with temperature controls the whole particle swelling, exerting an external pressure over the core, which in turn influences its size during the swelling process. Scaling laws for the correlation lengths were found with respect to temperature and polymer concentration. Finally, it has been encountered that for the collapsed microgel states, the particle surface seems to have a fractal character.

Journal Article↗

PubMed1999

Long-range density fluctuations in a symmetric diblock copolymer.

We present an experimental piece of evidence of the excess small-angle-scattering in a nearly symmetric diblock copolymer. This phenomenon is observed only in the two-phase coexistence region of the ordered and disordered phases near the order-disorder transition temperature, using the ultra-small-angle x-ray scattering method. This excess scattering is found to be caused by the difference of the density between the ordered and disordered phases in the coexistence region.

Journal Article↗