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PubMed2004

Partitioning of macrocyclic compounds in a cationic and an anionic micellar solution: a small-angle neutron scattering study.

Following a previous investigation on partitioning of some macrocycle compounds in sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) aqueous solutions and their effect on the micellar structure, a small-angle neutron scattering (SANS) study has been performed at fixed surfactant content (0.20 mol/L) and varying macrocycle concentrations from 0.20 up to 1.0 mol/L. Conductivity measurements have been also performed in order to evaluate the effect of the presence of macrocycles on the critical micellar concentration (cmc) of the two surfactants. SANS experimental data were fitted successfully by means of a core-plus-shell monodisperse prolate ellipsoid model. It has been found that 1,4,7,10,13,16-esaoxacyclooctadecane (18C6) and 4,7,13,16-tetraoxa-1,10-diazacyclooctadecane (22) do not interact with DTAB micelles whereas their sodium complexes interact with SDS aggregates and partially localize, as a consequence of electrostatic interaction, on the micellar surface or in the Stern layer. 2,5,8,11,14,17-Hexaoxabicyclo[16.4.0] dicosane (B18C6), as a consequence of the increased hydrophobic character with respect to 18C6, interacts with DTAB hydrocarbon chains and partially localizes in the inner part of micelles. This finding has been successfully used to justify the higher amount of B18C6 compared to the 18C6 one found in the SDS micellar phase. The substituted crown ether has been found localized both on the micelle surface via complex formation and in the inner part of micelles as a consequence of the increased hydrophobic character. For all systems, the aggregate size primarily decreases with the amount of macrocycle in the micellar phase. The interpretation of cmc trends as a function ofmacrocycle concentration gives information on its distribution between micellar and aqueous phases that is in line with SANS results.

Journal Article↗

PubMed1994

Interparticle interactions and structure in nonideal solutions of human serum albumin studied by small-angle neutron scattering and Monte Carlo simulation.

Moderately or highly concentrated nonideal solutions of macromolecules are very important systems e.g. in biology and in many technical processes. In this work we have used the small-angle neutron scattering technique (SANS) to study the interactions and interparticle structure in solutions of human serum albumin (HSA) up to a concentration of 0.26 g/cm3 in 1.08 M NaCl. In order to obtain a model for the interactions we have combined the SANS data with results obtained by Monte Carlo simulations where we calculate the structure factor S(Q) and the pair correlation function g(r). The advantage of using the Monte Carlo method is that completely general models for the particle shape and the interactions can be considered. It is found that the SANS data can be explained by a model where the shape of the HSA molecule is approximated by an ellipsoid of revolution with semiaxes a = 6.8 nm and b = c = 1.9 nm. The interaction potential between the HSA molecules consists of two parts: (a) the molecules are hard and impenetrable and (b) the molecules are surrounded by a spherically symmetric repulsive potential which can be expressed by an empirical formula containing the HSA concentration and the centre-centre distances as the only variables.

Humans↗

PubMed2002

SANS study on the effect of lanthanide ions and charged lipids on the morphology of phospholipid mixtures. Small-angle neutron scattering.

The structural phase behavior of phospholipid mixtures consisting of short-chain (dihexanoyl phosphatidylcholine) and long-chain lipids (dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol), with and without lanthanide ions was investigated by small-angle neutron scattering (SANS). SANS profiles were obtained from 10 degrees C to 55 degrees C using lipid concentrations ranging from 0.0025 g/ml to 0.25 g/ml. The results reveal a wealth of distinct morphologies, including lamellae, multi-lamellar vesicles, unilamellar vesicles, and bicellar disks.

Dimyristoylphosphatidylcholine↗

PubMed1986

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll.

Micelles of hydrated chlorophyll a (P740), bacteriochlorophyll a (P865), bacteriochlorophyll c (P750), and pheophytin a prepared in organic media have been studied by small-angle neutron scattering to determine their shape, size, and mass per unit length. All of the micelles are hollow cylinders of well-defined size. The P740 and P750 cylinders are essentially monolayers of macrocycles crosslinked by water, probably in an arrangement similar to that of crystals of chlorophyll derivatives. The P865 micelle is more nearly a bilayer of macrocycles. We show that the curvature necessary to form cylinders probably results from intrinsic curvature of the five-coordinated chlorophyll macrocycle. Studies of P740 micelle formation and the disaggregating effects of another nucleophile (pyridine) are described. As the P750 micelles are nearly identical in size and optical spectra to the rod-shaped structures observed in chlorosomes, and the P865 micelles have optical properties very similar to the in vivo properties of the long-wavelength antenna of purple photosynthetic bacteria, we propose that features of the hydrated cylindrical micelles of these chlorophylls provide good models for antenna chlorophyll in photosynthetic bacteria.

Journal Article↗

PubMed2006

Protein kinase A targeting and activation as seen by small-angle solution scattering.

We have studied the solution structures of the multi-functional protein kinase A using small-angle X-ray and neutron scattering and have found a remarkable structural diversity in the different isoforms of this multi-subunit enzyme, in spite of its having high sequence homology and a common domain organization within its sequences. The available high-resolution crystal and NMR structural data for the protein kinase A components have aided in the interpretation of the solution scattering data and enabled us to develop models that bring insights into protein kinase A activation and targeting mechanisms, such as the opening and closing of the catalytic cleft to facilitate substrate binding or inhibition, respectively, and the role of sequence segments that join functional domains in the R subunit in providing a structurally flexible scaffold for interactions with the C subunit and A kinase-anchoring proteins (AKAPs).

Cyclic AMP-Dependent Protein Kinase RIIalpha Subun↗

PubMedDate not supplied

Collagen fiber orientation as quantified by small angle light scattering in wounds treated with transforming growth factor-beta2 and its neutalizing antibody.

The purpose of this study was determine quantitative differences in collagen fiber orientation in a wound healing model in the presence of transforming growth factor-beta2 and anti-transforming growth factor-beta2,3 antibody. Full-thickness wounds were made in the paravertebral area of two young pigs. Wounds were treated once, topically, with either transforming growth factor-beta2 or anti-transforming growth factor-beta2 antibody, or with methylcellulose gel. Control wounds were left untreated. Tissue biopsies were obtained from each wound on days 7, 14 and 46 post wounding. Tissue sections were stained with hematoxylin and eosin, and collagen fiber preferred orientation was quantified using small angle light scattering. Our results indicated that wounds treated with transforming growth factor-beta2 and anti-transforming growth factor-beta2,3 antibody had a significantly higher degree of orientation of collagen fibers than normal unwounded skin on days 7, 14 and 46 (p < 0.001). Transforming growth factor-beta2- treated wounds had a higher degree of orientation of collagen fibers than control wounds on days 7 and 14 (p < 0.001), and control wounds displayed a higher degree of orientation than wounds treated with anti-transforming growth factor-beta2,3 and normal unwounded skin at all time points (p < 0.001). These results suggest that differences in the dermal collagen degree of orientation correlate with scarring, and show that small angle light scattering can be used quantitatively to assess differences in the collagen fiber architecture of dermal wounds.

Administration, Topical↗

PubMed2004

A small-angle neutron scattering study of adsorbed poly(ethylene oxide) on Laponite.

The adsorption of poly(ethylene oxide) (PEO) on synthetic anisotropic clay particles (Laponite) has been investigated as a function of the molecular weight. Contrast variation small-angle neutron scattering (SANS) measurements were used to characterize the distribution and adsorbed amount of polymer on the particles. These experiments show not only that polymer is present on the face of the clay particle but that it also extends or "wraps" over the edges. The edge layer was thicker than the face layer for all the molecular weights studied. The polymer layers are unusually thin, with a thickness and adsorbed amount that show little variation with molecular weight.

Journal Article↗

PubMed2005

Local structure of a phase-separating binary mixture in a mesoporous glass matrix studied by small-angle neutron scattering.

The mesoscopic structure of the binary system isobutyric acid + heavy water (D(2)O) confined in a porous glass (controlled-pore silica glass, mean pore width ca. 10 nm) was studied by small-angle neutron scattering at off-critical compositions in a temperature range above and below the upper critical solution point. The scattering data were analyzed in terms of a structure factor model similar to that proposed by Formisano and Teixeira [Eur. Phys. J. E 1, 1 (2000)], but allowing for both Ornstein-Zernike-type composition fluctuations and domainlike structures in the microphase-separated state of the pore liquid. The results indicate that the phase separation in the pores is shifted by ca. 10 K and spread out in temperature. Microphase separation is pictured as a transition from partial segregation at high temperature, due to the strong preferential adsorption of water at the pore wall, to a tube or capsule configuration of the two phases at low temperatures, depending on the overall composition of the pore liquid. Results for samples in which the composition of the pore liquid can vary with temperature due to equilibration with extra-pore liquid are consistent with this picture.

Journal Article↗

PubMed1990

The H(+)-induced dissociation of human plasma alpha 2-macroglobulin. An investigation using small-angle neutron scattering and test of trypsin binding activity.

The dissociation of the tetrameric alpha 2-macroglobulin molecule into two half-molecular fragments, which occurs at pH less than 4.5, has been investigated using the small-angle neutron scattering method, and test of trypsin binding activity. Best fit with the relative forward scattering of neutrons is obtained for a model where the dissociation of the protein is driven by the uptake of H+ on altogether four acid-base groups, one per monomeric subunit of alpha 2-macroglobulin. These groups are not (or only slightly) accessible in the native tetramer, but become exposed to the solvent after dissociation of the protein. The H(+)-binding constant obtained for these groups, after dissociation of the protein, log K1 in the range 4.2-4.5, suggests that they are most probably carboxylate groups. From the about 10% increase in the radius of gyration, which occurs when lowering the pH from 4.5 to 2.0, we can conclude that the dissociation is associated with a change in structure of the protein. Tests of trypsin binding show that there is also an irreversible loss in trypsin binding activity, which is directly related to the fraction of dissociated protein. Thus, at pH less than 4.5, there is a transition of alpha 2-macroglobulin which results simultaneously in dissociation, disorganisation of the conformation of the subunits and loss in activity.

Binding Sites↗

PubMed2005

A small-angle neutron scattering study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles.

Mixed micelle of protonated or deuterated sodium dodecyl sulfate (SDS and SDSd25, respectively) and poly(propylene oxide) methacrylate (PPOMA) are studied by small-angle neutron scattering (SANS). In all the cases the scattering curves exhibit a peak whose position changes with the composition of the system. The main parameters which characterize mixed micelles, i.e., aggregation numbers of SDS and PPOMA, geometrical dimensions of the micelles and degree of ionisation are evaluated from the analysis of the SANS curves. The position q(max) of the correlation peak can be related to the average aggregation numbers of SDS-PPOMA and SDSd25-PPOMA mixed micelles. It is found that the aggregation number of SDS decreases upon increasing the weight ratio PPOMA/SDS (or SDSd25). The isotopic combination, which uses the "contrast effect" between the two micellar systems, has allowed us to determine the mixed micelle composition. Finally, the SANS curves were adjusted using the RMSA for the structure factor S(q) of charged spherical particles and the form factor P(q) of spherical core-shell particle. This analysis confirms the particular core-shell structure of the SDS-PPOMA mixed micelle, i.e., a SDS "core" micelle surrounded by the shell formed by PPOMA macromonomers. The structural parameters of mixed micelles obtained from the analysis of the SANS data are in good agreement with those determined previously by conductimetry and fluorescence studies.

Journal Article↗

PubMed1997

Nucleotide cofactor-dependent structural change of Xenopus laevis Rad51 protein filament detected by small-angle neutron scattering measurements in solution.

Rad51 protein, a eukaryotic homologue of RecA protein, forms a filamentous complex with DNA and catalyzes homologous recombination. We have analyzed the structure of Xenopus Rad51 protein (XRad51.1) in solution by small-angle neutron scattering (SANS). The measurements showed that XRad51.1 forms a helical filament independently of DNA. The sizes of the cross-sectional and helical pitch of the filament could be determined, respectively, from a Guinier plot and the position of the subsidiary maximum of SANS data. We observed that the helical structure is modified by nucleotide binding as in the case of RecA. Upon ATP binding under high-salt conditions (600 mM NaCl), the helical pitch of XRad51.1 filament was increased from 8 to 10 nm and the cross-sectional diameter decreased from 7 to 6 nm. The pitch sizes of XRad51.1 are similar to, though slightly larger than, those of RecA filament under corresponding conditions. A similar helical pitch size was observed by electron microscopy for budding yeast Rad51 [Ogawa, T., et al. (1993) Science 259, 1896-1899]. In contrast to the RecA filament, the structure of XRad51.1 filament with ADP is not significantly different from that with ATP. Thus, the hydrolysis of ATP to ADP does not modify the helical filament of XRad51.1. Together with our recent observation that ADP does not weaken the XRad51.1/DNA interaction, the effect of ATP hydrolysis on XRad51.1 nucleofilament should be very different from that on RecA.

Adenine Nucleotides↗

PubMed2005

Small angle neutron scattering study of demixing in micellar solutions containing CTAC and a partially fluorinated cationic surfactant.

Demixing of fluorocarbon and hydrocarbon surfactants to form coexisting fluorocarbon-rich and hydrocarbon-rich micelles has been studied by small angle neutron scattering in aqueous solution, using an equimolar mixture of cetyltrimethylammonium chloride and the partially fluorinated cationic surfactant N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride, with a deuterated pyridinium headgroup. Measurements have been performed under varying experimental conditions: in both pure aqueous solutions and with salt (0.10 M NaCl), at several contrasts for neutrons obtained by varying the H(2)O/D(2)O ratio, mainly at 25 degrees C but also at 60 degrees C to promote mixing of the surfactants. The experiments show that a substantial residual scattering is retained at the solvent composition where the average scattering length density of mixed micelles would match that of the solvent. It is moreover observed that, in solutions without added salt, a prominent correlation peak observed in 100% D(2)O disappears at the match point. These observations are in accordance with a very broad composition distribution, possibly composed of two populations of mixed micelles of similar sizes but different compositions, but would not result from micelles with merely a highly inhomogeneous internal structure. Increasing the temperature from 25 to 60 degrees C reduces substantially the scattered intensity at zero angle at the match point, as expected for a less broad population of mixed micelles. In the numerical analysis, the scattering data for scattering vector q > or = 0.02 A(-1) were analyzed by the indirect Fourier transform method to give the scattering at zero angle. From these data, the average micelle aggregation number was obtained as 76 at 25 degrees C and 54 at 60 degrees C. The contrast variation results for the intensity at zero angle give a measure of the width of the micelle distribution, which is obtained as sigma = 0.33 at the lower temperature and sigma = 0.20 at 60 degrees C. The result at the low temperature is compatible with the formation of two populations that are polydisperse (sigma = 0.07) and centered around 18 and 82%; other broad distributions cannot be excluded.

Journal Article↗

PubMed2006

Colloidal dispersions of octadecyl grafted silica spheres in toluene: a global analysis of small angle neutron scattering contrast variation and concentration dependence measurements.

In this paper we report measurements of the form factor and the structure factor of a sterically stabilized colloidal dispersion consisting of silica spheres coated with octadecane in toluene by small angle neutron scattering (SANS). The phase diagram of this system shows the liquid-liquid coexistence line and also a jamming transition at higher concentrations, where the jamming line intersects the coexistence line roughly at the critical point. We have performed SANS experiments at a temperature well above the transition temperature and at various volume fractions phi, spanning from the very dilute regime (phi=0.2%) to the critical concentration (phi=16%) and the highly viscous regime (phi=39.2%). Except for the very dilute regime, we observe a structure factor S(q) in all other cases. We fitted our data over the whole concentration regime using a global fitting routine with a core-shell model for the form factor P(q), taking into account the structure factor, which we describe with the Robertus model for an adhesive polydisperse core-shell particle. At a volume fraction of phi=5% a SANS contrast variation experiment has been performed. From that the product of the volume of the shell and the amount of solvent within the corona of our core-shell particle could be determined. At the most probable shell thickness of 2.3 nm a solvent content of about 50% within the corona was found. Moreover we could conclude that the core is not interpenetrated by solvent molecules. From the contrast variation experiment followed that the structure factor at zero average contrast exhibits a strong q dependence, which is an effect of an inhomogeneous particle in combination with a size distribution.

Journal Article↗

PubMed2004

Structural features of molecular-colloidal solutions of C60 fullerenes in water by small-angle neutron scattering.

Highly stable and reproducible molecular-colloidal water solutions of C60 fullerenes (FWS) obtained by transferring fullerenes from an organic solution into an aqueous phase with the help of ultrasonic treatment are investigated by means of small-angle neutron scattering (SANS). A polydispersity in the size of detected particles up to 84 nm is revealed. These particles are slightly anisotropic and have a characteristic size of approximately 70 nm. Along with it, there are some indications that a significant part of fullerenes composes particles with the size of the order of 1 nm. The contrast variation based on mixtures of light and heavy water shows that the mean scattering length density of the particles is close to that of the packed fullerene associates as well as that the characteristic size of possible fluctuations of the scattering length density within the particles does not exceed 2 nm. A smooth surface resulting in the Porod law for the scattering is detected. A number of models discussed in the literature are considered with respect to the SANS data.

Journal Article↗

PubMed2006

Spatially resolved small-angle neutron scattering in the 1-2 plane: a study of shear-induced phase-separating wormlike micelles.

The shear-induced microstructure of a shear-induced phase separating (SIPS) wormlike micellar solution is measured by combining small-angle neutron scattering with a shear cell capable of gap-resolved measurement in the 1-2 (velocity-velocity gradient) plane. Quantitative results show evidence for shear-induced microphase separation accompanied by shear banding. The results suggest both concentration fluctuations and gradients in segmental alignment occurring during SIPS.

Journal Article↗

PubMed1991

Quantification of vertical-fiber defect in cattle hide by small-angle light scattering.

Vertical-fiber defect (VFD), an abnormal arrangement of collagen fibers in hides of certain cattle breeds, is still not fully understood. Prior work has been limited to subjective histological examinations from hide biopsies. A device using small angle light scattering (SALS) was used to quantify the collagen fiber orientation of sections taken from hide biopsies. Sections were chosen from the Hereford cattle breed and classified by conventional observation as belonging to either the normal, intermediate, or vertical phenotypes. The vertical fibers occur only in the upper reticular dermis, with the fibers in the lower reticular dermis lying parallel to the plane of the hide in all phenotypes. By SALS the vertical phenotype was found to be significantly different from the normal phenotype, whilst the intermediate phenotype was found to be structurally indistinguishable from the vertical one. No evidence was found for the existence of other phenotypes.

Animals↗

PubMed1997

Structure and thermodynamics of nonideal solutions of colloidal particles: investigation of salt-free solutions of human serum albumin by using small-angle neutron scattering and Monte Carlo simulation.

The understanding of the structural and thermodynamic properties of moderately or highly concentrated solutions is fundamental, e.g., in medicine and biology and also in many technical processes. In this work, we have used the small-angle neutron scattering method (SANS), in combination with Monte Carlo simulation, to study salt-free solutions of human serum albumin (HSA) in the concentration range up to 0.26 g ml-1. The model calculations of the theoretical SANS intensities are quite general, thus avoiding the approximation that the relative positions and orientations of the particles are independent of each other. The computation of the theoretical intensities also includes the calculation of a "thermodynamic' intensity scattered at zero angle, which is obtained via the nonideal part of the chemical potential. The latter quantity is obtained by applying the test particle method during the Monte Carlo simulations. It is found that the SANS data can be explained by a model where the HSA molecules behave as hard ellipsoids of revolution with semiaxes a = 6.8 nm, b = c = 1.9 nm. In addition to the hard core interaction, the particles are also surrounded by a soft, repulsive rectangular-shaped potential which is spherically oriented around the particles. The combination of SANS and statistical thermodynamics also allows a determination of the nonideal part of the chemical potential and the activity coefficient of HSA. As expected the activity coefficient deviates strongly from the value one (several powers of ten) already at fairly low concentrations: the effects are comparable to, or even larger than, for instance hydrophobic or van der Waals interaction.

Chemical Phenomena↗

PubMed1986

Small-angle neutron scattering study of the ternary complex formed between bacterial elongation factor Tu, guanosine 5'-triphosphate, and valyl-tRNAVal.

The formation of the ternary complex between bacterial elongation factor Tu, GTP, and valyl-tRNAVal has been studied by small-angle neutron scattering. Titrations of the protein with amino-acyl-tRNA solutions in both H2O and 70% D2O confirm the expected stoichiometry. The molecular weight obtained for the protein alone is significantly higher than expected and can be explained by postulating a monomer-dimer equilibrium. The titration data are then internally consistent with a dissociation of the dimer on ternary complex formation. The radius of gyration for the ternary complex and the calculation of the separation of the centers of mass of the protein and tRNA components suggest a compact model for the ternary complex.

Escherichia coli↗