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Results for “Scattering, Small Angle”

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At least 109 records · Page 6Linked to original sources

PubMed2002

High-pressure effects on horse heart metmyoglobin studied by small-angle neutron scattering.

Small-angle neutron scattering experiments were performed on horse azidometmyoglobin (MbN3) at pressures up to 300 MPa. Other spectroscopic techniques have shown that a reorganization of the secondary structure and of the active site occur in this pressure range. The present measurements, performed using various concentrations of MbN3, show that the compactness of the protein is not altered as the value of its radius of gyration remains constant up to 300 MPa. The value of the second virial coefficient of the protein solution indicates that the interactions between the molecules are always strongly repulsive even if their magnitude decreases with increasing pressure. Taking advantage of the pressure-induced contrast variation, these experiments allow the partial specific volume of MbN3 to be determined as a function of pressure. Its value decreases by 5.4% between atmospheric pressure and 300 MPa. In this pressure range the isothermal compressibility of hydrated MbN3 is found to be almost constant. Its value is (1.6 +/- 0.1) 10-4 MPa-1.

Animals↗

PubMed1998

Determination of the conformations of cAMP receptor protein and its T127L,S128A mutant with and without cAMP from small angle neutron scattering measurements.

Small angle neutron scattering (SANS) measurements were performed on solutions of cAMP receptor protein (CRP) and on solutions of the T127L,S128A double mutant of CRP (CRP*) in D2O K3PO4 buffer containing 0.5 M KCl, in the absence and presence of 3',5' cyclic adenosine monophosphate (cAMP). Energy-minimized structures of the CRP were calculated by minimization of the x-ray crystallographic structure of CRP in either the exclusively "closed" form where the alpha-helices of the carboxyl-terminal domain are folded close to the amino-terminal domain and in the exclusively "open" form where the alpha-helices of the carboxyl-terminal domain are folded away from the amino-terminal domain. Neutron scattering models show that the CRP SANS data follow closely the data curve predicted for unligated CRP in the open form, whereas the cAMP-ligated data are more in agreement with the data predicted for the minimized cAMP-ligated CRP structure in the closed form. Thus, it appears that CRP undergoes a conformational change from the open form to the closed form in solution upon ligation with cAMP. The SANS data from the CRP* and cAMP-ligated CRP* are coincidental, which implies that there is very little structural difference between the two species of CRP*. This is in agreement with in vivo results, which show that whereas CRP activates transcription in the cell only in the presence of cAMP, CRP* activates transcription in the absence of cAMP, implying that CRP* is already in the correct conformation for the activation of transcription.

Crystallography, X-Ray↗

PubMed2002

Anisotropy of the structure factor of magnetic fluids under a field probed by small-angle neutron scattering.

Small-angle neutron scattering is used to measure the two-dimensional diffraction pattern of a monophasic magnetic colloid, under an applied magnetic field. This dipolar system presents in zero field a fluidlike structure. It is well characterized by an interaction parameter K(0)(T) proportional to the second virial coefficient, which is here positive, expressing a repulsion of characteristic length kappa-10. Under the field a strong anisotropy is observed at the lowest q vectors. The length kappa-10 remains isotropic, but the interaction parameter K(T) becomes anisotropic due to the long-range dipolar interaction. However, the system remains stable, the interaction being repulsive in all directions. Thus we do not observe any chaining of the nanoparticles under magnetic field. On the contrary, the revealed structure of our anisotropic colloid is a lowering of the concentration fluctuations along the field while the fluidlike structure, observed without field, is roughly preserved perpendicularly to the field. It expresses a strong anisotropy of the Brownian motion of the nanoparticles in the solution under applied field.

Journal Article↗

PubMed2003

Shear- and magnetic-field-induced ordering in magnetic nanoparticle dispersion from small-angle neutron scattering.

Small-angle neutron scattering experiments have been performed to investigate orientational ordering of a dispersion of rod-shaped ferromagnetic nanoparticles under the influence of shear flow and static magnetic field. In this experiment, the flow and flow gradient directions are perpendicular to the direction of the applied magnetic field. The scattering intensity is isotropic in zero-shear-rate or zero-applied-field conditions, indicating that the particles are randomly oriented. Anisotropic scattering is observed both in a shear flow and in a static magnetic field, showing that both flow and field induce orientational order in the dispersion. The anisotropy increases with the increase of field and with the increase of shear rate. Three states of order have been observed with the application of both shear flow and magnetic field. At low shear rates, the particles are aligned in the field direction. When increasing shear rate is applied, the particles revert to random orientations at a characteristic shear rate that depends on the strength of the applied magnetic field. Above the characteristic shear rate, the particles align along the flow direction. The experimental results agree qualitatively with the predictions of a mean field model.

Journal Article↗

PubMed1991

Kappa-casein micelles: structure, interaction and gelling studied by small-angle neutron scattering.

Small-angle neutron scattering (SANS) measurements on dilute and concentrated dispersions of kappa-casein micelles in a buffer at pH = 6.7 were made using the D11 diffractometer in Grenoble. Results indicate that the micelles have a dense core with a fluffy outer layer. This outer layer appears to give rise to a steeply repulsive interaction on contact. In fact, the hard-sphere model best fits the measured scattering intensities. Adding chymosin to the dispersion initiated a fractal flocculation of the micelles and consecutively a coalescence of the micelles. This unexpected second process resembled that of spinodal demixing. The dispersion phase thus separates into a water and a protein phase on a time scale of hours. The observed phenomona contribute to the understanding of the cheese-making process.

Caseins↗

PubMed1998

Coherent small-angle scattering on a bending-magnet beamline at the ESRF.

A coherent X-ray beam is obtained from the D2AM bending-magnet beamline (BM2) of the European Synchrotron Radiation Facility. As this line has permanent convergent optics, monochromated by an Si(111) double monochromator, coherence conditions are satisfied by selecting a part of the beam close to the focal point. Low intensity (10(6) X-rays s(-1)) is partially compensated by a high degree of coherence and by the use of a high-resolution direct-illumination CCD area detector. The stability of the small-angle set-up makes quantitative analysis possible. The calculated and measured degrees of coherence are compared. The distributions of speckle intensities are explained by a beam composed of two-thirds coherent and one-third incoherent parts. This incoherent component is caused by Kapton windows, which will be removed in future experiments.

Journal Article↗

PubMed1978

X-ray and neutron small-angle scattering studies of the complex between protein S1 and the 30-S ribosomal subunit.

X-ray neutron solution scattering experiments have been done to investigate the influence of the binding of ribosomal protein S1 on the conformation of the 30-S ribosomal subunit of Escherichia coli. The following conclusions were made. 1. The alterations (if any) in conformation of the non-S1 parts of the 30-S subunit induced by S1 binding are too small to be detected (less than 0.1 nm change in radius of gyration). 2. The center of gravity of protein S1 bound to the 30-S subunit is quite far from the center of gravity of the particle (approximately 7.5 nm).

Escherichia coli↗

PubMed2002

Synchrotron X-ray and neutron small-angle scattering of lyotropic lipid mesophases, model biomembranes and proteins in solution at high pressure.

In this review we discuss the use of X-ray and neutron diffraction methods for investigating the temperature- and pressure-dependent structure and phase behaviour of lipid and model biomembrane systems. Hydrostatic pressure has been used as a physical parameter for studying the stability and energetics of lipid mesophases, but also because high pressure is an important feature of certain natural membrane environments and because the high pressure phase behaviour of biomolecules is of importance for several biotechnological processes. Using the pressure jump relaxation technique in combination with time-resolved synchrotron X-ray diffraction, the kinetics of different lipid phase transformations was investigated. The techniques can also be applied to the study of other soft matter and biomolecular phase transformations, such as surfactant phase transitions and protein un/refolding reactions. Several examples are given. In particular, we present data on the pressure-induced unfolding and refolding of small proteins, such as Snase. The data are compared with the corresponding results obtained using other trigger mechanisms and are discussed in the light of recent theoretical approaches.

Cell Membrane↗

PubMed1995

Nonideality of water-hexafluoropropanol mixtures as studied by X-ray small angle scattering.

Strong temperature dependent low angle X-ray scattering by trifluoroethanol- and hexafluoro-2-propanol-water mixtures was observed in conditions commonly used in NMR work on peptides. At least two types of molecular effects may contribute to the observed scattering: formation of clathrate hydrate-like aggregates of alcohol with water as have been proposed for the similar system tert-butanol-water (Iwasaki, K. and Fujiyama, T. (1976) J. Phys. Chem. 81, 1908-1912) and a further heterogeneity of the solution resulting from immiscibility of the two components.

1-Propanol↗

PubMed1998

Structural changes of the Escherichia coli GroEL-GroES chaperonins upon complex formation in solution: a neutron small angle scattering study.

We applied neutron scattering in conjunction with deuterium (D-) labeling in order to obtain information about the domain structure of GroEL and GroES isolated and in the complex. Each subunit of the heptameric GroES consists of two domains, a core domain (Met1 to Lys15 and Lys34 to Ala97) and an intervening loop region (Glu16 to Ala33). Neutron scattering shows that both regions change their conformation upon GroEL/GroES complex formation. The interdomain angle between the core regions of the heptameric GroES increases from 120 to 140 degrees, leading to a less dome-like shape of GroES, and the loop regions turn inwards by 75 degrees. The 23 C-terminal amino acids of the 14 GroEL subunits (Lys526 to Met548), which are unresolved in the crystal structure, are located either at the bottom of the cavity formed by the seven-membered GroEL ring or at the inner wall of the cavity. Upon complex formation the apical domains of GroEL move outwards, which facilitates binding of GroES at a Gro-EL-GroES center-to-center distance of (87 +/- 8) A. These structural changes may be important for the dissociation of the unfolded protein bound to the central cavity upon GroES binding. The overall structure determined by neutron scattering in solution tallies with the crystallographic model published after completion of this study. Differences in the conformation of GroES observed in the complex by the two methods support the view that the chaperonin complex is a flexible molecule which might switch in solution between different conformations.

Chaperonin 10↗

PubMed1991