Contrast Variation Method Research Articles

Neutron reflectivity at the solid/liquid interface: examples ofapplications in biophysics

Over the last 20 years, neutron reflection has emerged as a powerful techniquefor investigating inhomogeneities across an interface, inhomogeneities eitherin composition (Lu and Thomas 1998 J. Chem. Soc. Faraday Trans. 94 995) or magnetization (Felcher 1981 Phys. Rev. B 24 1995). By measuring the reflected over the incomingintensity of a well collimated beam striking at an interface, as a function ofthe incident angle and wavelength, the concentration profile giving rise to areflectivity curve is calculated. The success of neutron reflection arisesfrom the fact that, because of the short wavelengths available, it has aresolution of a fraction of a nanometre, so that information is gained at themolecular level. Unlike x-rays it is not destructive and can be used at buried interfaces, which are not easily accessible to other techniques,such as liquid/liquid or solid/liquid, as well as at solid/air and liquid/airinterfaces. It is particularly useful for soft-matter studies since neutronsare strongly scattered by light atoms like H, C, O and N of which most organicand biological materials are formed. Moreover, the nuclei of differentisotopes of the same element scatter neutrons with different amplitude andsometimes, as in the case of protons and deuterons, with opposite phase. Thisallows the use of the method of contrast variation, described below,and different parts of the interface may be highlighted. For biophysicsstudies, a major advantage of reflectivity over other scattering techniques isthat the required sample quantity is very small (<10-6 g) and it istherefore suitable for work with expensive or rare macromolecules.While specular reflection (angle of incoming beam equal to angle of reflectedbeam) gives information in the direction perpendicular to the interface, thelateral structure of the interface may be probed by the nonspecular scatteringmeasured at reflection angles different from the specular one (Sinha et al 1998 Phys. Rev. B 38 2297, Pynn 1992 Phys. Rev. B 45 602). Thistechnique is widely used with x-rays while there are far fewer data in theneutron case due to the smaller intensity of neutron beams. An examplerelevant in biophysics where the neutron technique has been applied is theoff-specular scattering from highly oriented multilamellar phospholipid membranes(Munster et al 1999 Europhys. Lett. 46 486).Neutron reflection is now being used for studies of surface chemistry(surfactants, polymers, lipids, proteins and mixtures adsorbed at liquid/fluidand solid/fluid interfaces), surface magnetism (ultrathin Fe films, magneticmultilayers, superconductors) and solid films (Langmuir-Blodgett films, thinsolid films, multilayers, polymer films). The number of reflectometers in theneutron facilities all around the world is increasing although the use of thetechnique is not yet very common because the availability of beam time isrestricted by cost.Since many biological processes occur at interfaces, the possibility of usingneutron reflection to study structural and kinetic aspects of model as well asreal biological systems is of considerable interest. However, the number ofsuch experiments so far performed is small. The reason for this is probablybecause it is well known that the most effective use of neutron reflectioninvolves extensive deuterium substitution and this is not usually an availableoption in biological systems. This problem may be partially solved bydeuteriating other parts of the interface as described by Fragneto et al (2000 Phys. Chem. Chem. Phys. 2 5214).In this paper we shall concentrate on the use of specular neutron reflection atthe solid/liquid interface, less studied than the solid/air or liquid/airinterfaces, although technologically more important.After a brief introduction to the theory and measurement of neutronreflectivity, solid/liquid interfaces both from hydrophilic and hydrophobicsolids will be described. Three examples of applications in biophysics will begiven: (1) the adsorption of two proteins, β-casein and β-lactoglobulin,on hydrophobic silicon; (2) the interaction of the peptide p-Antp43-58 with phospholipidbilayers deposited on silicon; (3) the fluid floating bilayer, a new model for biological membranes.

2P134中性子逆コントラスト変化法によるGM1/DPPC混合リポソームの熱構造転移の研究

2P134中性子逆コントラスト変化法によるGM1/DPPC混合リポソームの熱構造転移の研究

Application of small-angle neutron scattering to the study of porosity in energetic materials

Small-angle neutron scattering (SANS) and the method of contrast variation were used to measure porosity and crystallite surface area in the energetic system octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and to gauge the effects of mechanical deformation on the pore-size distribution and crystallite surface area. The crystallite surface area and the presence of voids (pores) in a high explosive system are known to affect its behavior and overall performance. Measures of these two quantities after an insult, resulting from various process and accident scenarios, can be used to predict the performance of an explosive system after process- and accident-related mechanical deformation. The contrast variation technique allows us to discriminate between internal pores and features that are on or contiguous with the crystallite surface. Measurements were conducted on loose powders of HMX (261 and 10 mm, volume averaged mean particle diameters) and pellets made by uniaxial consolidation to 7 and 10 vol% porosity, respectively. Analysis of the SANS data indicates significant alteration of the intragranular pore structure and systematic shifts in the surface area that are dependent upon mechanical deformation.

On the occurrence of ternary complexes in agarose gels as studied by the contrast variation method

The contrast variation method by isotopic labelling as used in small-angle neutron scattering experiments is applied to the case of agarose systems (sol and gels) in order to cast some light on the occurrence of complexes (crystallosolvates). This approach proves to be helpful for giving further support to the existence of ternary complexes in agarose/water/DMSO gels and in agarose/DMSO sols. The results are discussed in the light of temperature–composition phase diagrams established previously.

Anomalous Surface Conformation for Polymeric Gas-Hydrate-Crystal Inhibitors

AbstractWe have used both conventional small-angle neutron scattering and contrast variation techniques to characterize the polymer conformations of two non-ionic water soluble polymers: poly(ethylene oxide) and poly(N-vinyl-2-pyrollidone). The second of these is able to kinetically suppress hydrate crystallization, and an objective of these studies is to obtain a understanding of this inhibition mechanism. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows only a small difference between the polymers. The single- chain characteristics are unperturbed, but the hydrate inhibitor polymer seems to show an enhanced tendency to form aggregates in solution. This is evidenced by excess low-q scattering following a q-2.5 power law. Much more evident is the strong perturbation in the conformation of the inhibitor polymer upon crystallization of the hydrate. We utilize contrast variation methods to resolve the scattering of the polymer on the hydrate surface. Unlike expectations from polymer scaling laws, the resulting layer is considerably thicker (550) than the single- chain radius of gyration, 80. Also surprising, only 2 percent of the available crystal surface is covered. Within the covered areas, the polymer concentration is significantly enhanced over that in the surrounding solution, about 2.5c* (where c* is the overlap concentration). This suggests a coverage of clumps of polymer widely separated from one another. Consideration of the concentration and implied spacing of such clumps on the hydrate surface suggests that they can effectively inhibit crystal growth.

Internal structural change of ganglioside micelle depending on temperature observed by neutron solvent contrast variation

By using solvent contrast variation method of small-angle neutron scattering, we have studied the thermal structural transition of glycosphingolipid micelle (0.5% w/v monosialogangliosides in 50 mM Hepes buffer at pH 7). The present results clearly indicate that the thermal structural transition accompanies a change of the intermicellar scattering density distribution, which results from the dehydration of the polar head (oligosaccharide chain with acidic sugar) region of the micelle. The present experimental evidences strongly support our previous results obtained by using small-angle X-ray scattering and shell-modeling methods.

Application of contrast variation method in SANS experiments with ferrofluids

The contrast variation method, applied in a neutron small angle scattering experiment, was used to simultaneously obtain information about molecular and magnetic structure of ferrofluids. Limitations caused by size polydispersity of particles were considered.

Two concentric protein shell structure with spikes of silkworm Bombyx mori cytoplasmic polyhedrosis virus revealed by small-angle neutron scattering using the contrast variation method.

The overall and internal structures of the silkworm Bombyx mori cytoplasmic polyhedrosis virus was investigated by small-angle neutron scattering using the contrast variation method. Data were collected in aqueous buffer solutions containing 0, 50, 75, and 100% D2O in the q range of 0.002 to 0.0774 A-1 at 5 degrees C. The radius of gyration at infinite contrast was estimated to be 336 A. The contrast matching point of the virus was determined to correspond to about 50% D2O level, evidence that the virus is composed of protein and nucleic acid. The virus was basically spherical and had a diameter of about 700 A. The main feature of its structure is the clustering of protein into two concentric shells separated by about 100 A. Most of the RNA moieties are located in the central core and between these two protein shells. However, the distance distribution function P(r) showed a minor distribution beyond a distance of r = 700 A, with a maximum particle distance of the virus of 1350 A. This is indicative of an external structure region with very low scattering density, in addition to the basic spherical structure. This external region is thought to correspond to twelve pyramidal protruding spikes shown by electron microscopic studies.

Polar structure of lysozyme aggregates in unsaturated solution determined by small-angle neutron scattering – contrast variation method

The structure of lysozyme aggregates in unsaturated solution was studied by means of the small-angle neutron scattering – contrast variation method. The fact that lysozyme molecules aggregate even in unsaturated solutions and the size of aggregates increases with the increase of the NaCl concentration has been confirmed. Constituent polar molecules in aggregates align parallel with each other on aggregate formation. This indicates that the aggregates could not become precursors of nucleation since the parallel molecular arrangement is not the motif for the crystal of tetragonal form.

Sphere to Rod Transition of Micelles Formed by Amphiphilic Diblock Copolymers of Vinyl Ethers in Aqueous Solution

Amphiphilic block copolymers of vinyl ethers containing 2-hydroxyethyl vinyl ether (HOVE) and partially deuterated n-butyl vinyl ether (NBVE) were synthesized by living cationic polymerization. Four block copolymers with the same hydrophilic length but different hydrophobic lengths were prepared. The internal structures of the micelles formed by these copolymers in aqueous solution were investigated by the contrast variation method of small-angle neutron scattering (SANS) measurement. The molar volume of HOVE was estimated to be quite small from the dependence of the forward scattering intensity on the contrast, which comes from the hydration effect in the micellar shell. The SANS data were well described by the theoretical form factor of a core−shell model. The micellar shape was strongly dependent on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed the q-1 dependence, reflecting the formation of rodlike micelles. These scattering curves could be described well by a sphere−rod coexistence model. The volume fraction of the rodlike micelle was found to increase with increasing hydrophobic chain length.

Depth of water incorporation into float glass surfaces studied by neutron reflection

Specular neutron reflection is used to depth profile non-invasively water incorporation into float glass surfaces as a function of immersion time. The effective nanometre resolution of the technique, allied with the contrast variation method, has uniquely revealed the presence of two layers containing water. The first layer occurs at the glass surface and has a thickness that remains constant at 30 A under conditions of 100% relative humidity. The second layer penetrates ever deeper into the glass as the time of immersion lengthens, and is accompanied by a loss of material from the glass itself.

3PB019 コントラスト変調法を用いたカエル骨格筋の中性子回折実験

3PB019 コントラスト変調法を用いたカエル骨格筋の中性子回折実験

Anomalous Ultra-Small-Angle X-ray Scattering From Evolving Microstructures during Tensile Creep

ABSTRACTUltra-small-angle x-ray scattering provides quantitative and statistically significant information on the size distribution of electron density inhomogeneities with dimensions between ≈100 Å and ≈5 μm. All sizes are sampled simultaneously with a single experiment, removing the possibility of observational bias. In a material such as commercial silicon nitride, where the inhomogeneities are due to populations of intergranular secondary phases and voids of similar dimensions, the scattering contains contributions from each individual population. A single USAXS scan cannot separate overlapping populations of scatterers due to the different contrasts of the microstructural components. Anomalous USAXS (A-USAXS) is an elementspecific contrast variation method to vary the scattering contribution from one of the populations while holding that of the other populations fixed. To follow the size evolution under tensile creep of both the cavities and the Yb disilicate secondary phases, A-USAXS data was measured near the Yb LIII absorption edge. Creep cavity and disilicate size distributions were each determined as a function of deformation.

Surface and pore structure characterisation by neutron scattering techniques

Neutron scattering techniques can provide extensive information about the microstructure of porous materials and the properties of adsorbates within the pores. This information is obtained from measurements of both coherent and incoherent scattering. Coherent scattering comprises diffraction and small angle scattering (SAS) and has a counterpart in X-ray scattering. Incoherent scattering arises from a unique interaction of the neutron with the atomic nucleus and has no equivalent with either X-ray or light radiation. Here the application of small angle neutron scattering (SANS) is reviewed with particular emphasis on two recent developments. The first concerns contrast variation methods to investigate adsorption mechanisms, which include the capillary condensation of benzene in a mesoporous silica gel. The second concerns the examination of oriented anisotropic porous microstructures, as can occur in microporous carbon fibres. The applications of incoherent inelastic and quasielastic scattering together with neutron diffraction are also described. Here the investigation of the state of water in micro and mesoporous silica gels with a model pore structure is illustrated. It is shown that these techniques give evidence of a perturbed H-bonded structure of water due to confinement in pores. This affects both the diffusional behaviour and freezing properties of the water.

Small-angle neutron scattering and dynamic light scattering studies of N- and C-terminal fragments of ovotransferrin

In order to rationalize the physicochemical heterogeneities between the N- and C-lobes of ovotransferrin (OTf), we have analyzed the structural characteristics of the isolated fragments corresponding to the N- and C-terminal halves of OTf (OTf/2N and OTf/2C) with and without iron by means of small-angle neutron scattering (SANS) using the contrast variation method with solvents of various D 2O/H 2O mixtures, and dynamic light scattering (DLS) measurements. The analyses of the internal structural characteristics from SANS data revealed that the radius of gyration ( R g) for both fragments decreased to the same extent with iron binding, and the structural distortion of OTf/2C was smaller than that of OTf/2N, decreasing with iron uptake. The DLS studies showed that the change in the diffusion coefficient induced by iron binding to OTf/2C was greater than that to OTf/2N. It was inferred that the OTf/2C molecule tends to become more compact on the whole by iron binding as compared to the OTf/2N molecule.

Small-Angle Scattering from Particles Having an Irregular Inner Structure

A model is presented for the scattering pattern of a dilute dispersion of particles having an irregular inner structure, made of two phases randomly distributed. For isotropic particles in a dispersant medium, the correlation function of the inner structure has, in the limit of small distances, the same exponential form as that for a two-phase system, provided that the characteristic length scale of the inner structure is small compared to the particle dimension. The scattering pattern of spherical particles is presented in a form suitable for analysis using the contrast-variation method. The cross-term, related to the orientation average of the scattering amplitude, is absent for a random inner structure. The heterogeneous part of the scattering curve is the same as for a randomly distributed two-phase structure. The sphere method is used to simulate the scattering behaviour of particles having an irregular inner structure and to demonstrate the validity of the proposed model.

Small-Angle Neutron Scattering Studies on the Mesoporous Molecular Sieve MCM-41

A pure silica MCM-41 preparation with high crystalline order perpendicular to the channels has been examined using small-angle neutron scattering and the contrast variation method in both the calcined and template-containing forms. This defines various structural features such as template and water distributions and content, local silica density, and surface properties. Adsorption isotherm data, NMR, previous neutron scattering data, and the SANS data allow a choice between proposed models for the channel structure of MCM-41, in terms of the straightness of the channels and their surface roughness.

Micellization of vinyl ether amphiphilic block copolymers by small-angle neutron scattering

Several amphiphilic block copolymers based on vinyl ether derivatives were synthesized by living cationic polymerization. Different compositions of hydrophilic and hydrophobic segments in the copolymers were achieved by changing the monomer ratios of vinyl ether derivatives. Partially deuterated copolymers at ether substituent parts were especially prepared for SANS experiments. These copolymers formed micelles of core-shell structure in water. SANS measurements for aqueous solutions of various copolymers were performed by using a contrast-variation method. The morphology of these micelles was studied as a function of chemical composition of the copolymers.

An investigation of the porosity and surface roughness of liquid crystal alignment layers using neutron reflectivity

Neutron reflectivity has been used to investigate the porosity and surface roughness of three different liquid crystal alignment layers to elucidate how they orient the director. The absorption of hexane into these alignment layers was measured by neutron reflection and the volume fraction profile of the hexane within the layers was determined using the contrast variation method. Measurements were made on rubbed polyimide and silicon monoxide (SiO) evaporated at 5 and 30 to the substrate. The porosity and surface roughness of the alignment layers were found to be correlated with the induced pre-tilt of the director. The low pre-tilt rubbed polyimide and SiO 30 alignment layers were found to be smooth, uniform and impervious to hexane, whilst the high pre-tilt SiO 5 was porous and extremely rough. These observations suggest that both polyimide and SiO 30 rely on an anisotropic interaction with the surface rather than one induced by the surface topography. In contrast, the alignment of liquid crystals by SiO 5 probably originates from an interaction between the mesogens and the rough, porous surface.

Inner Structure of Inhomogeneous Copolymer Latex Particles

A small-angle X-ray scattering (SAXS) study of a model copolymer latex, composed of styrene and pentabromobenzyl acrylate (PBBA, 40 wt %), is presented. The contrast variation method employed in th...