A. Kh. Islamov

Scientific Reviews: Two-Detector System for Small-Angle Neutron Scattering Instrument

Most of the objects of small-angle neutron scattering (SANS) experiments require the measurements of a studied sample in a wide range of momentum transfer (Q-range). Larger Q-range means more reliable determination of a model of the investigated material as well as higher accuracy of its calculated structural parameters. The dynamic range of SANS instruments is normally determined by the size of the detector, which is limited mainly by technical reasons and by the wavelength range of available thermal neutrons in the neutron beam. Even in the case of the largest detector (1 m2) at one of the best beam lines—the D22 instrument at ILL—the dynamic range is about 50. Usually the problem of the Qrange is solved by a sequence of measurements with the detector at different positions. However, it leads to considerable increase in the data acquisition time. Moreover, the problem becomes critical when it is necessary to study processes in real time, especially irreversible processes.

Fractal nature of chromatin organization in interphase chicken erythrocyte nuclei: DNA structure exhibits biphasic fractal properties

Arrangement of chromatin in intact chicken erythrocyte nuclei was investigated by small angle neutron scattering. The scattering spectra have revealed that on the scales between 15 nm and 1.5 μm the interior of the nucleus exhibited properties of a mass fractal. The fractal dimension of the protein component of cell nucleus held constant at approximately 2.5, while the DNA organization was biphasic, with the fractal dimension slightly higher than 2 on the scales smaller than 300 nm and approaching 3 on the larger scales.

Structural hierarchy of chromatin in chicken erythrocyte nuclei based on small-angle neutron scattering: Fractal nature of the large-scale chromatin organization

The chromatin organization in chicken erythrocyte nuclei was studied by small-angle neutron scattering in the scattering-vector range from 1.5 × 10−1 to 10−4 Å−1 with the use of the contrast-variation technique. This scattering-vector range corresponds to linear dimensions from 4 nm to 6 μm and covers the whole hierarchy of chromatin structures, from the nucleosomal structure to the entire nucleus. The results of the present study allowed the following conclusions to be drawn: (1) both the chromatin-protein structure and the structure of the nucleic acid component in chicken erythrocyte nuclei have mass-fractal properties, (2) the structure of the protein component of chromatin exhibits a fractal behavior on scales extending over two orders of magnitude, from the nucleosomal size to the size of an entire nucleus, and (3) the structure of the nucleic acid component of chromatin in chicken erythrocyte nuclei is likewise of a fractal nature and has two levels of organization or two phases with the crossover point at about 300–400 nm.

The spatial structure of dendritic macromolecules

A low-resolution ab initio shape determination was performed from small-angle neutron and X-ray scattering (SANS and SAXS) curves from solutions of polycarbosilane dendrimers with the three-functional and the four-functional branching centre of the fourth, fifth, sixth, seventh and eighth generations. In all cases, anisometric dendrimer shapes were obtained. The overall shapes of the dendrimers with the three- and four-functional branching centres were oblate ellipsoids of revolution and triaxial ellipsoids, respectively. The restored bead models revealed a pronounced heterogeneity within the dendrimer structure. The density deficit was observed in the central part and close to the periphery of the dendrimers. The fraction of the overall volume of the dendrimers available for solvent penetration was about 0.2-0.3. These results may help in the design of new practical applications of dendrimer macromolecules.

The aggregation of branched polyethylenimine and cationic surfactants in aqueous systems

The aggregation of binary mixtures of branched polyethylenimine with a molecular mass of 30 000 and cationic surfactants, such as cetyltrimethylammonium bromide, cetyldimethylethylammonium bromide, or cetyldimethyl(2-hydroxyethyl)ammonium bromide, in aqueous solutions is studied. The critical micellization concentrations, as well as the sizes and shapes of aggregates, are determined by conductometry, small-angle neutron scattering, and dynamic light scattering methods. Catalytic activity of cationic surfactant-polyethylenimine compositions in the hydrolysis of tetracoordinated phosphorus acid 4-nitrophenyl esters is studied via spectrophotometry. The influence of spatial characteristics of aggregates on the catalytic activity and selectivity of nanostructured polymeric systems is revealed.

Analytical model for determination of parameters of helical structures in solution by small angle scattering: comparison of RecA structures by SANS

The filament structures of the self‐polymers of RecA proteins from Escherichia coli and Pseudomonas aeruginosa, their complexes with ATPγS, phage M13 single‐stranded DNA (ssDNA) and the tertiary complexes RecA::ATPγS::ssDNA were compared by small angle neutron scattering. A model was developed that allowed for an analytical solution for small angle scattering on a long helical filament, making it possible to obtain the helical pitch and the mean diameter of the protein filament from the scattering curves. The results suggest that the structure of the filaments formed by these two RecA proteins, and particularly their complexes with ATPγS, is conservative.

Application of wavelet analysis to data treatment for small-angle neutron scattering

This work presents an improvement in the resulting scattering spectra quality due to the use of spectrometer resolution during both wavelet filtering and traditional smoothing the small-angle neutron scattering (SANS) data. This result leads to a better fitting of the form-factor curve at the next step of data analysis. In addition, the wavelet analysis permits one to extract and analyze a background (noisy) component and what is more to carry out instrumental hardware corrections.

Studies of the structure of defects in In4Se3 crystals by small-angle neutron scattering

Small-angle neutron scattering is used to study the layered semiconductor In4Se3. It is observed that the samples contain inhomogeneities of colloidal dimensions, which are attributed to the precipitation of indium. Annealing reduces the concentration of inhomogeneities. Modeling the system using an ensemble of primary spherical inhomogeneities showed that a single cluster of a rectangular superlattice (2-2-6) contains 24 primary inhomogeneities of radius 13.5±1,5 Å and the distance between them is 70.9±1.5 Å.

Structure and properties of soil organic-mineral gel

Changes in the fractal dimension and scattering intensity of colloidal structures in a chernozem, soddy-podzolic soil, and a krasnozem were studied by small-angle neutron scattering at different temperatures and soil water contents. The character of the neutron scattering by soil colloids indicated that the latter were mass fractals in all of the soils studied; i.e., the colloidal particles were located apart from one another even in dry soils. The obtained results confirmed the supposition about the distribution of colloidal particles in the humus gel matrix. The changes in the fractal parameters of the soddy-podzolic soil and chernozem with increasing water contents were nonmonotonic in character, which indicated complex structural rearrangements of the colloidal component in these soils. From the results obtained, a conclusion was drawn that the destruction of the molecular network of reinforced humus gel occurred upon heating the soils to high temperatures: colloidal particles reinforcing the humus gel began to move and coagulate with the formation of dense aggregates. The electron-microscopic study of gel films released from the predried and then capillary wetted aggregates in water showed that the gel films were nonhomogeneous and included zones of humus gel reinforced by colloidal particles and zones almost free from these particles.

Physicochemical principles of the fractal organization of soil colloids

An electron-microscopic study was conducted of gel films collected from aggregates from humus-accumulative horizons of chernozem and soddy-podzolic soils. The aggregates were dried and then capillary-wetted and immersed in water, Solutions obtained by pressing from these soils were also studied. Based on the results obtained, a hypothetical mechanism was proposed for the development of fractal organization of soil colloids, which involves the fixation of micron-size mineral particles in the humus gel and their transformation under the effect of aggressive substances with the formation of colloidal particles of reaction products diffusing in the humus gel. Humus macromolecules contain many polar groups; therefore, the colloidal particles pass some distance and are then fixed on these groups. The greater the distance from a coarse particle in the center of a cluster the smaller the number of colloidal particles capable of traversing it. Therefore, the concentration of colloidal particles decreases when going from the cluster center to its periphery according to an exponential law, which results in the development of the fractal organization in the colloidal soil component. Results of soil studies using the small-angle neutron scattering method were analyzed in terms of the hypothesis proposed.