A. I. Kuklin

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.

The Effect of Fructan on the Phospholipid Organization in the Dry State

Fructans, a family of oligo- and polyfructoses, are implicated to play a drought-protecting role in plants. Inulin-type fructan is able to preserve the membrane barrier during dehydration. However, whether other fructans would be able to perform this function is unknown. In addition, almost nothing is known about the organization of these systems, which could give insight into the protective mechanism. To get insight into these questions the effect of different fructans on phosphatidylcholine-based model systems under conditions of dehydration was analyzed. Using a vesicle leakage assay, it was found that both levan- and inulin-type fructans protected the membrane barrier. This suggests that fructans in general would be able to protect the membrane barrier function. Furthermore, both fructan-types inhibited vesicle fusion to a large extent as measured using a lipid-mixing assay. Using x-ray diffraction, it was found that in the presence of both inulin- and levan-type fructans the lamellar repeat distance increased considerably. From this it was concluded that fructans are present between the lipid bilayers during drying. Furthermore, they stabilize the L(alpha) phase. In contrast to fructans, dextran did not increase the lamellar repeat distance and it even promoted L(beta) phase formation. These data support the hypothesis that fructans can have a membrane-protecting role during dehydration, and give insight into the mechanism of protection.

How a viscoelastic solution of wormlike micelles transforms into a microemulsion upon absorption of hydrocarbon: new insight

In this article, we investigate the effect of hydrocarbon addition on the rheological properties and structure of wormlike micellar solutions of potassium oleate. We show that a viscoelastic solution of entangled micellar chains is extremely responsive to hydrocarbons-the addition of only 0.5 wt % n-dodecane results in a drastic drop in viscosity by up to 5 orders of magnitude, which is due to the complete disruption of micelles and the formation of microemulsion droplets. We study the whole range of the transition of wormlike micelles into microemulsion droplets and discover that it can be divided into three regions: (i) in the first region, the solutions retain a high viscosity (∼10-350 Pa·s), the micelles are entangled but their length is reduced by the solubilization of hydrocarbons; (ii) in the second region, the system transitions to the unentangled regime and the viscosity sharply decreases as a result of further micelle shortening and the appearance of microemulsion droplets; (iii) in the third region, the viscosity is low (∼0.001 Pa·s) and only microemulsion droplets remain in the solution. The experimental studies were accompanied by theoretical considerations, which allowed us to reveal for the first time that (i) one of the leading mechanisms of micelle shortening is the preferential accumulation of the solubilized hydrocarbon in the spherical end caps of wormlike micelles, which makes the end caps thermodynamically more favorable; (ii) the onset of the sharp drop in viscosity is correlated with the crossover from the entangled to unentangled regime of the wormlike micellar solution taking place upon the shortening of micellar chains; and (iii) in the unentangled regime short cylindrical micelles coexist with microemulsion droplets.

Small-angle neutron scattering study of the n-decane effect on the bilayer thickness in extruded unilamellar dioleoylphosphatidylcholine liposomes

Dioleoylphosphatidylcholine (DOPC) and n-decane were mixed and hydrated afterwards in an excess of heavy water at 1 wt.% of DOPC. From this dispersion, unilamellar liposomes were prepared by extrusion through polycarbonate filter with 500-A pores. Small-angle neutron scattering (SANS) was conducted on these liposomes. From the Kratky-Porod plot ln[I(Q)Q2] vs. Q2 of SANS intensity I(Q) in the range of scattering vectors Q corresponding to the interval 0.001 A(-2) < or = Q2 < or = 0.006 A(-2), the liposome bilayer radius of gyration Rg and the bilayer thickness parameter d(g) = 12(0.5)Rg were obtained. The values of d(g) indicated that the bilayer thickness is within the experimental error constant up to n-decane/DOPC approximately 0.5 molar ratio, and then increases by 2.4 +/- 1.3 A up to n-decane/DOPC = 1.2 molar ratio.

Complementarity of small-angle neutron and X-ray scattering methods for the quantitative structural and dynamical specification of dendritic macromolecules

The structural characteristics of polycarbosilane dendrimers with different molecular architecture were determined in solutions by small angle neutron and X-ray scattering. The same linear dimensions were sized up for the dendrimers both in benzene and chloroform. A solvent molecules penetration inside dendrimer structure in amount up to 30 vol.-% was found from the comparison of the partial and effective scattering volume for the dendrimers in solution.

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.

Small-angle neutron scattering study of the lipid bilayer thickness in unilamellar dioleoylphosphatidylcholine vesicles prepared by the cholate dilution method: n-decane effect

Previous X-ray diffraction studies on fully hydrated fluid lamellar egg phosphatidylcholine phases indicated a approximately 10 A increase of bilayer thickness in the presence of excess n-decane [Biochim. Biophys. Acta 597 (1980) 455], while the small-angle neutron scattering (SANS) on unilamellar extruded dioleoylphosphatidylcholine (DOPC) vesicles detected substantially smaller 2.4+/-1.3 A bilayer thickness increase at n-decane/DOPC molar ratio of 1.2 [Biophys. Chem. 88 (2000) 165]. The purpose of the present study is to investigate the n-decane effect on the bilayer thickness in unilamellar DOPC vesicles prepared by the sodium cholate (NaChol) dilution method. Mixed DOPC+NaChol micelles at DOPC and NaChol concentrations of 0.1 mol/l were prepared in 2H(2)O containing 0.135 mol/l NaCl. This micellar solution was diluted in 0.135 mol/l NaCl in 2H(2)O to reach the final DOPC and NaChol concentrations of 0.008 mol/l. Thirty microliters of n-decane solution in methanol was added to 1 ml of this dispersion. After methanol evaporation, SANS was conducted on the dispersions. From the Kratky-Porod plot ln[I(Q)Q(2)] vs. Q(2) of SANS intensity I(Q) in the range of scattering vector values Q corresponding to interval 0.001 A(-2)<or=Q(2)<or=0.006 A(-2), the bilayer radius of gyration R(g) and the bilayer thickness parameter d(g)=12(0.5)R(g) were obtained. The values of d(g) indicate that the bilayer thickness increases by 2.4+/-0.9 A up to n-decane/DOPC molar ratio of 1.6. This result confirms the previous SANS findings obtained with extruded vesicles.

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.

Microstructure of stomaflex based magnetic elastomers

Stomaflex elastomers filled with two types of magnetic particles (nano- and micro-sized) were investigated. It was observed that doping with Fe3O4 nanoparticles and applying a magnetic field during the polymerisation process led to a significant change in the local structure of the elastomer. Decreases in the quasi-crystalline phase concentration, in the average size of the crystalline blocks, and in the ordering distance were observed after doping the elastomer with magnetite nanoparticles. After filling the polymer with Fe3O4 nanoparticles, yet the elastomer fractal dimension changes. For the elastomer filled with a large amount of Fe microparticles (75% particle concentration) a texture effect is observed, and this effect is larger for the samples polymerised in a magnetic field. At all microparticle concentrations, these elastomers exhibit surface fractal structure.