Tsendra O. Simulation of interaction between silica surface and some components of biological membranes

The thesis is devoted to the study on the characteristics of the interaction of biomolecules, main components of cell membranes, with the surface of silica adsorbents. This interaction is assessed by means of state-of-the-art quantum-chemical methods. The silica surface is simulated by multi-sized clusters resembling the structure of the <111> face of b-cristobalite. The adsorption of dimers of carboxylic acids (formic and acetic) on silica surface was studied theoretically. It was found that silanol groups or surface water disrupt the hydrogen bonds of the dimer and transform the octatomic structure of the dimers into a decatomic one. The existence of such adsorbed structures was proved by a correlation between calculated and experimentally obtained vibration spectra of the dimers. Hydrated complexes of methylphosphonic acid were also studied as a model of the phosphatidic acids of plasmatic membrane. Their electronic structure and geometry were determined. Tetra-, penta-, and hexahydrate molecular structures were found to coexist with their ion-pair associates. The interconversion between the two forms seems to involve the simultaneous transfer of two protons, a process with a low energy barrier. A special focus is put on the calculation of the equilibrium structures and the energies of formation of adsorption complexes and surface compounds of silica supported carbohydrates (lactose, saccharose), N-acetylneuraminic acid. It was found that the key role in the formation of this surface compounds is played by the formation of hydrogen bonds between the adsorbates and the electron donating silanol groups of silica. The results of calculations show that disaccharides (i.e. saccharose and lactose) can act as mediators of sorption of N-acetylneuraminic acid on silica. Disaccharides have high affinity to silica surface and tend to form complexes with N acetylneuraminic acid. These complexes exhibit higher adsorption heats than that of the acid alone . Finally, the interaction between hydroxylated silica particles and erythrocytes was studied. Both the silica surface and the molecule had negative surface charges due to electrolytic dissociation of surface functional groups. A probable interaction mechanism was suggested and subsequently verified by quantum-chemical calculations. The erythrocyte was represented by a lipid fragment of plasmatic membrane. It was found that at large distances between these negatively charged clusters the derived hydrated cations get between the clusters and promote their interaction. When the distance between objects reaches a critical value the localized negative charges of the silica anion and the lipid complex get saturated with protons from the hydrated cations. The resulting neutral components of the disperse system interact in such a way that water molecules get involved as intermediates of a network of hydrogen bonds.