Soloviov D. Effect of impurities on phase transitions and properties of lipid membranes

This research focuses on the effect of impurities of different chemical nature on phase transitions and properties of lipid membranes. Using small angle X-ray scattering and differential scanning calorimetry methods the effect of silver nitrate, urocanic acid, glycerol and oxyethylene glycol on structural and thermodynamic parameters of lipid bilayers was studied. High resolution inelastic X-ray scattering method used for study the dispersion curves of lipid membranes. It has been proved the existence of acoustic longitudinal and transverse phonon modes in membranes, which reflect two different mechanisms of sound propagation. It was found that in the lipid which is heated above temperature of the main phase transition, transverse acoustic mode has gap, due to the existence in membrane regions of local ordering, whose lifetime is several picoseconds. Based on experimental data, a model of the structure of the lipid membrane is proposed. According to this model the passive transport of molecules through lipid membranes realized by passing them between the specified ordered regions. Collective vibrations of molecules in two-component lipid membranes with the participation of cholesterol have been investigated. It was proved the existence of optical phonon modes in such systems, which is related to out of phase oscillations of lipid and cholesterol molecules. It has been found that the optical phonon mode in the lipid membrane has a gap, which may be explained by the presence of limited-sized lipid complexes consisting of several pairs of lipid molecules and cholesterol molecules. The appearance of lipid complexes has a threshold character as a function of the concentration of cholesterol in the membrane. The model of the phase separation in multicomponent lipid membranes is proposed. According to this model, any domain consists of identical stabile lipid complexes, which, under certain thermodynamic conditions, are combined with each other. Using time-resolved spectroscopy the photocycle of bacteriorhodopsin membrane protein was investigated as function of temperature and cholesterol concentration in lipid membrane. It is proved that increasing of cholesterol concentration in the membrane leads to acceleration of the photocycle of bacteriorhodopsin due to the formation of domains located around the protein molecules.