The effect of high external pressure on DPPC-cholesterol multilamellar vesicles: a pressure-tuning Fourier transform...

We have investigated the effect of incorporation of cholesterol on the barotropic phase behavior of aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) using Fourier-transform infrared spectroscopy (FTIR) in combination with the diamond anvil technique. Infrared spectral parameters, such as the frequencies, intensities, bandshapes and band splittings have been used to detect structural and dynamical changes upon incorporation of cholesterol into the DPPC bilayer. Analysis of these spectral parameters yields information on conformer population, reorientational fluctuations, interchain interaction, hydrogen bonding, interdigitation packing, and phase transformations of the DPPC/cholesterol mixtures. We present FTIR data of aqueous DPPC dispersions at 0, 10, 20, 30, 40, and 50 mol% cholesterol in the pressure range from 0.001 to 20 kbar at two temperatures, 25 degrees C and 55 degrees C. In addition, comprehensive temperature dependent measurements in the range from 20 degrees C to 80 degrees C were performed at ambient pressure. Analysis of the CH2 symmetric and antisymmetric stretching modes yields information of the effect of cholesterol concentration on the phase transition phenomena occurring in the lipid bilayer. Observation of the correlation field splittings of the CH2 bending and rocking modes monitors structural changes and dynamical properties of the lipid mixtures. Cholesterol induces more orientational disorder of the lipid molecules in terms of an increase of the reorientational fluctuations of the molecules and twisting/torsion motions of the acyl chains in the gel phase even at elevated pressures. It therefore appears that one important role of cholesterol is to make the membrane insensitive to changes in external environment, such as high hydrostatic pressure. Increase of pressure leads to a decrease in half width of the C = O band contour of pure DPPC and of DPPC/cholesterol mixtures, especially for cholesterol concentrations equal and higher than 30 mol%, which might be due to a marked increase in free carbonyl groups. At high pressure part of the bound water from the interfacial zone of the membrane is withdrawn. Increase of the cholesterol concentration and increase in pressure have opposite effects on the population of free and hydrated carbonyl ester groups of DPPC in the gel phases.