RAMAN-STUDY ON EFFECTS OF HIGH-PRESSURE ON THE STRUCTURE OF DPPC-CHOLESTEROL MULTILAMELLAR VESICLES

Raman spectra of aqueous solutions of DPPC-cholesterol mixtures with 0 , 10, 20, 30, and 30 mol % cholesterol are recorded at room temperatur e and at 60 degrees C within the pressure range from 1 bar to 30 kbar. In addition, at normal pressure a detailed temperature study is perfo rmed between 20 and 60 degrees C. Spectral parameters, such as frequen cies, intensities, band shapes, and band splittings are analyzed to pr ovide information on conformer population, reorientational fluctuation s, acyl chain interactions, and phase transformation of the DPPC-chole sterol mixtures. Evidence for the formation of the new phase, orthorho mbic GV phase, is provided and discussed. The correlation field splitt ing of the CH2 bending mode is used to discuss the structure, degree o f disorder, and relative orientation of acyl chains in various phases and in the presence of cholesterol. Cholesterol induces reorientationa l disorder in the lipid molecules. Increased cholesterol concentration results in phase transformations being observed over progressively br oader pressure ranges. Cholesterol makes the lipid membrane less sensi tive to the external parameters such as hydrostatic pressure. The diff erent pressure behavior of Raman and IR bands is explained in terms of the change in volume associated with Raman and IR active modes.