A Raman spectroscopic study on the effect of cholesterol on lipid packing in diether phosphatidylcholine bilayer dispersions.

For assessing lipid-sterol packing characteristics in model membrane systems, the vibrational Raman spectra of 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) multilamellar dispersions containing 18 mol% cholesterol were examined. The thermotropic behavior of the pure diether- and cholesterol-containing bilayers were studied in both the C-H stretching (2800-3100 cm-1) and C-C stretching (1000-1200 cm-1) mode regions, spectral intervals reflecting intermolecular and intramolecular order/disorder characteristics. Pure DHPC bilayers exhibit the pretransition and gel to liquid-crystalline phase transition temperatures at approx. 31 and 42.8 degrees C, respectively. In contrast to the temperature behavior of dipalmitoylphosphatidylcholine-cholesterol bilayers in which the primary phase transition Tm, is simply broadened, Tm is increased to approx. 49.6 degrees C in the DHPC (diether PC) liposomes containing 18 mol% cholesterol. As in the saturated chain diacyl-cholesterol bilayer systems, the sterol disorders the DHPC gel phase and eliminates the pretransition, while ordering the liquid-crystalline phase. Pure diether liposomes exhibit a headgroup dehydration effect as the multilamellar dispersions are cycled slowly over a temperature range including the gel and liquid-crystalline forms. That is, on multiple passages through the phase transitions, both the gel and liquid-crystalline bilayer forms assume an increased hydrocarbon chain order. In the cholesterol-containing diether bilayers, only the disordered intra- and interchain states are formed.