The effect of cholesterol on lipid dynamics and packing in diether phosphatidylcholine bilayers. X-ray diffraction...

In order to compare the lipid packing, conformation and dynamics of ether- and ester-linked phosphatidylcholines in the presence of equimolar concentrations of cholesterol, multilamellar dispersions of these lipid-sterol mixtures were investigated by X-ray diffraction and 2H-NMR. A comparison of the X-ray diffraction patterns at 22 degrees C of 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) dispersion, each containing 50 mol% cholesterol, demonstrate that the structural characteristics of DHPC and DPPC bilayers in the presence of cholesterol are essentially indistinguishable by X-ray diffraction. In contradistinction to the similar structural characteristics of DHPC and DPPC in the presence of cholesterol, the low-angle lamellar reflections of DHPC at 22 degrees C in the absence of cholesterol are indicative of an interdigitated phase, demonstrating that cholesterol facilitates the conversion from an interdigitated to a non-interdigitated phase. Above Tc in each lipid-sterol mixture, the quadrupolar splittings from the alpha-methylene segments of 1,2[1',1'-2H]DHPC and 1,2[2',2'-2H2]DPPC indicate that both chain inequivalence and magnetic inequivalence of any particular alpha-C2H2 deuteron pair are preserved and actually enhanced in the presence of cholesterol. Lowering of the temperature below Tc in 1,2[2',2'-2H2]DPPC/cholesterol dispersion leads to a progressive intensity loss of the sn-2 chain components, a thermotropic effect which is not observed in the corresponding components of the 1,2[1',1'-2H2]DHPC/cholesterol spectra.