The effect of cholesterol on lipid dynamics and packing in diether phosphatidylcholine bilayers. X-ray diffraction and 2H-NMR study

Abstract

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° C of 1,2- di-O- hexadecyl-sn- glycero-3- phosphocholine (DHPC) and 1,2- dipalmitoyl-sn- glycero-3- phosphocholine (DPPC) dispersions, 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° 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 T c in each lipid-sterol mixture, the quadrupolar splittings from the α-methylene segments of 1,2[1′,1′- 2H 2]DHPC and 1,2[2′,2′- 2H 2]DPPC indicate that both chain inequivalence and magnetic inequivalence of any particular α-C 2H 2 deuteron pair are preserved and actually enhanced in the presence of cholesterol. Lowering of the temperature below T c in 1,2[2′,2′- 2H 2]DPPC/cholesterol dispersions 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′- 2H 2]DHPC/cholesterol spectra.