The ordering and dynamics of the terminal methyl group on the lignoceric acid chain in cerebroside sulfate

Abstract

Cerebroside sulfate containing lignoceric acid deuterated at the terminal methyl group was prepared by a semi-synthetic method in order to study the ordering and motion of the fatty acyl chain by deuterium NMR spectroscopy. The lipid was hydrated with either 2 M KCl or 2 M LiCl. The NMR results are consistent with the formation of an interdigitated bilayer in the gel phase as suggested by spin label and X-ray diffraction studies, although they do not delineate whether the interdigitation is of the partial or mixed triple-chain type. In the low-temperature stable phase formed by this lipid on slow cooling from the liquid-crystalline phase, which has been shown by X-ray diffraction to be partially interdigitated, the dominant molecular motion is reorientation of the lipid about its along axis. The motion had a correlation time of the order of the deuterium quadrupole coupling constant (10 −6 s). The rate of axial diffusion is greater than that found for symmetric phospho- or glycosphingo-lipids in their crystalline subgel phases, but less than that of a lipid which forms a non-interdigitated gel phase bilayer such as DPPE, or a lipid which forms a fully interdigitated gel phase bilayer like DHPC. The rate of motion appears to be greater in a low temperature, more hydrated metastable phase formed by this lipid after cooling rapidly from the liquid crystalline phase. This phase is thought to be a mixed interdigitated bilayer based on spin label studies. The disorder observed at the C-terminal methyl group in the L α phase, which is probably partially interdigitated according to X-ray diffraction studies, is greater than that of a non-interdigitated bilayer formed by a symmetric lipid. In the CBS/Li + system at high temperature, there is evidence for a liquid-crystalline to liquid-crystalline phase transition, in which the average orientation of the terminal methyl group changes.