Conformational Order in Lipid Tubules Formed from a Diacetylenic Lecithin: A Raman Spectroscopic Study

摘要

The conformational characteristics of the monomeric lipids in the novel tubular structure formed from the diacetylenic phosphatidylcholine DC8,9PC have been determined by laser Raman spectroscopy. It is found that DC8, 9PC tubules are generally much more crystalline than other lipid microstructures such as liposomes characterized at the same reduced temperature and it is suggested that this high degree of lipid chain ordering may be a key factor in the driving force for tubule formation. It is found that the diacetylenic unit decouples the two halves of the acyl chains into approximately m methylene segments (upper half of the lipid) and (n+diacetylenic unit) segments (lower half of the lipid molecules). The m portion of the chains is always fully extended in the all-trans conformations in freshly prepared tubes as determined by observation of a strong longitudinal acoustic mode, while the n segments are initially somewhat less well ordered. However, a very high degree of order can be induced by annealing the tubules at low temperature, resulting in both halves of the chains adopting a rigid, fully extended all-trans conformation quantitatively by analysis of one of the skeletal optical modes in terms of the fraction of trans bonds present. In addition, it is determined that laterally adjacent lipids in thermally annealed tubules are packed very closely next to each other, as are the terminal methyl portions of lipids in opposing monolayers, and that the crystal lattice sub-cell is probably orthorhombic or monoclinic.