Influence of highly polyunsaturated lipid acyl chains of biomembranes on the NMR order parameters

We investigate the effect of specific conformations of double-bond segments in highly polyunsaturated acyl chains on the deuterium H-2 NMR order param eters of a fully hydrated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phospho choline (SDPC, 18:0/22:6 PC) lipid bilayer. The system is analyzed by perfo rming a molecular dynamics simulation study at ambient conditions in the fl uid lamellar phase. By separately calculating the different partial contrib utions to the total order parameter profiles measurable experimentally, we are able to get insights into the molecular origin of earlier experimental and theoretical observations. The effect of the position of the different c onformations of double-bond segments along the polyunsaturated acyl chain i s also examined. As in experiments performed in a series of lipid bilayers with an increasing number of cis double bonds per lipid molecule [Holte, L. L., et al. Biophys. J. 1995, 68, 2396], we find that unsaturations influen ce mainly the order of the bottom half of the saturated chain. Specific con formations of the polyunsaturated chain close to the lipid headgroups have a distinct effect on the order of the bottom half of the saturated chain an d on the top half of the polyunsaturated chain. Our results indicate that f or SDPC the conformation of the region of the polyunsaturated chain located between the first three cis double bonds is responsible for the major effe cts on the orientational order of both the saturated and the polyunsaturate d chains.