The effects of phosphatidylethanolamine and phosphatidylglycerol headgroups on bilayer dynamics: a slow-motion ESR study

Abstract

A study of the effect of phosphatidylethanolamine and phosphatidylglycerol headgroups on the behaviour of cholestane spin-label (CSL) molecules in three lipid bilayer configurations: planar multibilayers, multilamellar liposomes and small unilamellar vesicles (SUV) is presented. The results are compared with those obtained from the corresponding configurations of phosphatidylcholines with identical acyl chain composition. A comparison of the parameters describing the orientational order and dynamics of CSL in palmitoyloleoylphosphatidyl-ethanolamine (POPE) and palmitoyloleoylphosphatidylcholine (POPC) bilayer configurations indicates that the hydration of the lipid headgroups is the most important factor determining the rates of rotational motion. The orientational order of CSL is found to be lower in fully hydrated multilamellar liposomes and SUVs of negatively charged dioleoylphosphatidylglycerol (DOPG), than in the same configurations of the zwitterionic dioleoylphosphatidylcholine (DOPC). However, the rotational diffusion is not affected by the difference in the lipid headgroup. Apparently, the electrostatic repulsion in DOPG multilamellar liposomes and SUVs causes the orientational order to decrease, but without affecting the rates of rotational diffusion.