Headgroup interactions in mixed phospholipid bilayers

Abstract

2 H NMR methods have been used to study how bilayer-forming phospholipids interact with each other at the membrane surface. Aqueous dispersions of dimyristoyl- sn -phosphatidylcholine (Myr 2 -PtdCho), dimyristoyl- sn -phosphatidylethanolamine (Myr 2 -PtdEtn), and dimyristoyl- sn -phosphatidyl-3-glycerol, specifically deuterated at different positions in their headgroups, give well-resolved 2 H NMR powder spectra. These spectra are characteristic of a lipid bilayer with quadrupole splittings that are sensitive to the amplitude of headgroup motion. In binary mixed bilayers of deuterated lipids with an unlabeled component, all parts of the deuterated headgroup monitor the presence of the second lipid from changes in the measured quadrupole splittings. The headgroups of the charged lipids, dimyristoyl- sn -phosphatidylserine and dimyristoyl- sn -phosphatidyl-3-glycerol, interact to the largest extent with the choline moiety of Myr 2 -PtdCho and the ethanolamine moiety of Myr 2 -PtdEtn, whereas a somewhat smaller but still marked alteration in headgroup motion was observed in Myr 2 -PtdCho/Myr 2 -PtdEtn mixtures. The large changes in the deuterium quadrupole splittings for the zwitterionic lipids after addition of a charged lipid suggest that either a strong perturbation in the hydrogen bonding occurs or changes take place in the water structure at the membrane surface, or possibly both.