Lipid Diffusion In Domain-forming Bilayers Studied By Pfg-nmr

The pulsed field gradient (pfg)-NMR method for measurements of translational diffusion of molecules in aligned lipid bilayers is presented. Lateral phase separation of lipids has been successfully studied as well as their dynamics within the bilayer organization. Support was obtained for that the lateral diffusion depends on lipid packing and acyl chain ordering. Therefore, investigations of order parameters of perdeuterated acyl chains, using 2H NMR quadrupole splittings, were useful complements. Here, some of our recent achievements on lipid membranes will be summarized. In particular, bilayers exhibiting two-phase coexistence of liquid disordered (ld) and liquid ordered (lo) phases are considered in detail. Among our major results are that the lateral diffusion is the same for all components, independent of the molecular structure (including cholesterol (CHOL)), if they reside in the same domain in the membrane. Furthermore, quite unexpectedly CHOL seems to partition into the ld and lo phases to roughly the same extent, indicating that CHOL has no strong preference for any of these phases. We propose that the lateral phase separation in bilayers containing one high Tm and one low Tm lipid together with CHOL is driven by the increasing difficulty of incorporating an unsaturated or prenyl lipid into the highly ordered bilayer formed by a saturated lipid and CHOL, i.e. the phase transition is entropy driven to keep the disorder of the hydrocarbon chains of the unsaturated lipid.