10: Lateral & Transverse Diffusion of Phospholipids in Lipid Bilayer Membranes
In 1972 Philippe Devaux devised a clever technique to determine the lateral diffusion coefficient of a spin labeled phospholipid in bilayer membranes (147). He made a series of reference mixtures of spin-labeled phospholipids having different concentrations of spin label. Due to exchange and dipolar interactions between the labeled molecules, the reference mixtures show different line shapes and provide a set of reference spectra. Devaux then made a concentrated spot of spin labeled lipid in a large membrane and analyzed the time dependence of the spectra, in terms of a lipid diffusion coefficient, that turned out to be about 2 square microns/sec, a result now verified hundreds of times in the literature using different methods.
Published back-to-back with our paper (147) in J. Am. Chem. Soc. was a series of three papers by Sackmann and Trӓuble[22-24] who made a theoretical analysis of line shape vs. spin label composition, and reached the same conclusion as to the lipid diffusion coefficient! I admit I was totally surprised at this close agreement, especially given the theoretical assumptions made in their line shape analysis. (Roger Kornberg had made an earlier NMR experiment with a spin labeled phospholipid and had shown rapid diffusion but was unable to determine a diffusion constant from his data (145).)
Nowadays the subject of lateral diffusion of phospholipids in bilayers has again become a subject of current interest, this time in cholesterol containing lipid mixtures near a miscibility critical point. Here large fluctuations in composition can have large effects on lipid diffusion. Experimental work in this area has been carried out by Sarah Keller and her research group. (See Honerkamp-Smith et al.). I have used the theoretical work of Inaura and Fujitani to show that diffusion in the critical region may be biphasic, fast at short times and slow at longer times (481). This result is especially interesting in view of the possibility that some cell membrane lipids may be near a critical miscibility point. (See Veatch et al.). During the past few years I have also attempted to interpret the deuterium NMR of phospholipids in bilayers as the miscibility critical point is approached from above, as described in experiments by Klaus Garwich, Sarah Veatch and Sarah Keller. See (480) for leading references.
My graduate student, Roger Kornberg, used a spin-labeled phospholipid to measure the transmembrane motion of a phosphatidyl choline phospholipid in a lipid bilayer, finding the half time was long, of the order of eight hours (139). Interestingly, in later years Devaux discovered a flip-flop enzymatic activity in cell membranes and showed how in some cases this is related to cell function. Devaux has served as an editor of a book dealing with transmembrane lipid asymmetry.
During the course of studies of lipid bilayers, I felt it would be desirable to know the phase diagrams of these mixtures, and a potentially good source of information on this subject might be obtained using freeze fracture electron microscopy. My application to NSF for the necessary equipment was turned down because “McConnell had no experience in the field.” I obtained private funds, bought the equipment, and obtained beautiful results, largely due to the efforts of a graduate student, Bruce Copeland and others (236, 209, 201,191,184, 180).