Translational diffusion of lipids in liquid crystalline phase phosphatidylcholine multibilayers. A comparison of experiment with theory

Vaz, W. L.; Clegg, R. M.; Hallmann, D.

doi:10.1021/bi00324a037

A systematic study of the translational diffusion of the phospholipid derivative N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) was undertaken in liquid crystalline phase phosphatidylcholine bilayers by using the fluorescence recovery after photobleaching technique. This work was done with the intention of comparing the experimental results with the predictions of theoretical models for diffusion in membranes. The following is shown. For NBD-PE, the dependence of the translational diffusion coefficient (Dt) upon the acyl chain length of the diffusant is not that predicted by continuum fluid hydrodynamic models for diffusion in membranes. Plots of Dt vs. 1/T (Arrhenius plots) are nonlinear in dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) bilayers where the acyl chain composition of the NBD-PE is matched with that of the host bilayer lipid. This suggests that a free volume model may be appropriate for the description of lipid diffusion in lipid bilayers. In bilayers of phosphatidylcholines with saturated acyl chains at the same reduced temperature, the magnitude of D1 follows the order distearoylphosphatidylcholine > DPPC > DMPC > DLPC. This is the inverse of what may be expected from the hydrodynamic model but is in agreement with the free volume in these bilayers. A free volume model that takes into account the frictional drag forces acting upon the diffusing NBD-PE at the membrane-water interface and also at the bilayer midplane is shown to adequately describe the diffusion results for NBD-PE in DLPC, DMPC, DPPC and POPC bilayers in the liquid-crystalline phase.