Lateral diffusion in phospholipid bilayer membranes and multilamellar liquid crystals

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Abstract
The lateral-diffusion coefficients (D) of a fluorescent lipid analog, 3,3''-dioctadecylindotricarbocyanine (diI), were measured in black lipid membranes (BLM), in large (20-50 .mu.m diameter) solvent-free bilayer and multilayer membrane vesicles, and in multilamellar liquid crystals of dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine and egg lecithin. They show that D changes by several orders of magnitude at the liquid-crystal transformations of the solvent-free bilayers and multilayers. In all BLM, D .apprxeq. 10-7 cm2/s with only weak temperature dependence even near the putative phase-transformation temperatures, Tt. In the corresponding liquid crystals and large vesicles, D .apprxeq. 10-8 cm2/s above Tt, decreasing by about 2 orders of magnitude to D .apprxeq. 10-10 cm2/s below Tt. The changes of D in bilayer vesicles suggest that the expected liquid-crystal phase transitions from smectic A to a higher ordered state do persist in isolated bilayers. Retained solvent in black lipid membranes formed by both Mueller-Rudin and Montal-Mueller methods appears to enhance lateral diffusion. A simple method of forming small numbers of large solvent-free vesicles is described.