LATERAL DIFFUSION IN MODEL MEMBRANES IS INDEPENDENT OF THE SIZE OF THE HYDROPHOBIC REGION OF MOLECULES

We have systematically investigated the probe size and shape dependenc e of lateral diffusion in model dimyristoyl phosphatidylcholine membra nes. Linear hydrophobic polymers, which differ in length by an order o f magnitude, were used to explore the effect on the lateral diffusion coefficient of hydrodynamic restrictions in the bilayer interior. The polymers employed are isoprenoid alcohols-citronellol, solanesol, and dolichol. Tracer lateral diffusion coefficients were measured by fluor escence photobleaching recovery. Despite the large difference in lengt hs, the nitrobenzoxadiazole labelled alcohols all diffuse at the rate of lipid self-diffusion (5.0 x 10(-12) m2 s-1, 29-degrees-C) in the li quid crystal phase. Companion measurements in isotropic polymer soluti on, in gel phase lipid membranes and with nonpolar fluorescent polyaro matic hydrocarbons, show a marked dependence of the lateral diffusion coefficient on the probe molecule size. Our results in the liquid crys tal phase are in accord with free area theory which asserts that later al diffusion in the membrane is restricted by the surface-free area. P robe molecules which are significantly longer than the host phospholip id, seven times longer in the case of dolichol, are still restricted i n their lateral motion by the surface properties of the bilayer in the liquid crystal phase. Fluorescence quenching experiments indicate tha t the nitrobenzoxadiazole label does not reside at the aqueous interfa ce, although it must reside in close proximity according to the diffus ion measurements.