Models are proposed for lateral selfhyphen;diffusion of lipid species in bilayers and transmembrane diffusion of small permeants. The rate determining step for lateral diffusion is assumed to be separation of two neighboring head groups by one chain diameter, allowing inhyphen;plane displacement of the adjacent test molecule head group. Expressions for the activation energy and frequency factors of diffusion are derived from an earlier treatment of chain motion in polymers. The model predicts the magnitudes of the lateral diffusion coefficients for fluid phase one and two chains species, within a factor of sim;2, and suggests a time scale for lateral diffusion of sim;10minus;8s. Transverse diffusion of small molecules is presumed to occur by hopping between kinks formed on three neighboring chains in a quasihexagonal lattice. The experimentally inferred diffusion coefficients for O2inLagr;phase lipids are consistent with this mechanism, assuming the single chain bond rotation rates deduced in part II from application of the crankshaft model to NMR relaxation data, or those derived independently from molecular dynamics simulations of polyethylene chains.
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