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Lateral Diffusion of Flexible Lipid Chains. A Dynamic Monte-Carlo study.

Sintes T., A. Baumgartner, and Y. K. Levine
Intl. J. of Modern Phys. C 10, 341-354 (1999)

Recent experimental studies of lateral diffusion of lipid
molecules in bilayers using single-molecule detection techniques
(Schütz et al., 1997. {sl Biophysical J.} 73:1073-1080),
gave rise to speculation about confinement effects due
to anomalous diffusion. Here we report a computer simulation
study of the lateral diffusion of conformationally
disordered lipid monolecules in a monolayer structure.
Simulations were performed using dynamic Monte Carlo methods
employing two different representations of the internal
motions of the lipid chains. No evidence for anomalous
diffusion effects are found in the homogeneous monolayer
studied.The classical Cohen-Turnbulltheory is found to
provide a good description of the simulated lateral
diffusion coefficients at high densities, above a certain
threshold $c^*$. At densities $c < c^*$, where the diffusion
of the flexible lipids is governed by their "soft core"
repulsion, we found an entropically activated diffusion
according to $D sim exp(- gamma sqrt{c})$ over a large
concentration regime $0.03 < c < 0.6$.
The consequences of the prsent findings are discussed
with respect to experiments.