Lateral Mobility of Pyrene in Model Membranes of Phospholipids with Different Chain Length

The coefficients of lateral diffusion, Dp, of pyrene in lipid bilayers of several 1,2‐diacyl‐phosphatidyl cholines (distearyllecithin (DSL), di‐palmitoyllecithin (DPL), dimyristoyllecithin (DML) and dilauryllecithin (DLL) above the corresponding lipid phase transition were determined. Dp was obtained from the second order rate constant, ka, of the pyrene excimer formation. It is found that the lateral mobility of an apolar molecule (such as pyrene) in the hydrophobic region of lipid lamella does not depend on the hydrocarbon chain length (Dp ≈︁ 2 · 10−7 cm2/sec at 60°C). The pyren mobility is characterized by an activation energy of ΔEp+ ≈︁ 10 kcal/mol. The experimental data are interpreted in terms of a simple model that is based on the assumption that the hopping frequency of a diffusing particle is determined by the rate of formation of free volume in the hydrocarbon moiety of the membrane. The latter is created by thermal activation of rotational isomers (essentially gtg‐kinks) in the hydrocarbon chains. The hopping frequency of the pyrene molecule is related to the jump frequency of the kinks along the hydrocarbon chains.