Chain length and pressure dependence of lipid translational diffusion.

The translational diffusion of pyrene, pyrene butyric acid and pyrene decanoic acid has been determined in phosphatidylcholine bilayers of different chain length and under pressure up to 200 bars. In the liquid crystalline phase and at a given temperature the diffusion decreases with increasing chain length. At a constant reduced temperature, Tred (about 10 K above the transition temperature), long chain lipids exhibit the fastest diffusion which is in disagreement with hydrodynamic models but favours free volume models for diffusion in lipid bilayers. The volume of activation, Vact, calculated from the decrease of the diffusion coefficient with pressure, delta lnD/delta P, depends on lipid chain length. Vact decreases with decreasing lipid chain length at a given temperature, T = 65 degrees C, and increases at the reduced temperature. These results are again in agreement with the dependence of the diffusion on lipid chain length and therefore with the free volume model.