Distribution of Hydrophobic Probe Molecules in Lipid Bilayers. 1. Monte Carlo Dynamics Computer Simulations

Lattice dynamics techniques are used to simulate the orientational dynamics of the hydrophobic probe molecules 1,6-diphenyl-1,3,5-hexatriene DPH and perylene in lipid bilayer systems. It is shown that these molecules are essentially found in two distinct environments: in the middle of the bilayer and in the two regions near the bilayer interfaces. This partition arises from the competition between the entropic effects of free volume and the interaction of the probes with the lipid molecules. Each probe population exhibits a distinct orientational and dynamical behavior. The probe molecules move through the bilayer on a time scale slow compared to their rotational motions such that the two populations are effectively in dynamic equilibrium. The simulations reveal, furthermore, inherent ambiguities in the analysis of time-resolved fluorescence anisotropy measurements on such heterogeneous systems. This problem is illustrated in the accompanying paper, where a detailed analysis of time-resolved fluorescence anisotropy of perylene molecules in lipid vesicle systems is presented.