DISTRIBUTION OF HYDROPHOBIC PROBE MOLECULES IN LIPID BILAYERS .1. MONTE-CARLO DYNAMICS COMPUTER-SIMULATIONS

Lattice dynamics techniques are used to simulate the orientational dyn amics of the hydrophobic probe molecules 1,6-diphenyl-1,3,5-hexatriene DPH and perylene in lipid bilayer systems. It is shown that these mol ecules are essentially found in two distinct environments: in the midd le of the bilayer and in the two regions near the bilayer interfaces. This partition arises from the competition between the entropic effect s of free volume and the interaction of the probes with the lipid mole cules. Each probe population exhibits a distinct orientational and dyn amical behavior. The probe molecules move through the bilayer on a tim e scale slow compared to their rotational motions such that the two po pulations are effectively in dynamic equilibrium. The simulations reve al, furthermore, inherent ambiguities in the analysis of time-resolved fluorescence anisotropy measurements on such heterogeneous systems. T his problem is illustrated in the accompanying paper, where a detailed analysis of time-resolved fluorescence anisotropy of perylene molecul es in lipid vesicle systems is presented.