A computer simulation study of the relation between lipid and probe behaviour in bilayer systems.

Computer simulations are presented of the behaviour of elongated probe molecules anchored to the interface of lipid bilayers above the phase transition of the hydrocarbon chains. The simulations thus mimic the behaviour of the fluorescent probe 1-(4-(trimethylammonio)phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) and Cholestane spin label in lipid systems. In contrast to any experimental technique the simulations follow the behaviour of both the lipid molecules and the probe within the bilayer structure. Thus, the relation between the behaviour of the probe molecules and the order and dynamics of the lipid chains can be studied in detail. We find that the presence of probe molecules, at the low concentrations used experimentally, causes only a marginal perturbation in the intrinsic properties of the lipid chains. The simulations presented support the conventional prescription for describing the orientational behaviour of probe molecules in lipid bilayers in terms of a local effective orienting potential. They indicate, however, that the potential arises from the confinement of the probe molecules between long segments of lipid chains in elongated free-volume cavities within the bilayer structure. In this sense the orienting potential concept needs to be refined in order to take into account the combined effect of the restricted free rattling motions of the probes within the free-volume cavities and the orientations of the cavities themselves relative to the normal to the bilayer plane. The time scale of the motions of the cavities within the bilayer is determined by the rotational motions of long segments of the lipid chains. These observations justify the use of rigid probe molecules such as TMA-DPH and Cholestane spin labels for monitoring the orientational order and dynamics in lipid bilayer systems.