Intrinsic Molecules in Fluid Phospholipid Bilayers. Fluorescence Probe Studies

Fluorescence probe data using 1,6-diphenyl-1,3,5-hexatriene for various concentrations of intrinsic molecules (cholesterol, gramicidin A amd cytochrome oxidase) within fluid lipid bilayers have been examined. The polarization value increases with increasing concentration of intrinsic molecule and then approaches a limiting value. Empirical curve-fitting of the experimental data, change of polarization with concentration, shows that each system can be fitted approximately by an exponential curve. A theory has been constructed based upon the assumption that only one intrinsic molecule need be adjacent to a fluorescent probe molecule to affect its motion drastically. The change in probe motion then depends upon the probability p of all positions next to a lipid chain being free of intrinsic molecules. The value of the probability p has been calculated and it is shown that (formula: see text) depending on whether the intrinsic molecule spans the lipid bilayer or not. The approximation p = e-Mx gives a good fit to the data for all x, thereby explaining the observed phenomenological fit. The fluorescent probe data is interpreted to show that protein-protein contacts increase as the intrinsic protein concentration increases within the lipid bilayer. An apparent dichotomy between the results from the fluorescence probe and from the deuterium magnetic resonance is explained in terms of a dominant affect on the probe being its hindrance to motion by interaction with the intrinsic molecule (protein) whilst individual C2H2 groups of the chain may exhibit greater disorder.