LIPID CLUSTERING IN BILAYERS DETECTED BY THE FLUORESCENCE KINETICS AND ANISOTROPY OF TRANS-PARINARIC ACID

This work confirms the ability of the fluorescent probe trans-parinari c acid to detect gel/fluid heterogeneity in lipid bilayers and shows a simple and useful method to quantify this heterogeneity. Taking advan tage of the maximum entropy method, we have resolved the probe fluores cence lifetime distributions in homogeneous solutions and in single an d two-component lipid bilayers at different temperatures. A precise de scription of the emission kinetics was obtained as a function of visco sity in the homogeneous solution and as a function of the phase compos ition (gel/fluid) in the lipid bilayers. These data show, unambiguousl y, that the same distribution pattern, with two well resolved lifetime classes, is observed both in pure solvents and in fluid bilayers. Thi s distribution is modified during the thermotropic phase transition, w ith the appearance of a long lifetime component. The anisotropy experi ments confirm that the amplitude of this component is proportional to the fraction of probe located in the gel phase. From this fraction we have quantified the amount of gel phase in the binary bilayer system d imyristoyl phosphatidylcholine/dipalmitoyl phosphatidylcholine and det ermined the thermotropic phase diagram of the mixture. This phase diag ram agrees well with that calculated assuming ideal mixing of the lipi ds (Marbrey, S., and J. M. Sturtevant. 1976. Proc. Natl. Acad. Sci. US A. 73:862-3866).