Excimer-forming lipids in membrane research.

Pyrenedecanoic acid and pyrene lecithin are optical probes well suited to investigate lipid bilayer membranes. The method is based on the determination of the formation of excited dimers or excimers. The rate of excimer formation yields information on the dynamic molecular properties of artificial as well as of natural membranes. This article will review applications of the excimer-forming probes. Pyrene lipid probes are used to determine the coefficient of the lateral diffusion in fluid lipid membranes. Results in artificial membranes are comparable to the values obtained in erythrocyte membranes. Moreover, the excimer formation rate is a very sensitive measure of changes in membrane fluidity. Membrane fluidity is an important regulator of membrane functional proteins. For example, there is a correlation between membrane fluidity and enzyme activities of the adenylate cyclase system. The excimer formation technique is not restricted to the measurement of lateral mobility in membranes. It can also be used to determine the transversal mobility, that is, the lipid exchange between the lipid layers of one bilayer or between bilayers of different vesicles. Again, artificial as well as natural membranes can be investigated by this technique. Another important area of investigation in membrane research is the interaction between lipids and proteins. Lipids, in the presence of a protein, show a different dynamic behavior from free lipids. Because of changes in fluidity and a modified solubility of the pyrene probes within different membrane regions, our methods could also be applied to the examination of phase separation phenomena and to lipid-protein interactions.