Modulation of mobilities of fluorescent membrane probes by adsorption of a hydrophobic polyelectrolyte

We have used three different fluorescence methods to examine the effect of the pH-dependent adsorption of a hydrophobic polyelectrolyte, poly(2-ethylacrylic acid), on the mobility of fluorescent probes in phosphatidylcholine membranes. Measurements of lateral diffusion, made by either pyrene excimer formation or fluorescence photobleaching recovery, show reduced probe mobility on polymer adsorption; formation of intramolecular excimers by 1,1'-propylenebis(pyrene) is also reduced. Fluorescence depolarization measurements, however, show increased rotational freedom for diphenylhexatriene and other depolarization probes. Extensive polymer adsorption solubilizes membranes into micelles, but mobilities do not exhibit sharp transitions upon membrane reorganization. Polymer adsorption appears to affect probe mobilities locally; the global structure of the membrane does not play a direct role.