The focus of this proposal is the experimental examination of lipid exchange rates between 'gel- and 'fluid-like' lipid clusters in membranes using model membrane systems. Theoretical modeling of lipid dynamics suggest that long range thermodynamic fluctuations of lipid chain conformations are responsible for several important membrane phenomena such as enhanced passive transport of sodium ion through the bilayer at the lipid 'melt' temperature and increased two-dimensional lateral diffusion rates. However little direct experimental evidence of lipid structural fluctuations is available. Submicrosecond lipid reorientational processes which exists in the bilayer may be studied using polarized fluorescence measurements. The fluorescence 'time-window' can be extended to span this critical submicrosecond region, by using fluorescence probes with lifetimes on the order of several hundreds of nanoseconds to detect submicrosecond acyl-chain motions. This extended submicrosecond time window, which has not previously been explored, may provide the key to understanding important lipid membrane mediated events e.g.ion transport, fusion and membrane signaling.
