Time-resolved emission spectroscopy of the dansyl fluorescence probe.

Using a cavity-dumped laser system as the light source, we have recorded time-resolved fluorescence spectra for the dansyl group in a variety of systems. The fluorescence spectrum of dansylphosphatidylethanolamine incorporated into a variety of lipid bilayer systems at ambient temperatures shows very marked time dependence, with red shifts of up to 40 nm over a 40-ns period. Similarly, large red shifts are observed for dansylamide in 1-butanol at low temperatures, which can be tentatively attributed to the initial rapid formation of an intramolecular charge transfer state, followed by reorientation of the polar solvent molecules. The time-resolved behavior in the lipid bilayer system can, by comparison, also be at least partly explained by solvent reorientation, but the possibility of heterogeneity of the binding site must also be considered. The complex time dependence of the fluorescence of the dansyl group in these lipid bilayer systems considerably complicates the interpretation of fluorescence quenching and fluorescence energy transfer experiments.