Binding of an indole derivative to micelles as quantified by phase-sensitive detection of fluorescence

Phase-sensitive detection of fluorescence was used to measure the fractional binding of an amphipathic derivative of indole to detergent micelles. By selection of the detector phase angle, the emissions from the free and bound indole could be individually suppressed. The phase-sensitive intensity was then proportional to the concentrations of the bound and free indole, respectively. Also, the ratio of the phase-sensitive intensities, recorded at the phase angles for suppression, revealed the ratio of the fluorophore present in each environment. As the detergent concentration was increased, the measurements revealed a cooperative formation of detergent micelles and binding of the indole derivative to these micelles. The theory and procedures for this specific analysis can be applied to the analysis of any association reaction in which one of the species is fluorescent, and in which the fluorescence lifetime is altered by the binding process. An advantage of this procedure is that binding can be quantified without physical separation of the individual species.