Lifetime and Reorientation Measurements of 7-Azaindole and 7-Azatryptophan in Aqueous Adipic Acid Solutions: The Significance of Pendant Functionalities in Solution Phase Association Processes

Abstract

We report on the ability of the chromophores 7-azatryptophan (7AT) and 7-azaindole (7AI) to sense aggregation of adipic acid in aqueous solution. We have studied the fluorescence lifetime and reorientation dynamics of 7AT and 7AI in water and aqueous adipic acid solutions from subsaturation through supersaturation. These probe molecules, differing by the presence (7AT) or absence (7AI) of an amino acid side group, each possess a labile proton on the chromophore ring system that renders them sensitive to changes in pH, and both chromophores are quenched collisionally in buffer solution. The dependence of the 7AI fluorescence lifetime on adipic acid concentration differs fundamentally from that of 7AT, indicating complexation of the 7AI heterocyclic nitrogen(s) with adipic acid. 7AT exhibits an increase in reorientation time near and above adipic acid saturation concentration and this trend is absent for 7AI. The 7AT amino acid side group interacts with adipic acid with a characteristic persistence time of < 50 ps. For 7AI the interaction with adipic acid is too short-lived to be seen by reorientation measurements.