PHOTOPHYSICS OF A NOVEL OPTICAL PROBE - 7-AZAINDOLE

7-Azaindole is the chromophoric side chain of the nonnatural amino acid 7-azatryptophan, which we have shown can be incorporated into bacterial protein and is amenable to peptide synthesis. Time-resolved fluorescence measurements of 7-azaindole are performed as a function of solvent, pH, and temperature in order to characterize its behavior and to establish criteria for the interpretation of its photophysics when it is incorporated into, or interacts with, proteins. The first time-resolved measurements of 7-azaindole in water are presented. The dependence of the fluorescence properties of 7-azaindole in water with respect to that in various solvents of differing polarity and the temperature dependence of the fluorescence lifetimes of 7-azaindole in H2O and D2O, and in CH3OH and CH3OD, suggest that the fuorescent species of 7-azaindole in water is a tautomerized excited-state solute-solvent complex. Time-resolved fluorescence measurements as a function of temperature verify the existence in methanol of a ground-state precursor to the 7-azaindole "tautomer" species. Upon optical excitation, this precursor decays into the tautomer in less than 30 ps. Our results are used to rationalize the sensitivity of the fluorescence lifetime of a synthetic peptide containing 7-azatryptophan alone in aqueous solution and in complex with a protein.