SINGLE-EXPONENTIAL FLUORESCENCE DECAY OF THE NONNATURAL AMINO-ACID 7-AZATRYPTOPHAN AND THE NONEXPONENTIAL FLUORESCENCE DECAY OF TRYPTOPHAN IN WATER

The fluorescence decay of an optical probe, the nonnatural amino acid 7-azatryptophan, is measured as a function of pH, in varying mixtures of H2O and D2O and in various nonaqueous solvents. The spectroscopic distinguishability of 7-azatryptophan is demonstrated by the comparison of its fluorescence lifetime in mixtures of N-acetyltryptophanamide (NATA) with that of mixtures of 5-hydroxytryptophan in NATA. The observation of single-exponential fluorescence decay for 7-azatryptophan in water is discussed in terms of nonradiative processes that compete effectively with charge transfer from the excited-state 7-azaindole to the side chain groups and in terms of the dependence of the charge-transfer reaction on the excited-state energy of 7-azaindole. We propose that the absence of nonexponential fluorescence decay (owing to the relative insignificance of charge transfer to the side chain as a nonradiative process in 7-azatryptophan in water) arises from an unfavorable free energy of reaction. This free energy is determined largely by the energy of the fluorescent state, which lies 46 nm (9.8 kcal/mol) below that of tryptophan when the solvent is water.