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 H 2 O and D 2 O and in various nonaqueous solvents. The spectroscopic distinguisbability 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