The development of a new intrinsic optical probe of protein structure and dynamics, Nl-methyl-7- azatryptophan (1M7AT), is reported. The utility of this nonnatural amino acid derivative lies in its single-exponential, long-lived fluorescence decay (21.7 f 0.4 ns) and in its high fluorescence quantum yield (0.53 f 0.07). Its absorption and emission maxima are red-shifted 10 and 65 nm, respectively, from those of tryptophan. These characteristics permit its unambiguous detection with unprecedented discrimination against emission from multiply occurring native tryptophan residues. In a mixture of these two amino acids, no tryptophan signal is detected until the tryptophan: N1-methyl-7-azatryptopharna tio exceeds 75: 1. Consequently, NI-methyl-7-azatryptophains ideal for studying the interactions of small peptides containing it with large proteins.