A major use of photophysical apparatus is the study of biological systems, for example, peptides, proteins, and DNA, that contain one or more chromophores. In recent years the incorporation of nonnatural fluorescent compounds into these systems has gained great popularity. A number of optical 'tags' have been suggested as detection tools of structure and function; we propose the use of the tryptophan derivative, 7-azatryptophan, as an intrinsic probe in photophysical analyses of proteins. Not only is this chromophore spectroscopically distinct from tryptophan and all other natural amino acids, but we have demonstrated the successful incorporation of 7-azatryptophan into peptide sequences, biological cofactors, and bacterial proteins;We have performed exhaustive steady-state and time-resolved studies of the chromophoric moiety of 7-azatryptophan, 7-azaindole. Such work is necessary to interpret fully data obtained in different locales within a biological system since the photophysical characteristics of the nonnatural amino acid are unusually sensitive to its surrounding environment. We also have analyzed tri- and octapeptides that mimic substrate active-site sequences. These peptides contain 7-azatryptophan or N1-methyl-7-azatryptophan at a variety of sites. We have performed binding studies and photophysical analyses of these peptides alone and bound to [alpha]-chymotrypsin or H-2K b (a class I MHC molecule). In addition, we have tagged enzymatic cofactors with 7-azatryptophan or N1-methyl-7-azatryptophan and studied the behavior of these probes in a protein environment. Lastly, we have begun direct incorporation of this nonnatural amino acid into a variety of bacterial proteins, substituting a tryptophan residue with 7-azatryptophan;In short, we are currently developing new probes and techniques for the photophysical study of proteins and other biological systems, primarily by direct incorporation of nonnatural amino acids such as 7-azatryptophan and its analogs.