Nonexponential Fluorescence Decay of 7-Azatryptophan Induced in a Peptide Environment

7-Azatryptophan is an alternative to tryptophan as an optical probe of protein structure and dynamics. 7-Azatryptophan is synthetically incorporated into an octapeptide (NAc-Lys-Ala-Cys-Pro-7-azatryptophan-Asn-Cys-Asp-NH₂) that mimics the active site of potato chymotrypsin inhibitor II, which is known to be a strong inhibitor of α-chymotrypsin. The synthetic octapeptide retains some of this inhibitory activity. This is the first compound containing the 7-azaindole chromophore to display a nonexponential fluorescence decay (well fit to two exponentials) in water when fluorescence is collected over the entire emission band. The effect of external quenchers on the fluorescence decay is monitored and seen to differ markedly for the two components. These results are discussed in terms of the solvation of the 7-azaindole chromophore itself, which promotes or impedes excited-state tautomerization. The fluorescence quenching of free indole and 7-azaindole are compared. The fluorescence quenching of octapeptides containing both chromophores is also compared. It is the thesis of this article that the nonexponential fluorescence decay of the 7-azatryptophan-containing octapeptide is a consequence of excited-state tautomerization of the 7-azaindole chromophore. This tautomerization is suggested to be promoted by solvent reorganization induced by the peptide backbone or by direct interactions of the 7-azaindole with neighboring amino acid side chains.