Heterogeneous Tryptophan Environments in the Cyclic Peptides Tyrocidines B and C

Phosphorescence spectroscopy has been used to probe the microenvironments of the tryptophan residues of the cyclic antibiotic decapeptides tyrocidine B (which contains a single tryptophan residue) and tyrocidine C (which contains 2 tryptophan residues). Both peptides exhibited two distinct, overlapping tryptophan phosphorescence emission spectra under appropriate solvent conditions. The two emissions had characteristically different wavelength positions, phosphorescence lifetimes, and excitation spectra, and their relative intensities were strongly solvent-dependent. In contrast, an open chain derivative of tyrocidine B, which has no biological activity, showed only a single phosphorescence emission from tryptophan. This indicated that the environmental heterogeneity of tryptophan residues in the intact peptides arose not from their primary structures but from higher structural features. Factors which are known to increase the self-association of the tyrocidines, such as higher peptide concentrations or greater amounts of water in alcoholwater solvent mixtures, generally led to enhanced emission of the lower energy phosphorescence component relative to the higher energy one. We conclude that the higher energy component of the phosphorescence emission comes from solvent-exposed tryptophan residues of monomeric peptide molecules, and the lower energy component from tryptophan residues buried in the hydrophobic interior of peptide aggregates. These cyclic peptides reveal many of the phosphorescence complexities observed with native proteins and thus are useful as model compounds in correlating tryptophan microenvironments with emission characteristics.