The Fluorescence of Tyrosyl and Tryptophanyl

SUMMARY The influence of the peptide group on the quantum yield of phenol and indole has been evaluated from fluorescence measurements on the diketopiperazines of the two chromo- phores. Strong quenching of both groups occurs in water which is almost completely eliminated in dioxane and di- methyl sulfoxide. Polarization of fluorescence measure- ments indicate that the diketopiperazines have folded con- formations in 95% glycerol solutions. It is suggested that the low yields of tyrosyl and tryptophanyl emission in the native state of proteins may be accounted for, in part, by their interactions with the peptide group. The aromatic diketopiperazines are interesting models for the evaluation of the optical behavior of the aromatic and peptide chromophores in proteins. The cyclic dipeptide ring eliminates not only the charged end groups of linear peptides but also the rotational freedom of the peptide backbone. In addition, Kopple and Marr (1) have shown by nuclear magnetic resonance that the tyrosyl group in c .Gly-L-Tyrl lies over the peptide ring, face to face, forming a rigid, folded structure. This closed form is in equilibrium with an open form, in which there is rotational freedom between the two rings. Increasing temperature favors the open form. The native conformation of globular proteins is stabilized principally by the hydrophobic interactions of the nonpolar moieties (2). Among the latter the aromatic chromophores should make an important contribution. It is increasingly clear that the fluorescent properties of the aromatic residues of proteins offer a new and sensitive probe of their molecular environment, inter-residue distances, and configurational changes (3). The aromatic diketopiperazines offer a model system of * Permanent address, The Weizmann Institute of Science,