Conformational Effects on Tryptophan Fluorescence in Cyclic Hexapeptides

Abstract The peptide bond quenches tryptophan fluorescence by excited-state electron transfer, which probably accounts for most of the variation in fluorescence intensity of peptides and proteins. A series of seven peptides was designed with a single tryptophan, identical amino acid composition, and peptide bond as the only known quenching group. The solution structure and side-chain χ 1 rotamer populations of the peptides were determined by one-dimensional and two-dimensional 1 H-NMR. All peptides have a single backbone conformation. The ϕ -, ψ -angles and χ 1 rotamer populations of tryptophan vary with position in the sequence. The peptides have fluorescence emission maxima of 350–355nm, quantum yields of 0.04–0.24, and triple exponential fluorescence decays with lifetimes of 4.4–6.6, 1.4–3.2, and 0.2–1.0ns at 5°C. Lifetimes were correlated with ground-state conformers in six peptides by assigning the major lifetime component to the major NMR-determined χ 1 rotamer. In five peptides the χ 1 =−60° rotamer of tryptophan has lifetimes of 2.7–5.5ns, depending on local backbone conformation. In one peptide the χ 1 =180° rotamer has a 0.5-ns lifetime. This series of small peptides vividly demonstrates the dominant role of peptide bond quenching in tryptophan fluorescence.