Correlation of tryptophan fluorescence spectral shifts and lifetimes arising directly from heterogeneous environment.

Tryptophan (Trp) fluorescence is potentially a powerful probe for studying the conformational ensembles of proteins in solution, as it is highly sensitive to the local electrostatic environment of the indole side chain. However, interpretation of the wavelength-dependent complex fluorescence decays of proteins has been stymied by controversy about two plausible origins of the typical multiple fluorescence lifetimes: multiple ground-state populations or excited-state relaxation. The latter naturally predicts the commonly observed wavelength−lifetime correlation between decay components, which associates short lifetimes with blue-shifted emission spectra and long lifetimes with red-shifted spectra. Here we show how multiple conformational populations also lead to the same strong wavelength−lifetime correlation in cyclic hexapeptides containing a single Trp residue. Fluorescence quenching in these peptides is due to electron transfer. Quantum mechanics−molecular mechanics simulations with 150-ps trajectories...