Correlation of Tryptophan Fluorescence Spectral Shifts and Lifetimes Arising Directly from Heterogeneous Environment

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• 作者：Chia-Pin Pan ; Pedro L. Muin虄o ; Mary D. Barkley ; Patrik R. Callis
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：March 31, 2011
• 年：2011
• 卷：115
• 期：12
• 页码：3245-3253
• 全文大小：1004K
• 年卷期：v.115,no.12(March 31, 2011)
• ISSN：1520-5207

文摘

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鈭抣ifetime 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鈭抣ifetime correlation in cyclic hexapeptides containing a single Trp residue. Fluorescence quenching in these peptides is due to electron transfer. Quantum mechanics鈭抦olecular mechanics simulations with 150-ps trajectories were used to calculate fluorescence wavelengths and lifetimes for the six canonical rotamers of seven hexapeptides in aqueous solution at room temperature. The simulations capture most of the unexpected diversity of the fluorescence properties of the seven peptides and reveal that rotamers having blue-shifted emission spectra, i.e., higher average energy, have an increased probability for quenching, i.e., shorter average lifetime, during large fluctuations in environment that bring the nonfluorescent charge transfer state and the fluorescing state into resonance. This general mechanism should also be operative in proteins that exhibit multiexponential fluorescence decays, where myriad other sources of conformational heterogeneity besides rotamers are possible.