Correlation of Tryptophan Fluorescence Spectral Shifts and Lifetimes Arising Directly from Heterogeneous Environment
Correlation of Tryptophan Fluorescence Spectral Shifts and Lifetimes Arising Directly from Heterogeneous Environment
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DOI：
10.1021/jp111925w
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发表时间：
2011-03-31
影响因子：
3.3
通讯作者：
Callis, Patrik R.
中科院分区：
文献类型：
--
作者：
Pan, Chia-Pin;Muino, Pedro L.;Callis, Patrik R.
色氨酸（Trp）荧光对吲哚侧链的局部静电环境高度敏感，是研究溶液中蛋白质构象群的有力探针。然而，对蛋白质波长依赖的复杂荧光衰减的解释一直受到关于典型多重荧光寿命的两个似是而非的起源的争议：多个基态种群或激发态弛豫。后者自然地预测了通常观察到的衰变组分之间的波长寿命相关性，它将短寿命与蓝移发射光谱联系起来，将长寿命与红移光谱联系起来。在这里，我们展示了多个构象种群如何导致含有单个色氨酸残基的环六肽具有相同的强波长寿命相关性。这些多肽中的荧光猝灭是由于电子转移。采用量子力学-分子力学模拟方法，计算了7个六肽的六个典型旋体在室温水溶液中的荧光波长和寿命。模拟捕获了7种多肽荧光特性的大部分意想不到的多样性，并揭示了具有蓝移发射光谱的旋转体，即更高的平均能量，在使非荧光电荷转移状态和荧光状态进入共振的环境的大波动中，具有更高的猝灭概率，即更短的平均寿命。这种一般机制也适用于表现出多指数荧光衰减的蛋白质，在这些蛋白质中，除了旋转体之外，还有无数其他构象异质性的可能来源。
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 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.