Impact of Electronic State Mixing on the Photoisomerization Time Scale of the Retinal Chromophore

Impact of Electronic State Mixing on the Photoisomerization Time Scale of the Retinal Chromophore

Spectral data show that the photoisomerization of retinal protonated Schiff base (rPSB) chromophores occurs on a 100 fs time scale or less in vertebrate rhodopsins, it is several times slower in microbial rhodopsins and it is between one and 2 orders of magnitude slower in solution. These time scale variations have been attributed to specific modifications of the topography of the first excited state potential energy surface of the chromophore. However, it is presently not clear which specific environment effects (e.g., electrostatic, electronic, or steric) are responsible for changing the surface topography. Here, we use QM/MM models and excited state trajectory computations to provide evidence for an increase in electronic mixing between the first and the second excited state of the chromophore when going from vertebrate rhodopsin to the solution environments. Ultimately, we argue that a correlation between the lifetime of the first excited state and electronic mixing between such state and its higher neighbor, may have been exploited to evolve rhodopsins toward faster isomerization and, possibly, light-sensitivity.

光谱数据显示，在脊椎动物视紫红质中，视黄醛质子化席夫碱（rPSB）生色团的光异构化在100飞秒或更短的时间尺度内发生，在微生物视紫红质中则慢几倍，而在溶液中则慢1到2个数量级。这些时间尺度的变化归因于生色团第一激发态势能面地形的特定改变。然而，目前尚不清楚是哪些特定的环境效应（例如静电、电子或空间位阻）导致了表面地形的改变。在此，我们使用量子力学/分子力学模型和激发态轨迹计算来提供证据，证明当从脊椎动物视紫红质到溶液环境时，生色团的第一激发态和第二激发态之间的电子混合增加。最终，我们认为第一激发态的寿命以及该状态与其更高能级相邻态之间的电子混合之间的相关性，可能已被利用来使视紫红质朝着更快的异构化以及可能的光敏感性方向进化。