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.