QM/MM Photodynamics of Retinal in the Channelrhodopsin Chimera C1C2 with OM3/MRCI

The photoisomerization of all-trans retinal in channelrhodopsins triggers the opening of a cation channel in the membrane of certain green algae. The stimulus resulting from cell depolarization enables the algae to perceive and react to the present lighting conditions. This property makes channelrhodopsins especially interesting for neuroscientific applications in optogenetics. We investigate the initial photoreaction in the channelrhodopsin chimera C1C2 with a combined quantum mechanical/molecular mechanical (QM/MM) strategy. For geometry optimizations, the OM3/RHF and OM3/MRCI/Amber protocols are used. Trajectory surface hopping calculations are performed with OM3/MRCI/Amber and a statistically relevant number of molecules to obtain reaction rates and quantum yields. The utilized models include a direct hydrogen bond of the retinal Schiff base to the proposed counterions in C1C2, i. e. either toward glutamate or aspartate. Static and dynamic aspects are compared to experimental findings including time constants and spectral traces. The calculations provide a rationale for the observed short and long time components in the photokinetics of C1C2 and yield detailed insight into the photoreaction mechanisms that include bicycle pedal and hula twist motions.