Relationship between the Excited State Relaxation Paths of Rhodopsin and Isorhodopsin

The pigment Isorhodopsin, an analogue of the visual pigment Rhodopsin, is investigated via quantum-mechanics/molecular-mechanics computations based on an ab initio multiconfigurational quantum chemical treatment. The limited <5 kcal mol^-1 error found for the spectral parameters allows for a nearly quantitative analysis of the excited-state structure and reactivity of its 9-cis-retinal chromophore. We demonstrate that, similar to Rhodopsin, Isorhodopsin features a shallow photoisomerization path. However, the structure of the reaction coordinate appears to be reversed. In fact, while the coordinate still corresponds to an asynchronous crankshaft motion, the dominant isomerization component involves a counterclockwise, rather than clockwise, twisting of the 9-cis bond. Similarly, the minor component involves a clockwise, rather than counterclockwise, twisting of the 11-trans bond. Ultimately, these results indicate that Rhodopsin and Isorhodopsin relax along a common excited-state potential energy valley starting from opposite ends. The fact that the central and lowest energy region of such valley runs along a segment of the intersection space between the ground and excited states of the protein explains why the pigments decay at distinctive conical intersection structures.