10.1021/ja0749082.s002 Angela Strambi Angela Strambi Pedro B. Coto Pedro B. Coto Luis Manuel Frutos Luis Manuel Frutos Nicolas Ferré Nicolas Ferré Massimo Olivucci Massimo Olivucci Relationship between the Excited State Relaxation Paths of Rhodopsin and Isorhodopsin American Chemical Society 2008 counterclockwise energy valley energy region ab initio multiconfigurational quantum chemical treatment pigments decay IsorhodopsinThe pigment Isorhodopsin isomerization component asynchronous crankshaft motion photoisomerization path pigment Rhodopsin conical intersection structures Isorhodopsin features intersection space Excited State Relaxation Paths bond 2008-03-19 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Relationship_between_the_Excited_State_Relaxation_Paths_of_Rhodopsin_and_Isorhodopsin/2950624 The pigment Isorhodopsin, an analogue of the visual pigment Rhodopsin, is investigated via quantum-mechanics/molecular-mechanics computations based on an <i>ab initio</i> multiconfigurational quantum chemical treatment. The limited <5 kcal mol<sup>-1</sup> error found for the spectral parameters allows for a nearly quantitative analysis of the excited-state structure and reactivity of its 9-<i>cis</i>-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-<i>cis</i> bond. Similarly, the minor component involves a clockwise, rather than counterclockwise, twisting of the 11-<i>trans</i> 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.