Effect of opsin on the shape of the potential energy surfaces at the conical intersection of the Rhodopsin chromophore

In order to disentangle the role of the protein in the control of the photoisomerization of the chromophore of the visual pigment Rhodopsin, we compare the structure of the ground and excited potential energy surfaces of gas-phase and opsin-embedded 11-cis retinal chromophore at the corresponding (lowest energy) conical intersections. It is shown that, along the branching plane, the asymmetric opsin environment destabilizes one of the ground state relaxation channels emerging from the conical intersection. This suggests that opsin promotes the formation of the product (bathorhodopsin) via enhanced decay probability along the all-trans exit channel. In contrast, in the gas-phase no significant structural difference has been found for the channels that lead towards the 11-cis or all-trans forms of the chromophore.