Understanding the Relationship Between Thermal and Photochemical Isomerization in Visual Receptors

Visual sensitivity in vertebrae depends on the photochemical and thermal processes occurring in visual pigments of the retina. Indeed, the efficiency of the primary event in vision, the cis-trans photoinduced isomerization of the chromophore embedded inside visual pigments, must be high to achieve such sensitivity. Meanwhile, thermal processes are responsible for generating spontaneous electrical signals even in the absence of light, thus imposing a limit on visual sensitivity. While the photochemical process in visual pigments has been extensively studied both experimentally and computationally, there are fewer studies targeting the thermal process, and such studies are often conflicting. However, the observation of a relationship between the maximum absorption wavelength (λmax), a photochemical property, and the activation kinetic constant (k), a thermal property, of visual pigments suggests that the thermal and photochemical processes are related. The understanding of these processes has been largely aided by computational studies. Indeed, computer simulations can be used to investigate certain features of the potential energy surfaces (PESs) driving these processes. However, a quantitative (or even qualitative) description of these PESs requires computational methods capable of correctly describing the ground (S0) and excited (S1) state regions of concern. Therefore, in the following, we first employ a reduced model of the chromophore of visual pigments (the penta-2,4-dieniminium cation) to design a stringent test of the performance of quantum mechanical methods along regions of interest on the S0 and S1 PESs. We then select a suitable method and employ it in a full hybrid quantum mechanical / molecular mechanical model of the prototypal bovine rod pigment, rhodopsin, to provide a molecular-level understanding of the thermal process in such pigments and its relation to the photochemical isomerization mechanism. We find that the transition state mediating thermal activation has the same electronic structure as the photoreceptor excited state, thus creating a direct link between λmax and k. Such a link appears to be the manifestation of intrinsic chromophore features associated with the existence of a conical intersection between its ground and excited states