Exploring the Opsin shift with ab initio methods: Geometry and counterion effects on the electronic spectrum of retinal

To study the effect of the charged chromophore environment on the absorption spectrum of rhodopsin, we have calculated excited state energies of chromophore models using multi-configurational second-order perturbation theory. Taking advantage of the recently solved crystal structures of rhodopsin we have considered different chromophore geometries and their interaction with the Glu113 counterion, water and Glu181 in different protonation states. We observe a strongly blueshifted S1 state upon inclusion of Glu113/water to the wave function; the effect of the additional carboxyl group appears to be overbalanced by the complex counterion.