Effects of water re-location and cavity trimming on the CASPT2//CASSCF/AMBER excitation energy of Rhodopsin

We have used quantum-mechanics/molecular-mechanics computations based on ab initio multiconfigurational perturbation theory to determine and rationalize the effect of the re-location of one crystallographic water molecule on the vertical excitation energy of the visual pigment rhodopsin. It is found that the re-location of one water molecule to the opposite side of the 11-cis retinal chromophore leads to a large 0.7-0.8 Å contraction in the chromophore-counterion salt-bridge distance. In spite of this structural effect, the change in excitation energy is found to be limited (< 1.5 kcal mol -1). Through an analysis of different rhodopsin models in terms of "components" (isolated chromophore, isolated chromophore-counterion ion-pair and models deprived of the counterion charges) we show that the limited change of the excitation energy can be related to a displacement of the retinal chromophore to a different spot of the protein cavity. © Springer-Verlag 2007.