About the intrinsic photochemical properties of the 11-cis retinal chromophore: Computational clues for a trap state and a lever effect in Rhodopsin catalysis

CASPT2//CASSCF/6-31G*computations are used on the singlet S ₁ and S ₂ states to map the photoisomerization process of the 11-cis retinal protonated Schiff base in vacuo and to characterize its optical properties. It is shown that the spectroscopic observations recorded in Rhodopsin are reproduced quite well, calling for a substantially neutral effect of the protein. Furthermore, a rationale is proposed for the unreactive population recently observed in Rhodopsin, which is here addressed to the accessible S ₂ state, behaving as a trap. The experimental transient absorption and (absorption-wavelength dependent) emission are discussed and interpreted under the light of this novel model. Finally, a planarization of the β-ionone ring is observed on S ₁, which may cause a steric lever effect into the protein pocket, thus assisting photoisomerization catalysis. The reported results constitute a solid reference for further studies aimed to rationalize the effect of the environment on the photochemical reactivity of retinal chromophores.