6533b85dfe1ef96bd12be850

RESEARCH PRODUCT

Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization

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In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state ( S 0 ) and the first two singlet excited-state ( S 1 , S 2 ) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4- cis -γ-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all- trans -hepta-2,4,6-trieniminium cation in isolated conditions. The computed S 2 and S 1 energy profiles do not show any avoided crossing feature along the S 1 reaction path and maintain an energy gap >20 kcal⋅mol −1 . In addition, the analysis of the charge distribution shows that there is no qualitative change in the S 2 and S 1 electronic structure along the path. Thus, the S 1 state maintains a prevalent ionic (hole–pair) character whereas the S 2 state maintains a covalent (dot–dot) character. These results, together with the analysis of the S 1 reaction coordinate, support a two-state, two-mode model of the photoisomerization that constitutes a substantial revision of the previously proposed models.