title: Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization.
abstract: 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-gamma-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 small middle dotmol(-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.
authors.affiliation: Departamento de Quimica-Fisica, Universitat de València, Valencia, Spain.
authors.forename: R
authors.initials: R
authors.lastname: González-Luque
authors.forename: M
authors.initials: M
authors.lastname: Garavelli
authors.forename: F
authors.initials: F
authors.lastname: Bernardi
authors.forename: M
authors.initials: M
authors.lastname: Merchán
authors.forename: M A
authors.initials: MA
authors.lastname: Robb
authors.forename: M
authors.initials: M
authors.lastname: Olivucci
mesh_terms: D000818:Animals; D001436:Bacteriorhodopsins / Q000737:chemistry / Q000378:metabolism; D003198:Computer Simulation; D005453:Fluorescence; D007536:Isomerism; D008956:Models, Chemical; D010777:Photochemistry; D017785:Photons; D012243:Rhodopsin / Q000737:chemistry* / Q000378:metabolism*; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D001436:Bacteriorhodopsins; D012243:Rhodopsin