title: Nonadiabatic ab initio dynamics of a model protonated Schiff base of 9-cis retinal.
abstract: The dynamics of the photoisomerization of a model protonated Schiff base of 9-cis retinal in isorhodopsin is investigated using nonadiabatic molecular dynamics simulation combined with ab initio quantum chemical calculations on-the-fly. The quantum chemical part is treated at the complete-active space self-consistent field level for six electrons in six active pi orbitals with the 6-31G basis set (CASSCF(6,6)/6-31G). The probabilities of nonadiabatic transitions between the S(1) ((1)pipi*) and S(0) states are estimated in light of the Zhu-Nakamura theory. The photoinduced cis-trans isomerization of 9-cis retinal proceeds slower than that of its 11-cis analogue and at a lower quantum yield, confirming experimental observations. An energetic barrier in the excited state impedes the elongation and twist of the C(9)=C(10) stretch and torsion coordinates, respectively, resulting in the trapping of trajectories before transition. Consequently, the isomerization takes longer time and the transition more often occurs at smaller twist angle of =C(8)-C(9)=C(10)-C(11)=, which leads to regeneration of the 9-cis reactant. Thus, neither the smaller twist observed in the X-ray crystal nor the slower movement of nuclei in the transition region would be the main reason for the longer reaction time and lower yield. A well-known space-saving asynchronous bicycle pedal or crankshaft photoisomerization mechanism is found to be operational in 9-cis retinal. The simulation in vacuo suggests that the excited-state barrier and the photoisomerization itself are intrinsic properties of the visual chromophore and not triggered mainly by the protein environment that surrounds the chromophore.
authors.affiliation: Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-nishihirakicho, Kyoto 606-8103, Japan.
authors.forename: Wilfredo Credo
authors.initials: WC
authors.lastname: Chung
authors.forename: Shinkoh
authors.initials: S
authors.lastname: Nanbu
authors.forename: Toshimasa
authors.initials: T
authors.lastname: Ishida
mesh_terms: D004224:Diterpenes; D005277:Fenfluramine; D007700:Kinetics; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory; D012172:Retinaldehyde / Q000737:chemistry*; D012243:Rhodopsin / Q000737:chemistry; D012545:Schiff Bases / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D004224:Diterpenes; D011522:Protons; D012545:Schiff Bases; C042882:isorhodopsin; D005277:Fenfluramine; C031390:9-cis-retinal; D012243:Rhodopsin; D012172:Retinaldehyde