Nonadiabatic ab initio dynamics of a model protonated Schiff base of 9-cis retinal.
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.
Chung
Wilfredo Credo
WC
Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-nishihirakicho, Kyoto 606-8103, Japan.
Nanbu
Shinkoh
S
Ishida
Toshimasa
T
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
D004224:Diterpenes; D011522:Protons; D012545:Schiff Bases; C042882:isorhodopsin; D005277:Fenfluramine; C031390:9-cis-retinal; D012243:Rhodopsin; D012172:Retinaldehyde