4A04
ロドプシンとイソロドプシンの光応答性に関する
非断熱 ab initio トラジェクトリ法による理論的研究(京都大福井セ 1、上智大学理工 2) Wilfredo C. Chung1、南部伸孝 2、石田俊正 1

A theoretical study on photo-responses of rhodopsin and isorhodopsin(FIFC, Kyoto Univ. 1, Sophia Univ. 2)Wilfredo C. Chung1, Shinkoh Nanbu2 and Toshimasa Ishida1

Rhodopsin (Rh) has 11-cis retinal as chromophore and is the photosensitive chemical found on theouter segment of rod-like cells in the retina, the light-sensing structure of eyes. Isorhodopsin (isoRh) isan Rh analogue that contains 9-cis retinal embedded in the same opsin environment. Despite theirsimilarity, the photoisomerization period and quantum yield are largely different. Rhodopsin
photoisomerization is experimentally known to be faster and more efficient
Here, we carried out Quantum Mechanics/Molecular Mechanics (QM/MM) trajectory surface
hopping (TSH) direct dynamics calculations for Rh and isoRh with 162 runs for each in order tounderstand the origin of discrepancies in the rate and efficiency[1]. The transition probability isestimated on the basis of Zhu-Nakamura (ZN) theory[2]. Comparison is made with our previous in
vacuo calculations[3,4]. The QM treatment was at the CASSCF(6,6) level with the mechanicalembedding using the 6-31G basis set. The MM part is described by AMBER.
The simulation reproduced faster and more efficient isomerization in rhodopsin than inisorhodopsin. The reaction times in Rh and isoRh were 187 and 344 fs, and the quantum yields were0.52 and 0.31. The corresponding experimental values are 200 and 600 fs for the reaction times and0.65 and 022 for the yields. The opposite rotation of ϕ9 and ϕ11 (‘wring-a-wet-towel’ motion) takes
place upon photoexcitation, which also does without opsin. The wring-a-wet-towel motion is
dynamically enhanced in comparison with the one expected from locations of the MECI.
Figure 1 shows the diagram of
the active twist angle and the length of
the active bond for five typicaltrajectories for Rh and isoRh. Fast andstraightforward dynamics in Rh isshown

inFigure

1(a)whereascomplicated excited-state dynamics is

Figure 1. Change in length of the active bond -Cn-Cn+1- against theillustrated in the isoRh case in Figure

absolute value of the dihedral angle Cn-1-Cn-Cn+1-Cn+2 for typical (a)1(b). The faster and more efficientRh (n=11) and (b) isoRh (n=9) trajectories leading to the all-transphotoisomerization of Rh than ofform. The solid and dashed lines show that trajectories are in the

isoRh is due to a straightforward andexcited and ground states and the black triangles correspond to

fast excited-state dynamics for Rh intransition from the excited state to the ground state.

contrast with a complicated dynamics in a back and forth fashionespecially in the excited state for isoRh. This difference would be
mainly due to the dihedral that needs to be twisted in the isoRhcase (-C9=C10-) is situated within the narrow gap between Thr118
and Tyr268 as shown in Figure 2(b). In Rh, the twisting area(-C11=C12-) is off from the two sandwiching residues as shown inFigure 2(a).
Photoexcitation of Rh gives only bathoRh (and the reactant)whereas the isoRh excitation yields a 9,11-dicis analog in addition
to bathoRh (and the reactant). The rigorous selectivity in rhodopsinwould be another reason why rhodopsin is selected biologically.
Comparison with our previous opsin-free investigations
reveals that opsin tends to confine the twist of the active dihedral
to only one direction When the opsin environment was totally
ignored, the calculated quantum yield was only 0.27 and 0.13,
respectively in sharp contrast with the values of 0.51 and 0.31 in
opsin, respectively. This would be mainly due to the unidirectionalrotation in opsin environment. The twist-confinement totally
blockssimultaneous

twistingof

ϕ9and

ϕ11(=C10–C11=C12–C13=) and enhance the quantum yields. Also
found is the fact that the opsin funnels transitions into the vicinityof minimum energy conical intersections (MECI). The opsin

Figure 2. Superimposed structuresof Frank-Condon geometry (gray)and geometry at the MECI (pink)
for isorhodopsin. The twisting parts
ofretinal(redcircle)

andsandwiching Thr118 and Tyr268(yellow circles) are highlighted. The
twisting part is sandwiched between
the two residues.

would prevent inefficient, premature hops especially in the isoRhcase.
The present simulation reveals that the Weiss-Warshel model for cis-trans photoisomerization is
not applicable for rhodopsin because the branching ratio after transition is crucial. In the model, it isassumed that a trajectory goes back and forth around the crossing point in the excited state. In theirscheme, if the transition takes place when the trajectory go forth, the product is generated and if it does
when the trajectory go back, the ground state reactant is regenerated. The analysis of trajectories,
however, reveals that the picture totally breaks down, and that branching ratio at the first transition iscrucial.
The present ZN-QM/MM-TSH is found to be a promising approach to investigate reactions inbiomolecules.
[1] W. C. Chung, S. Nanbu, and T. Ishida, Chem. Lett. 40, 1395 (2011); J. Phys. Chem. B, 116 , 8009 (2012). [3]
H. Nakamura, Nonadiabatic Transition: Concepts, Basic Theories and Applications, Second Ed., World
Scientific, Singapore, 2012. [4] T. Ishida, S. Nanbu, and H. Nakamura, J. Phys. Chem. A. 113, 4356(2009).[5] W.
C. Chung, S. Nanbu, and T. Ishida, J. Phys. Chem. A, 114, 8190(2010).