Ishida Toshimasa, Nanbu Shinkoh, Nakamura Hiroki
Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-nishihirakicho, Kyoto 606-8103, Japan.
J Phys Chem A. 2009 Apr 23;113(16):4356-66. doi: 10.1021/jp8110315.
On-the-fly classical dynamics calculations combined with ab initio quantum chemical computations are carried out for two models of protonated Schiff base retinal in vacuo. The models are the 6pi system of 2-cis-penta-2,4-dieneimminium cation and the 12pi system in which two methyl groups are removed from the Schiff base of retinal. The CASSCF(6,6) level with the 6-31G basis set was employed for the quantum chemical part and the velocity Verlet algorism is utilized for time evolution of trajectories. The probabilities of nonadiabatic transition between the excited and ground state are estimated by the Zhu-Nakamura formulas. The 9-cis form product in addition to the all-trans one is generated in the present gas phase calculation for the 12pi model, despite the 9-cis generation being suppressed in protein. We have found that energy relaxation on the ground state occurs in two steps in the 12pi model. In the first step a metastable intermediate state is formed at approximately 100 fs after photoexcitation at the energy around 20-40 kcal/mol down from the excited potential energy surface, then it further relaxes to the energy around 60-80 kcal/mol from the excited surface, leading to the final state (second step). This relaxation pattern can be seen in all the three pathways to the all-trans, 9-cis, and (reverted) 11-cis form. Fourier transformation analysis reveals that the effective vibrational frequencies of the intermediate state are 1600-2000 cm(-1), which can be attributed to the conjugate CC bond frequencies in the electronic ground state. The two-step relaxation may be due to dynamical barriers. The two-step relaxation is not revealed in the smaller 6pi model. The crank-shaft motion of the C11C12 and C9C10 bonds is found in the isomerization, which indicates the motion is intrinsic in retinal, not due to the surrounding protein. The branching ratio is about 1:1:2 for the all-trans, 9-cis, and 11-cis form generation. The ratio is different from earlier works where Tully's fewest switching scheme was employed. The bond length and the dihedral angle at the transitions are also analyzed to investigate the transition mechanism.
对质子化席夫碱视黄醛的两种真空模型进行了实时经典动力学计算与从头算量子化学计算相结合的研究。这两种模型分别是2-顺式-戊-2,4-二烯亚胺阳离子的6π体系和从视黄醛席夫碱上去除两个甲基的12π体系。量子化学部分采用6-31G基组的CASSCF(6,6)水平，并利用速度Verlet算法进行轨迹的时间演化。通过朱-中村公式估计激发态和基态之间非绝热跃迁的概率。在目前对12π模型的气相计算中，除了全反式产物外，还生成了9-顺式产物，尽管在蛋白质中9-顺式的生成受到抑制。我们发现，在12π模型中，基态的能量弛豫分两步进行。第一步，在光激发后约100飞秒，从激发势能面下降约20-40千卡/摩尔的能量处形成一个亚稳中间态，然后它从激发面进一步弛豫到约60-80千卡/摩尔的能量处，从而达到最终状态（第二步）。这种弛豫模式在通向全反式、9-顺式和（反转）11-顺式形式的所有三条途径中都可以看到。傅里叶变换分析表明，中间态的有效振动频率为1600-2000厘米-1，这可归因于电子基态中的共轭CC键频率。两步弛豫可能是由于动力学势垒。在较小的6π模型中未发现两步弛豫。在异构化过程中发现了C11C12和C9C10键的曲轴运动，这表明该运动是视黄醛固有的，而不是由于周围的蛋白质。全反式、9-顺式和11-顺式形式生成的分支比约为1:1:2。该比例与早期采用塔利最少开关方案的研究不同。还分析了跃迁时的键长和二面角，以研究跃迁机制。
