González-Luque R, Garavelli M, Bernardi F, Merchán M, Robb M A, Olivucci M
Departamento de Quimica-Fisica, Universitat de València, Valencia, Spain.
Proc Natl Acad Sci U S A. 2000 Aug 15;97(17):9379-84. doi: 10.1073/pnas.97.17.9379.
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.
在本文中，我们使用从头算多组态二阶微扰理论来建立视网膜发色团的本征光异构化路径模型。这是通过计算视紫红质发色团模型4-顺式-γ-甲基壬-2,4,6,8-四烯亚胺阳离子和细菌视紫红质发色团模型全反式-庚-2,4,6-三烯亚胺阳离子在孤立条件下沿严格确定的光异构化坐标的基态（S(0)）以及前两个单重激发态（S(1)、S(2)）能量来实现的。计算得到的S(2)和S(1)能量分布在沿S(1)反应路径上未显示任何避免交叉特征，并且保持大于20千卡·摩尔⁻¹的能隙。此外，电荷分布分析表明，沿该路径S(2)和S(1)电子结构没有定性变化。因此，S(1)态保持普遍的离子（空穴对）特征，而S(2)态保持共价（点点）特征。这些结果，连同对S(1)反应坐标的分析，支持了光异构化的双态、双模模型，这对先前提出的模型进行了实质性修订。
