De Vico Luca, Page Christopher S, Garavelli Marco, Bernardi Fernando, Basosi Riccardo, Olivucci Massimo
Dipartimento di Chimica, Università di Siena, via Aldo Moro, Siena, I-53100 Italy.
J Am Chem Soc. 2002 Apr 17;124(15):4124-34. doi: 10.1021/ja017502c.
Multiconfigurational second-order perturbation theory computations and reaction path mapping for the retinal protonated Schiff base models all-trans-nona-2,4,6,8-tetraeniminium and 2-cis-nona-2,4,6,8-tetraeniminium cation demonstrate that, in isolated conditions, retinal chromophores exhibit at least three competing excited-state double bond isomerization paths. These paths are associated with the photoisomerization of the double bonds in positions 9, 11, and 13, respectively, and are controlled by barriers that favor the position 11. The computations provide a basis for the understanding of the observed excited-state lifetime in both naturally occurring and synthetic chromophores in solution and, tentatively, in the protein environment. In particular, we provide a rationalization of the excited-state lifetimes observed for a group of locked retinal chromophores which suggests that photoisomerization in bacteriorhodopsin is the result of simultaneous specific "catalysis" (all-trans --> 13-cis path) accompanied by specific "inhibition" (all-trans --> 11-cis path). The nature of the S(1) --> S(0) decay channel associated with the three paths has also been investigated at the CASSCF level of theory. It is shown that the energy surfaces in the vicinity of the conical intersection for the photoisomerization about the central double bond of retinal (position 11) and the two corresponding lateral double bonds (positions 9 and 13) are structurally different.
对视网膜质子化席夫碱模型全反式-壬-2,4,6,8-四烯亚胺阳离子和2-顺式-壬-2,4,6,8-四烯亚胺阳离子进行的多组态二阶微扰理论计算和反应路径映射表明，在孤立条件下，视网膜发色团至少展现出三条相互竞争的激发态双键异构化路径。这些路径分别与9、11和13位双键的光异构化相关，并受有利于11位的势垒控制。这些计算为理解溶液中以及初步在蛋白质环境中天然存在和合成的发色团中观察到的激发态寿命提供了基础。特别是，我们对一组锁定视网膜发色团观察到的激发态寿命进行了合理化解释，这表明细菌视紫红质中的光异构化是同时发生的特定“催化”（全反式→13-顺式路径）伴随着特定“抑制”（全反式→11-顺式路径）的结果。还在CASSCF理论水平上研究了与这三条路径相关的S(1)→S(0)衰变通道的性质。结果表明，视网膜中心双键（11位）和两个相应侧链双键（9位和13位）光异构化的锥形交叉点附近的能量表面在结构上是不同的。
