Altun Ahmet, Yokoyama Shozo, Morokuma Keiji
Cherry L. Emerson Center for Scientific Computation, Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
J Phys Chem B. 2008 Dec 25;112(51):16883-90. doi: 10.1021/jp807172h.
We have investigated photoabsorption spectra of bovine rhodopsin and its mutants (E122Q and E113Q) by hybrid quantum mechanical/molecular mechanical (QM/MM) calculations as well as retinal in vacuo by pure QM calculations, employing multireference (MR) ab initio and TD-B3LYP methods. The sophisticated MR-SORCI+Q and MRCISD+Q methods extrapolated with respect to adopted approximations can reproduce the experimental absorption maxima of retinal very well. The relatively inexpensive MR-DDCI2+Q method gives absorption maxima blue-shifted by ca. 65 nm from experimental values; however, this error is systematic and thus MR-DDCI2+Q can be used to estimate spectral shifts. In MR calculations, the ground-state energy of retinal at B3LYP geometry is significantly lower than that at CASSCF geometry. Therefore, B3LYP geometry is more reliable than CASSCF geometry, which has a blue-shift error as large as 100 nm in the gas phase. The effect of ground-state geometry on the excitation energies is less critical in the polarizing field of protein environments. At the B3LYP geometry, there is no significant charge transfer upon vertical excitation to the S1 excited-state either from Glu1 13 to retinal or from Schiff-base terminal to beta-ionone ring through the polyene chain. All-trans to 11-cis isomerization of retinal in the gas phase has no influence on the calculated S1 absorbing state, in agreement with experiment. The shoulder of the experimental absorption spectrum of retinal in vacuo at the S1 absorbing band appears to be the second electronic transition (S2) in our calculations, contrary to previous tentative assignment to vibrational state of S1 or to the S1 band of a retinal isomer.
我们采用多参考（MR）从头算和TD - B3LYP方法，通过混合量子力学/分子力学（QM/MM）计算研究了牛视紫红质及其突变体（E122Q和E113Q）的光吸收光谱，以及通过纯QM计算研究了真空中的视黄醛。采用的近似方法外推的精密MR - SORCI + Q和MRCISD + Q方法能够很好地重现视黄醛的实验吸收最大值。相对廉价的MR - DDCI2 + Q方法给出的吸收最大值比实验值蓝移约65 nm；然而，这个误差是系统性的，因此MR - DDCI2 + Q可用于估计光谱位移。在MR计算中，视黄醛在B3LYP几何结构下的基态能量明显低于在CASSCF几何结构下的能量。因此，B3LYP几何结构比CASSCF几何结构更可靠，CASSCF几何结构在气相中有高达100 nm的蓝移误差。在蛋白质环境的极化场中，基态几何结构对激发能的影响不太关键。在B3LYP几何结构下，垂直激发到S1激发态时，无论是从Glu113到视黄醛，还是从席夫碱末端通过多烯链到β - 紫罗兰酮环，都没有明显的电荷转移。气相中视黄醛从全反式到11 - 顺式的异构化对计算的S1吸收态没有影响，这与实验结果一致。真空中视黄醛在S1吸收带的实验吸收光谱的肩峰在我们的计算中似乎是第二个电子跃迁（S2），这与之前将其初步归为S1的振动态或视黄醛异构体的S1带不同。
