Yamazaki Shohei, Domcke Wolfgang, Sobolewski Andrzej L
J Phys Chem A. 2008 Nov 27;112(47):11965-8. doi: 10.1021/jp806622m.
We present the excited-state potential energy profiles of the biologically relevant 9H-keto-amino tautomer of guanine with respect to the radiationless decay via the out-of-plane deformation of the six-membered ring as well as the dissociation of NH bonds. The CASPT2//CASSCF method is employed for the reaction-path calculations. The reaction path for the out-of-plane deformation in the (1)pi pi* state leads in a barrierless way to a conical intersection with the electronic ground state. For the NH dissociation via the (1)pi sigma* state, the 9H-keto-amino tautomer is shown to have lower energy barriers than the 7H tautomers which we have studied recently. These two radiationless decay mechanisms explain the unexpected missing of the biologically relevant form in the resonant two-photon ionization spectrum of guanine in a supersonic jet. It is suggested that these ultrafast deactivation processes provide the biologically relevant tautomer of guanine with a high degree of photostability.
我们展示了鸟嘌呤的生物学相关9H-酮氨基互变异构体的激发态势能面，涉及通过六元环的面外变形以及NH键的解离的无辐射衰变。采用CASPT2//CASSCF方法进行反应路径计算。(1)ππ态的面外变形反应路径以无障碍方式通向与电子基态的锥形交叉点。对于通过(1)πσ态的NH解离，9H-酮氨基互变异构体的能垒低于我们最近研究的7H互变异构体。这两种无辐射衰变机制解释了在超声速射流中鸟嘌呤的共振双光子电离光谱中生物学相关形式意外缺失的现象。有人认为，这些超快失活过程为鸟嘌呤的生物学相关互变异构体提供了高度的光稳定性。
