Department of Chemistry, University of Gdańsk, Sobieskiego 18, 80-952 Gdańsk, Poland.
Chemphyschem. 2010 Mar 15;11(4):880-8. doi: 10.1002/cphc.200900810.
The photoelectron spectrum of the anion of the guanine...cytosine base pair (GC)(*-) is recorded for the first time. The observed variation in the spectral peak-height ratios with the source conditions suggests the presence of two or more anionic isomers. Two maxima of the broad bands in the photoelectron spectrum were measured at about 1.9 and about 2.6 eV. These values are very well reproduced by the vertical detachment energies of the B3LYP/6-31++G(d,p) calculated low-energy anionic structures, which are 1) the Watson-Crick base-pair anion with proton transferred from N1 of guanine to N3 of cytosine, 2) its analogue in which the proton is transferred from N9 of guanine to N7 of guanine, and 3) the global minimum geometry, which is formed from the latter anion by rotation of guanine about the axis approximately defined by C2 of guanine and C4 of cytosine. Furthermore, a minor difference in the stabilities of the two lowest energy anions explains the experimentally observed source (temperature) dependence of the PES spectrum. A rational procedure, based on the chemistry involved in the formation of anionic dimers, which enables the low-energy anions populated in the photoelectron spectrum to be identified is proposed. In contrast to the alternative combinatorial approach, which in the studied case would lead to carrying out quantum chemical calculations for 2000-2500 structures, the procedure described here reduces the computational problem to only 15 geometries.
首次记录到鸟嘌呤-胞嘧啶碱基对（GC）（*-）阴离子的光电子能谱。观察到光谱峰高比随源条件的变化表明存在两种或更多种阴离子异构体。在光电子光谱中，两个宽带的最大值被测量到约 1.9 和约 2.6 eV。这些值与 B3LYP/6-31++G(d,p)计算的低能阴离子结构的垂直离解能非常吻合，这些结构分别为：1）Watson-Crick 碱基对阴离子，质子从鸟嘌呤的 N1 转移到胞嘧啶的 N3；2）其类似物，质子从鸟嘌呤的 N9 转移到鸟嘌呤的 N7；3）全局最小几何形状，由后者阴离子通过围绕鸟嘌呤的 C2 和胞嘧啶的 C4 大致定义的轴旋转形成。此外，两种最低能量阴离子的稳定性差异较小，解释了实验观察到的 PES 光谱的源（温度）依赖性。提出了一种基于形成阴离子二聚体涉及的化学的合理程序，该程序可识别光电子光谱中存在的低能阴离子。与替代的组合方法形成对比，在研究的情况下，该方法将导致对 2000-2500 种结构进行量子化学计算，而此处描述的方法将计算问题减少到仅 15 种几何形状。
