Harańczyk Maciej, Rak Janusz, Gutowski Maciej
Chemical Sciences Division, Fundamental Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Phys Chem A. 2005 Dec 22;109(50):11495-503. doi: 10.1021/jp0535590.
We characterized valence anionic states of 1-methylcytosine using various electronic structure methods. We found that the most stable valence anion is related to neither the canonical amino-oxo nor a rare imino-oxo tautomer, in which a proton is transferred from the N4 to N3 atom. Instead, it is related to an imino-oxo tautomer, in which the C5 atom is protonated. This anion is characterized by an electron vertical detachment energy (VDE) of 2.12 eV and it is more stable than the anion based on the canonical tautomer by 1.0 kcal/mol. The latter is characterized by a VDE of 0.31 eV. Another unusual low-lying imino-oxo tautomer with a VDE of 3.60 eV has the C6 atom protonated and is 3.6 kcal/mol less stable than the anion of the canonical tautomer. All these anionic states are adiabatically unbound with respect to the canonical amino-oxo neutral, with the instability of 5.8 kcal/mol for the most stable valence anion. The mechanism of formation of anionic tautomers with carbon atoms protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to the C5 or C6 atom. The six-member ring structure of anionic tautomers with carbon atoms protonated is unstable upon an excess electron detachment. Indeed the neutral systems collapse without a barrier to a linear or a bicyclo structure, which might be viewed as lesions to DNA or RNA. Within the PCM hydration model, the anions become adiabatically bound with respect to the corresponding neutrals, and the two most stable tautomers have a carbon atom protonated.
我们使用各种电子结构方法对1-甲基胞嘧啶的价态阴离子态进行了表征。我们发现，最稳定的价态阴离子既不与标准的氨基-氧代互变异构体相关，也不与罕见的亚氨基-氧代互变异构体相关，在后者中质子从N4原子转移到N3原子。相反，它与一种亚氨基-氧代互变异构体相关，其中C5原子被质子化。这种阴离子的特征在于电子垂直脱离能（VDE）为2.12 eV，并且它比基于标准互变异构体的阴离子稳定1.0 kcal/mol。后者的VDE为0.31 eV。另一种具有3.60 eV的VDE的异常低位亚氨基-氧代互变异构体的C6原子被质子化，并且比标准互变异构体的阴离子稳定性低3.6 kcal/mol。所有这些阴离子态相对于标准氨基-氧代中性体都是绝热非束缚的，最稳定的价态阴离子的不稳定性为5.8 kcal/mol。碳原子质子化的阴离子互变异构体的形成机制可能涉及分子间质子转移或对标准中性互变异构体的解离电子附着，随后氢原子无障碍地附着到C5或C6原子上。碳原子质子化的阴离子互变异构体的六元环结构在过量电子脱离时是不稳定的。实际上，中性体系会无障碍地坍缩成线性或双环结构，这可能被视为对DNA或RNA的损伤。在PCM水合模型中，阴离子相对于相应的中性体变为绝热束缚，并且两个最稳定的互变异构体有一个碳原子被质子化。
