Institut de Química Computacional i Catàlisi, Universitat de Girona, Campus de Montilivi, 17071 Girona, Spain.
Photochem Photobiol Sci. 2013 Aug;12(8):1401-8. doi: 10.1039/c3pp50061f.
The photochemistry and photophysics of the amino-oxo (AO) and imino-oxo (IO) tautomers of 1-methylcytosine are investigated with ab initio calculations, using the CASPT2//CASSCF approach. Our aim is to rationalize recent experimental results which show that the AO tautomer has an excited state lifetime of about 1 ps, similar to cytosine (J.-W. Ho, H.-C. Yen, W.-K. Chou, C.-N. Weng, L.-H. Cheng, H.-Q. Shi, S.-H. Lai and P.-Y. Cheng, J. Phys. Chem. A, 2011, 115, 8406-8418), and that irradiation with wavelengths shorter than 308 nm induces the AO→IO tautomerization (I. Reva, M. J. Nowak, L. Lapinski and R. Fausto, J. Phys. Chem. B, 2012, 116, 5703-5710). For the canonical AO tautomer we find two analogous decay mechanisms to those described previously for cytosine, involving a conical intersection of ethylenic type and one where the amino group is bent out of plane. Decay through these intersections provides an unreactive return path to the AO ground state species. More importantly, we have identified new decay paths that lead from the two intersections to the trans-IO tautomer, without a barrier. These paths provide the possibility of forming the IO tautomer, presumably in small yields, as a side product of the radiationless decay. Thus, we have established for the first time computationally the mechanism of the UV-induced tautomerization, which is compatible with the well-established decay mechanism for cytosine. For the IO tautomer, we also find a mechanism for the excited state interconversion of the cis and trans forms and for efficient radiationless decay through a conical intersection where the imino group is perpendicular to the ring and bent out of the plane. These results are likely to be valid also for cytosine.
使用从头算（ab initio）计算，采用 CASPT2//CASSCF 方法，研究了 1-甲基胞嘧啶的氨基-氧代（AO）和亚氨基-氧代（IO）互变异构体的光化学和光物理性质。我们的目的是合理说明最近的实验结果，这些结果表明 AO 互变异构体的激发态寿命约为 1 ps，类似于胞嘧啶（J.-W. Ho、H.-C. Yen、W.-K. Chou、C.-N. Weng、L.-H. Cheng、H.-Q. Shi、S.-H. Lai 和 P.-Y. Cheng，J. Phys. Chem. A，2011，115，8406-8418），并且用波长小于 308nm 的光照射会诱导 AO→IO 互变异构化（I. Reva、M. J. Nowak、L. Lapinski 和 R. Fausto，J. Phys. Chem. B，2012，116，5703-5710）。对于典型的 AO 互变异构体，我们发现了两种与先前描述的胞嘧啶类似的衰减机制，涉及到双键型的锥形交叉和氨基基团离开平面的弯曲。通过这些交叉点的衰减提供了一条非反应性的返回 AO 基态物种的途径。更重要的是，我们已经确定了从两个交叉点到反式 IO 互变异构体的新的衰减途径，没有障碍。这些途径提供了形成 IO 互变异构体的可能性，可能是辐射衰减的副产物，产量很小。因此，我们首次从计算上确定了 UV 诱导互变异构化的机制，该机制与已确立的胞嘧啶衰减机制兼容。对于 IO 互变异构体，我们还发现了顺式和反式形式的激发态互变机制，以及通过一个锥形交叉点的有效非辐射衰减机制，其中亚氨基基团垂直于环并离开平面弯曲。这些结果可能也适用于胞嘧啶。
