Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China.
Institute of Physics, University of São Paulo, Rua do Matão 1371, São Paulo, SP 05508-090, Brazil.
J Phys Chem B. 2021 Feb 25;125(7):1778-1789. doi: 10.1021/acs.jpcb.0c10855. Epub 2021 Feb 11.
The excited-state properties and relaxation mechanisms after light irradiation of 6-selenoguanine (6SeG) in water and in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach with the multistate complete active space second-order perturbation theory (MS-CASPT2) method. In both environments, the S (nπ) and S (ππ) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T (ππ) and T (nπ), are found energetically below the S state. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S state and destabilizes the S, due to hydrogen-bonding interactions, but it does not affect the order of the states. The optimized minima, conical intersections, and singlet-triplet crossings are very similar in water and in DNA, so that the same general mechanism is found. Additionally, for each excited state geometry optimization in DNA, three kind of structures ("up", "down", and "central") are optimized which differ from each other by the orientation of the C═Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S state, 6SeG evolves to the S minimum, near to a S/S conical intersection that allows for internal conversion to the S state. Linear interpolation in internal coordinates indicate that the "central" orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S/S conical intersection. From the S state, 6SeG can further decay to the T (ππ) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T minimum and the T/S crossing point. Although this general S → T mechanism takes place in both media, the presence of DNA induces a steeper S potential energy surface, that it is expected to accelerate the S → S internal conversion.
使用量子力学/分子力学（QM/MM）方法结合多态完全活性空间二级微扰理论（MS-CASPT2）方法，研究了 6-硒代鸟嘌呤（6SeG）在水和 DNA 中经光照后的激发态性质和弛豫机制。在这两种环境中，S（nπ）和 S（ππ）态分别预测为光谱暗态和亮态。发现两个三重态 T（ππ）和 T（nπ）态在 S 态之下能量较低。将 QM 区域扩展到包含 6SeG-Cyt 碱基对会略微稳定 S 态，并由于氢键相互作用而使 S 态不稳定，但不会影响态的顺序。在水中和 DNA 中，优化的最小值、锥形交叉和单重态三重态交叉非常相似，因此发现了相同的一般机制。此外，对于 DNA 中每个激发态几何结构的优化，优化了三种不同构象（“上”、“下”和“中”），它们彼此之间的区别在于 C═Se 基团相对于周围鸟嘌呤和胸腺嘧啶碱基的取向不同。经 S 态激发后，6SeG 演变为 S 最小值，靠近允许内部转换回 S 态的 S/S 锥形交叉。在内部坐标中的线性插值表明，“中”构象不太有利，因为需要额外的能量来克服高势垒，才能到达 S/S 锥形交叉。从 S 态，6SeG 可以通过系间窜越进一步衰减到 T（ππ）态，在那里由于 T 最小值和 T/S 交叉点之间存在相当大的能量势垒，它将被捕获。尽管这种一般的 S→T 机制在两种介质中都发生，但 DNA 的存在会诱导出更陡峭的 S 势能表面，这有望加速 S→S 内部转换。
