González-Luque Remedios, Climent Teresa, González-Ramírez Israel, Merchán Manuela, Serrano-Andrés Luis
Instituto de Ciencia Molecular, Universitat de València, Apartado 22085, ES-46071 Valencia, Spain.
J Chem Theory Comput. 2010 Jul 13;6(7):2103-14. doi: 10.1021/ct100164m.
The present study provides new insight into the intrinsic mechanisms for the population of the triplet manifold in DNA nucleobases by determining, at the multiconfigurational CASSCF/CASPT2 level, the singlet-triplet states crossing regions and the main decay paths for their lowest singlet and triplet states after near-UV irradiation. The studied singlet-triplet interacting regions are accessible along the minimum energy path of the initially populated singlet bright (1)ππ* state. In particular, all five natural DNA/RNA nucleobases have, at the end of the main minimum energy path and near a conical intersection of the ground and (1)ππ* states, a low-energy, easily accessible, singlet-triplet crossing region directly connecting the lowest singlet and triplet ππ* excited states. Adenine, thymine, and uracil display additional higher-energy crossing regions related to the presence of low-lying singlet and a triplet nπ* state. These funnels are absent in guanine and cytosine, which have the bright (1)ππ* state lower in energy and less accessible nπ* states. Knowledge of the location and accessibility of these regions, in which the singlet-triplet interaction is related to large spin-orbit coupling elements, may help to understand experimental evidence such as the wavelength dependence measured for the triplet formation quantum yield in nucleobases and the prevalence of adenine and thymine components in the phosphorescence spectra of DNA.
本研究通过在多组态CASSCF/CASPT2水平上确定单重态-三重态交叉区域以及近紫外光照射后其最低单重态和三重态的主要衰变路径，为DNA核碱基中三重态流形的填充内在机制提供了新的见解。所研究的单重态-三重态相互作用区域可沿着初始填充的单重态亮态(1)ππ态的最小能量路径到达。特别是，所有五种天然DNA/RNA核碱基在主要最小能量路径的末端且靠近基态和(1)ππ态的锥形交叉点处，都有一个低能量、易于到达的单重态-三重态交叉区域，直接连接最低的单重态和三重态ππ激发态。腺嘌呤、胸腺嘧啶和尿嘧啶还显示出与低能单重态和三重态nπ态的存在相关的额外高能交叉区域。鸟嘌呤和胞嘧啶中不存在这些通道，它们的亮态(1)ππ态能量较低且nπ态较难到达。了解这些区域的位置和可达性，其中单重态-三重态相互作用与大的自旋-轨道耦合元素有关，可能有助于理解诸如核碱基中三重态形成量子产率的波长依赖性以及DNA磷光光谱中腺嘌呤和胸腺嘧啶成分的普遍性等实验证据。
