DNA环境中硒代胸腺嘧啶和腺嘌呤碱基对的激发态性质和光物理机理的理论研究

应用高精度的多态完全活化自洽场二级微扰理论方法,在量子力学/分子力学组合方法的理论框架QM(MS-CASPT2//CASSCF)/MM下,系统研究了DNA环境中2-硒和4-硒取代胸腺嘧啶和腺嘌呤碱基对(2SeT-A和4SeT-A)的最低5个电子态(S₀, S₁, S₂, T₂和T₁)的结构、性质和光物理过程. QM(MS-CASPT2//CASSCF)/MM计算揭示了DNA环境中2SeT-A和4SeT-A碱基对激发态性质和光物理过程差异性的来源,提出的机理将有助于理解DNA类似物的光物理过程,在光动力学治疗中具有潜在的应用.

Theoretical Study on the Excited State Properties and Photophysical Mechanism of Selenothymine and Adenine Base Pairs in DNA Environment

Multi-state second-order perturbation approach was applied， the combination of quantum mecha-nics/molecular mechanics methods， under the theoretical framework of the QM（MS-CASPT2//CASSCF）/MM， to study the relevant minima， conical intersections， crossing points， and excited-state relaxation paths in the lowest five electronic states（i.e.， S₀， S₁， S₂， T₂， and T₁） of 2- and 4-selenothymine and adenine bases（2SeT-A and 4SeT-A） in DNA. Although 2SeT-A and 4SeT-A share similar electron excitation states， their electron structural characteristics are different due to the different n and π orbitals involved， resulting in a higher vertical and adiabatic excitation energy for 2SeT-A than for 4SeT-A. In addition， both 2SeT-A and 4SeT-A have different minimum energy structures and inactivation pathways in the DNA environment. For 2SeT-A， after excitation， the system first reaches the S₂（ππ^*） state in the Franck-Condon region， then relaxes to the S₂ state minimal energy structure， and finally to the S₁（nπ^*） state. Next， the S₁→T₁ process with a large S₁/T₁ spin-orbit coupling reaches the T₁（ππ^*） state， which can be arrived to the S₀ state again through the T₁/S₀ intersection. But the minimum energy structure of T₁ and the barrier of 22.6 kJ/mol between T₁/S₀ intersection make the system stay in the lowest T₁ triplet state for a period of time. 4SeT-A has similar photophysical process of S₂→S₁→T₁， but its S₂→S₁ internal transformation process needs to overcome the energy barrier of 60.7 kJ/mol. The results reveals the different sources of the excitation state properties and photophysical mechanisms of 2SeT-A and 4SeT-A base pairs in DNA environment， and provides insight into the mechanism， which will help to understand the excitation state properties and photophysical mechanisms of bases substi- tuted by selenium atoms in complex environment.