Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University , Beijing 100875, China.
J Phys Chem B. 2017 Nov 22;121(46):10467-10478. doi: 10.1021/acs.jpcb.7b09046. Epub 2017 Nov 14.
Semisynthetic alphabet can potentially increase the genetic information stored in DNA through the formation of unusual base pairs such as d5SICS:dNaM. However, recent experiments show that near-visible-light irradiation on the d5SICS and dNaM chromophores could lead to genetic mutations and damages. Until now, their photophysical mechanisms remain elusive. Herein, we have employed MS-CASPT2//CASSCF and QM(MS-CASPT2//CASSCF)/MM methods to explore the spectroscopic properties and excited-state relaxation mechanisms of d5SICS, dNaM, and d5SICS:dNaM in DNA. We have found that (1) the S state of d5SICS, the S state of dNaM, and the S state of d5SICS:dNaM are initially populated upon near-visible-light irradiation and (2) for d5SICS and d5SICS:dNaM, there are several parallel relaxation pathways to populate the lowest triplet state, but for dNaM, a main relaxation pathway is uncovered. Moreover, we have found that the excited-state relaxation mechanism of d5SICS:dNaM in DNA is similar to that of the isolated d5SICS chromophore. These mechanistic insights contribute to the understanding of photophysics and photochemistry of unusual base pairs and to the design of better semisynthetic genetic alphabet.
半合成字母表可以通过形成不寻常的碱基对（如 d5SICS:dNaM）来潜在地增加 DNA 中存储的遗传信息。然而，最近的实验表明，近可见光照射 d5SICS 和 dNaM 生色团可能导致遗传突变和损伤。到目前为止，它们的光物理机制仍然难以捉摸。在此，我们采用 MS-CASPT2//CASSCF 和 QM(MS-CASPT2//CASSCF)/MM 方法研究了 DNA 中 d5SICS、dNaM 和 d5SICS:dNaM 的光谱性质和激发态弛豫机制。我们发现：(1) 近可见光照射后，d5SICS 的 S 态、dNaM 的 S 态和 d5SICS:dNaM 的 S 态最初被占据；(2) 对于 d5SICS 和 d5SICS:dNaM，有几种平行的弛豫途径来占据最低三重态，但对于 dNaM，则揭示了一种主要的弛豫途径。此外，我们发现 DNA 中 d5SICS:dNaM 的激发态弛豫机制与分离的 d5SICS 生色团的激发态弛豫机制相似。这些机制见解有助于理解不寻常碱基对的光物理和光化学，并有助于设计更好的半合成遗传字母表。
