QM and QM/MM Studies on Excited-State Relaxation Mechanisms of Unnatural Bases in Vacuo and Base Pairs in DNA.

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 S2 state of d5SICS, the S1 state of dNaM, and the S2 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.