Mechanistic insight into photocrosslinking reaction between triplet state 4-thiopyrimidine and thymine

4-Thiopyrimidine (e.g., 4-thiouracil (4-TU) and 4-thiothymine (4-TT)) is a typical kind of thiobase. With the sulfur substitution at the C4 position of the canonical pyrimidine nucleobase, 4-thiopyrimidine displays unique photophysical and photochemical properties such as red-shifted maximum absorption peaks and efficient triplet state populations. One of the properties is the photocrosslinking reaction between 4-thiopyrimidine and pyrimidine base, which plays important roles in photochemotherapy and photolabeling applications. By using density functional theory (M06-2X), we have explored the potential energy profiles of the photocrosslinking reaction between 4-thiopyrimidine (4-TU and 4-TT) and thymine in the S₀ and T₁ states as well as the interaction between the two states. For both (6-4) and (5-4) photocrosslinking reactions, multiple nonadiabatic pathways via minimum energy crossing points (MECPs) between potential surfaces (PESs) of the T₁ and S₀ states greatly facilitate the proceeding of photocrosslinking reactions and lead to the relatively stable thietane intermediate in the S₀ state. The subsequent H migration in the thietane intermediate takes place solely in the S₀ state with surmountable energy barriers in bulk solution, resulting in the formation of the photocrosslinked product. This research provides not only a new mechanistic insight into the photocrosslinking reaction for 4-thiopyrimidines but also a rational explanation for the experiments of UVA irradiated Tp^4ST dinucleotide and 4-thiothymidine-containing oligonucleotides, facilitating the deep understanding of the synergistic cytotoxicity of 4-thiopyrimidines and UVA as well as the development of alternative phototherapeutic agents and photolabeling probes.