Pentachlorophenol, a widespread environmental pollutant that is possibly carcinogenic to humans, is metabolically oxidized to tetrachloroquinone （TCBQ） which can result in DNA damage. We have investigated the photochemical reaction dynamics of TCBQ with two pyrimidine type nucleobases （thymine and uracil） upon UVA （355 ran） excitation using the technique of nanosecond time-resolved laser flash photolysis. It has been found that 355 nm excitation populates TCBQ molecules to their triplet state 3TCBQ＊, which are highly reactive towards thymine or uracil and undergo two parallel reactions, the hydrogen abstraction and electron transfer, leading to the observed photoproducts of TCBQH. and TCBQ.- in transient absorption spectra. The concomitantly produced nucleobase radicals and radical cations are expected to induce a series of oxidative or strand cleavage damage to DNA afterwards. By characterizing the photochemical hydrogen abstraction and electron transfer reactions, our results provide potentially important molecular reaction mechanisms for understanding the carcinogenic effects of pentachlorophenol and its metabolites TCBQ.
Among all the DNA components, extremely redox-active guanine （G） and adenine （A） bases are subject to facile loss of an electron and form cation radicals （G＋＂ and A＋＇） when exposed to irradiation or radical oxidants. The subsequent deprotonation of G＋＇ and A＋＇ can invoke DNA damage or interrupt hole transfer in DNA. However, compared with intensive reports for G＋, studies on the deprotonation of A＋ are still limited at present. Herein, we investigate the deprotonation behavior of A＋. by time-resolved laser flash photolysis. The deprotonation product of A（N6-H）＇ is observed and the deprotonation rate constant, （2.0±0.1）×10 7 s-1, is obtained at room temperature. Further, the deprotonation rate con- stants of A＋. are measured at temperatures varying from 280 K to 300 K, from which the activation energy for the N6-H deprotonation is determined to be （17.1±1.0） kJ/mol by Arrhenius equation. In addition, by incorporating the aqueous solvent effect, we perform density functional theory calculations for A＋ deprotonation in free base and in duplex DNA. Together with experimental results, the deprotonation mechanisms of A＋ in free base and in duplex DNA are revealed, which are of fundamental importance for understanding the oxidative DNA damage and designing DNA-based electrochemical devices.
