Photophysical and photochemical properties of the T1 excited state of thioinosine

UV–VIS absorption, circular dichroism, room-temperature emission and nanosecond transient absorption measurements as well as steady-state photochemical methods were used to determine the photophysical and photochemical properties of the lowest excited triplet state (T1) of 2′,3′,5′-tri-O-acetyl-thioinosine (6-thiopurine 2′,3′,5′-tri-O-acetyl-riboside, TI) in acetonitrile (ACN) solutions. The experimental data were supplemented by ab initio quantum chemical calculations on 9-methyl-6-thiopurine (Me9TP), a model molecule for the nucleoside. The ground state tautomeric structures were computed at the MP2/aug-cc-pVTZ level of theory, and the CPCM model was used for an evaluation of the solvent effects. The results show that, in contrast to the gas phase, the thione tautomer is about 6 kcal/mol more stable than the thiol form in ACN solution. The predominance of the thione tautomer of TI in solution was confirmed by the good agreement between the measured UV absorption spectrum of TI and the calculated singlet electronic transitions and intensities of the thione form of Me9TP. In addition there was a very close resemblance between the experimental UV spectrum of TI and that of its derivative fixed by the methyl group in the thione tautomeric form. The T1 state of TI was characterized by its energy, phosphorescence (ϕp0), nonradiative process (ϕnr), photochemical reaction (ϕpch) quantum yields, intrinsic lifetime (τT0), rate constants of self-quenching (ksq), phosphorescence (kp), and nonradiative processes (knr). The rate constants for the quenching of the T1 state of TI by standard triplet quenchers (O2, KI) and by common constituents of nucleic acids (pyrimidine and purine nucleosides) were also determined. The results show that the T1 state of TI exhibits the properties typical of the 3(ππ*) states of aromatic thiocarbonyl compounds, i.e. weak room-temperature phosphorescence (ϕp0=2×10−4), fast self-quenching process (ksq = 7.4 × 109 M−1 s−1) and high reactivity towards O2 (kq ∼ 6.8 × 109 M−1 s−1). Mechanistic studies of the steady-state photolysis of TI in air-equilibrated ACN solutions revealed that TI acts as a sensitiser and an acceptor of singlet oxygen.