Direct evidence of singlet molecular oxygen [O₂ (¹Δg)] production from UVA excited 6-thioguanine

6-Thioguanine (6-TGua) is one of the thiopurines used as a cytostatic drug. When internalized by cells, 6-TGua is metabolized through the purine salvage pathway and readily incorporated into DNA. Patients treated with these thiopurines are more prone to develop squamous cell carcinoma of the skin. The absorbance spectrum of 6-TGua, in contrast to guanine (Gua) or any other canonical base, has a maximum absorbance at 342 nm. Therefore, 6-TGua undergoes photoexcitation upon exposure to UVA radiation, with maximum absorption at 340 nm. In this study, the used approach unequivocally demonstrates the generation and quenching of ¹O₂ by 6-TGua via the direct spectroscopic detection of ¹O₂ monomol light emission at 1270 nm. Chemiluminescence-based methods were employed for the determination of the ¹O₂ generation quantum yield (Φ¹O₂) and the total ¹O₂ quenching rate constant (k_t). For the ¹O₂ quantum yield, we found values of 0.22 ± 0.03 for 6-TGua and 0.12 ± 0.03 for 2'-deoxy-6-thioguanosine (6-TdGuo). These compounds presented k_t values of 1.5 × 10⁷ L mol^-1 s^-1 and 1.1 × 10⁷ L mol^-1 s^-1, respectively. Through the comparison of these values with the ones obtained for 2'-deoxyguanosine (dGuo) and its oxidation product 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo), we aim to provide new insights into the 6-TGua-containing DNA (DNA-6-TGua) reactivity towards ¹O₂ in a biological context.