Direct Observation and Quantitative Characterization of Singlet Oxygen in Aqueous Solution upon UVA Excitation of 6-Thioguanines

Abstract

The incorporation of 6-thioguanine (6-TG) into DNA increases the risk of ¹O₂-initiated skin cancer. We herein provide the first report on quantitative characterization of the photoactivity of 6-thioguanines including 6-TG and 6-thioguanosine. Time-resolved singlet oxygen luminescence was observed directly for the first time after UVA irradiation of 6-thioguanines in both CHCN₃ and aqueous solutions. Their photosensitization was characterized by the quantum yield of singlet oxygen production, showing a dramatic decrease over time from the initial 0.49−0.58 to zero. Experiments performed on both 6-TG and 6-thioguanosine did not show any significant difference in the quantum yield of singlet oxygen production, indicating that there was no potential participation of 7H- and 9H-tautomers. Our findings provide a primary basis for a better understanding of molecular events of thiopurine drugs in biological systems.