Irradiation of oxygenated aqueous solutions of 6-mercaptopurine with near-u.v. light leads to purine-6-sulphinate as primary product. The sulphinate is itself unstable to u.v. light and is further oxidized to purine-6-sulphonate. Hypoxanthine is a minor product (<10%) of direct photolysis, but also arises from degradation of the sulphonate. Sensitized photo-oxidation using dyes (Methylene Blue, Rose Bengal, or Rhodamine B) gives the same products, and reaction in D2O occurs at a greater rate, indicating that the primary photoreaction occurs via addition of singlet molecular oxygen.
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Hemmens et al. (1984) studied this question.