Spiroiminodihydantoin Is the Major Product of the 8-Oxo-7,8-dihydroguanosine Reaction with Peroxynitrite in the Presence of Thiols and Guanosine Photooxidation by Methylene Blue

[reaction: see text]. The potent oxidant, peroxynitrite, will oxidize 8-oxo-7,8-dihydroguanosine to give several products. In the presence of a thiol agent, the major final product has been determined to be a spiroiminodihydantoin compound. Additionally, we have found that the spiroiminodihydantoin, and not the previously reported 4-hydroxy-8-oxo-4,8-dihydroguanosine, is the major final product formed during the methylene blue-mediated photooxidation of guanosine.