5-Guanidino-4-nitroimidazole is a stable product from the peroxynitrite induced one-electron oxidation of guanine. Reaction mechanisms to form the 5-guanidino-4-nitroimidazole as well as 8-nitroguanine, through the combination of the guanine radical cation and nitrogen dioxide radical and through the combination of the deprotonated neutral guanine radical and nitrogen dioxide radical, have been investigated by the use of the B3LYP method of density functional theory. Our calculations suggest that the guanine radical cation mechanism is preferred over the neutral guanine radical mechanism and that a water molecule is involved in the reaction as a catalyst or as a reactant.