8-Nitroguanine (8-NitroG) is known to be a potent mutagenic product obtained from the guanine base of DNA that induces GC to AT transversion mutation. Its formation by the reaction of guanyl radical with nitrogen dioxide radical (NO;) has been observed experimentally. The reaction was studied here theoretically considering different reactant complexes, transition states, intermediate complexes, and product complexes. The solvent effect due to one, two, three. and six specific water molecules and the hulk solvent effect of aqueous media on the reaction were considered. With one, two, and three complexed water molecules, geometry optimization calculations were performed using the B3LYP and BHandHLYP functionals of density functional theory along with the 6-31G (d,p) and the AUG-cc-pVDZ basis sets in gas phase which was followed by single point energy calculations at the MP2/AUG-cc-pVDZ level. With six complexed water molecules, geometry optimization was performed at the BHandHLYP/6-31G(d,p) level of theory followed by single point energy calculations at all the other levels of theory mentioned above. Solvation in bulk aqueous media was treated at all the above-mentioned levels of theory using the polarizable continuum model (PCM). Specific water molecules were found to play catalytic roles in proton transfer processes that usually lowered the barrier appreciably.