The radiation damage of adenine base was studied by B3LYP and MP2 methods in the presence of hydroxyl radicals to probe the reactivities of five possible sites of an isolated adenine molecule.Both methods predict that the C8 site is the more vulnerable than the other sites.For its bonding covalently with the hydroxyl radicals,B3LYP predicts a barfierless pathway,while MP2 finds a transition state with an energy of 106.1 kJ/mol.For the hydroxylation at the C2 site,the barrier was calculated to be 165.3 kJ/mol using MP2 method.For the dehydrogenation reactions at five sites of adenine,B3LYP method predicts that the free energy barrier decreases in the order of H8 ＞ H2 ＞ HN62 ＞ HN61 ＞ HN9.