Multiconfigurational ab initio methods are used in this study to examine two initial reactions that take place during the OH radical attack of the DNA base guanine: a ring opening reaction and a hydrogen transfer reaction. The same reactions are also studied in the presence of a single water molecule. The ring opening reaction has a moderate barrier height of similar to 20-25 kcal/mol that is relatively insensitive to the presence of water. The barrier of the H-transfer reaction, on the other hand, is lowered from similar to 50 to similar to 22 kcal/mol when one water molecule is added, thus becoming comparable to the barrier height of the ring opening reaction.