The acidities of multiple sites in Cu + -guanine and Cu 2+ -guanine complexes have been investigated theoretically. To compare, the acidities of guanine (G) and guanine radical cation (G Å+ ) have also been included. The results clearly indicate that the acidities of CAH or NAH groups in Cu +/2+ -guanine are significantly enhanced relative to the neutral guanine. The acidic order for guanine derivatives is as follows: Cu 2+ -guanine > G Å+ P Cu + -guanine > G. For Cu +/2+ -guanine, N7-coordination exhibits N1-H acid, while for N3-coordination, Cu + -guanine and Cu 2+ -guanine behave as N2-H2b and N9-H acid, respectively. Moreover, for the given coordination site, N7-coordination greatly decreases the gap between the most acidic group and the least one with respect to that in the neutral guanine. Additionally, it is found that C8-H and N2-H2b groups are significantly acidified with respect to the other groups in the coordination complexes. Natural population analysis assumes the deprotonation process of Cu +/2+ -guanine complexes and interprets the highest acidity of Cu 2+ -guanine of all the guanine derivatives studied. Consequent NBO analyses confirm this assumption. Also, electrostatic potential calculations of [G(-H)] À and [G(-H)] Å well reproduce the geometrical characters of the deprotonated structures.