Deprotonation studies of Cu+–guanine and Cu2+–guanine complexes by theoretical investigation

The acidities of multiple sites in Cu+–guanine and Cu2+–guanine complexes have been investigated theoretically. To compare, the acidities of guanine (G) and guanine radical cation (G+) have also been included. The results clearly indicate that the acidities of CH or NH groups in Cu+/2+–guanine are significantly enhanced relative to the neutral guanine. The acidic order for guanine derivatives is as follows: Cu2+–guanine > G+ ⩾ Cu+–guanine > G. For Cu+/2+–guanine, N7-coordination exhibits N1–H acid, while for N3-coordination, Cu+–guanine and Cu2+–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 Cu2+–guanine of all the guanine derivatives studied. Consequent NBO analyses confirm this assumption. Also, electrostatic potential calculations of [G(–H)]− and [G(–H)] well reproduce the geometrical characters of the deprotonated structures.