The effects of copper ions coordinated to N7 of adenine on the intramolecular protontransfer in adenine and between its base pairs were studied at the B3LYP／631+G* level. The results showed that protontransfer in copperadenine complex occurred very easily, and the effects were more pronounced for the divalent copper than for the univalent one. The monohydration of the copper ions disfavored the reaction due to the screening of electrostatic effects. For Cu+adenine system, the waterassisted intramolecular protontransfer in Cu+adenine occurred more easily. On the other hand, for Cu2+AT (or Cu2+AU) system, two possible reaction pathways were calculated: deprotonation before protonation of adenine (Path1) and protonation before deprotonation of adenine (Path2). First, for Cu2+AT (or Cu2+AU), the divalent copper interaction could stabilize the ion pair structure derived from singleprotontransfer (SPT) reaction. Moreover, the SPT reaction was largely favored due to the increased acidity of the base monomers. Second, for the Cu2+-AT system, the structure resulting from the double-proton-transfer (DPT) reaction (N1-NT and N6-OT) was not stable contrasting to the ［AT］+system observed. Moreover, the analysis of the potential energy surface and the energies indicated that the better reaction pathway tend to Path2. The ion pair structure derived from the N1-NT SPT reaction could easily transform to Cu2+-AT by surmounting a very small barrier. So for the Cu2+-AT system, the Cu2+-AT without proton transfer was the dominating species in the gas phase. However, both SPT and DPT in the Cu2+-AU system could all occur, of which the latter was less favorable than the former. The same result was also found in the ［AU］+system. The calculated results showed that the two SPT pathways existed competition effect, and the DPT reaction tended more to Path2 than what was observed for the ［AU］+system.
