Abstract:

The effects of copper ions coordinated to N7 of adenine on the intramolecular protontransfer in adenine and between its base pairs were studied at the B3LYP／631+G* level. The results showed that protontransfer in copperadenine complex occurred very easily, and the effects were more pronounced for the divalent copper than for the univalent one. The monohydration of the copper ions disfavored the reaction due to the screening of electrostatic effects. For Cu+adenine system, the waterassisted intramolecular protontransfer 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 singleprotontransfer (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.

Key words: copper ions; adenine; base pair; proton-transfer; DFT