Theoretical study of cytosine-Al, cytosine-Cu and cytosine-Ag (neutral, anionic and cationic).

The binding of cytosine to Al, Cu and Ag has been analyzed using the hybrid B3LYP density functional theory method. The three metals all have open shell electronic configuration, with only one unpaired valence electron. Thus it is possible to study the influence of electronic configuration on the stability of these systems. Neutral, cationic and anionic systems were analyzed, in order to assess the influence of atomic charge on bond formation. We argue that in the case of anions, nonconventional hydrogen bonds are formed. It is generally accepted that the hydrogen bond A-H...B is formed by the union of a proton donor group A-H and a proton acceptor B, which contains lone-pair electrons. In this study, we found that in the case of (Cu-cytosine)(-1) and (Ag-cytosine)(-1), N-H...Cu and N-H...Ag bonds are geometrically described as nonconventional hydrogen bonds. Their binding energies fall within the range of -20.0 to -55.4 kcal/mol (depending on the scheme of the reaction) and thus they are classified as examples of strong (>10 kcal/mol) hydrogen bonds.