complex structure. The ab initio calculations were performed at B3LYP/6-31G(d) level.

The Cu-binding site in the [Cu·dCMP·dCMP-H] 1− complex was investigated. The tandem mass (MS/MS) spectra of the [Cu·dCMP·dCMP-H] 1− parent ion showed [dCMP·Cu·H 2 PO 4 + CONH] 1− fragment ions. Therefore, we propose that the Cu cation is simultaneously coordinated to the phosphate site and cytosine moiety in the stable geometry of the [Cu·dCMP·dCMP-H] 1− complex. Three geometries for the complex were considered in an attempt to optimize the structure of the [Cu·dCMP·dCMP-H] 1− complex. The ab initio calculations were performed at the B3LYP/6-311G** level.