The Cu-binding site in the $[Cu{cdot}dCMP{cdot}dCMP-H]^{1-}$ complex was investigated. The tandem mass (MS/MS) spectra of the [$[Cu{cdot}dCMP{cdot}dCMP-H]^{1-}$ parent ion showed $[dCMP{cdot}Cu{cdot}H_2PO_4+CONH]^{1-}$ 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 $[Cu{cdot}dCMP{cdot}dCMP-H]^{1-}$ complex. Three geometries for the complex were considered in an attempt to optimize the structure of the $[Cu{cdot}dCMP{cdot}dCMP-H]^{1-}$ complex. The ab initio calculations were performed at the $B3LYP/6-311G^{**}$ level.