Analysis of the [CuL n ]2+ and [CuG n ]2+ (n = 2–4) complex structures: Comparison with CID experiment and DFT calculation

The collision-induced dissociation (CID) of the copper-cytidine complex [CuL n ]2+ (L is cytidine, n = 2–4) shows that the inter-ligand proton transfer (PT) is the dominating process. This is quite different from the CID of copper-guanosine complex [CuG n ]2+ (G is guanosine, n = 2–4), in which the inter-electron transfer (ET) dominates. The possible structures and zero-point energies for the majority of these structures were calculated using density functional theory (DFT) methods, and the ΔG Gibbs energy analysis of the CID processes also demonstrated the difference between the two complex structures. The results show that the steric hindrance effects and the intermolecular hydrogen bonds are the main reasons that cause the coordination between Cu(II) and the ligands of the cytidine and guanosine molecule. Cu(II) coordinates 4 ligands for [CuL n ]2+, forming the primary order sphere and presenting non-covalent bonding of the ligands. A greater steric effect was observed in the purine ring, which could be unstable if the guanosine molecules distributed around the copper ions to form coordination compounds. The [CuG n ]2+ complex tends to form intermolecular hydrogen bonds to enhance its structural stability by amino N and carbonyl O of purine ring.