Collision-induced dissociation of singly and doubly charged CuII–cytidine complexes in the gas phase: an experimental and computational study

Abstract

The complexes of doubly charged cations, [CuL_n]²⁺, [CuL(MeOH)_n]²⁺ and single charged cations, [Cu(L–H)L_n]⁺, [L+H]⁺, [L₂+H]⁺, and [B+H]⁺, etc. (L = cytidine, B = cytosine) were observed via electrospray of water and methanol solutions of Cu(NO₃)₂ and cytidine. The peak distribution for [CuL_n]²⁺ shows that the [CuL₂]²⁺ and [CuL₄]²⁺ are the highest peaks, then followed by [CuL₃]²⁺, [CuL₅]²⁺, etc.Collision-induced dissociation (CID) of [Cu(L–H)L_n]⁺(n = 0,1), [CuL_n]²⁺(n = 2–6) and [CuL(MeOH)_n]²⁺(n = 1, 2) was investigated using the ESI-MS/MS instrument, and the structure and thermal energy for the most of these complexes were also calculated with ab initio and DFT methods through Gaussian 09 program. For [Cu(L–H)L_0-1]⁺, the primary dissociation only involved a ribose group, including ribose group lost, ribose side chain and ribose ring broken. The calculation result shows that the deprotonated site for [Cu(L–H)]⁺ is the N-atom in the amino group of the cytidine base. The inter-ligand proton transfer was observed to dominate the CID of all doubly charged Cu^II–cytidine complexes, [CuL_n]²⁺ (n = 2–6), and other channels, including charge reduction dissociation, lost neutral ligands were also observed. The protonated cytidine dimer, [L₂+H]⁺, produced in the CID of [CuL_n]²⁺ (n = 4–6), can further break into [L+ H]⁺ and neutral L, and the structure is proposed to be from the interaction of two cytidinesviahydrogen bonds. The protonated trimeric cytidine was also observed in the dissociation of [CuL₆]²⁺. Three charge reduction dissociation channels were observed in the CID of [CuL(MeOH)]²⁺. Unlike the Cu^II–guanosine complex, the radical cation [CuL_n]⁺ was not observed in the CID process of the [CuL_n]²⁺, and the reasons are attributed to the higher ionization energy and proton affinity, which makes the inter-ligand proton transfer more favorable than the electron transfer from ligand to Cu²⁺.