Formation of polynuclear copper complexes of guanine-based nucleobases in the gas phase studied by ESI-MS

Abstract

Electrospray ionization of water/methanol solutions containing the DNA nucleoside deoxyguanosine and Cu(II) sulphate provide direct experimental evidence for the formation of a series of charged clusters containing up to four Cu(II) ions and up to three sulphate anion plus a variable number of deoxyguanosine ligands, which we refer to as polynuclear copper complexes. The complexes are of the type [Cum(dGuo)n(SO4)m−1]2+ with m=2–4 and n=4–11. Complexes containing two and three Cu(II) ions have relative abundances in the range 1–13%, thus representing a significant fraction of all the Cu-containing ion species. Their chemical composition is determined by high resolution mass measurements and confirmed by comparison of experimental and simulated isotope patterns. Complex stabilities and some structural information are assessed by collision induced fragmentation experiments. In some specific cases ion mobility measurements have been exploited to resolve the contribution of singly and doubly charge ions overlapping in the same m/z region. Some feasible structures are proposed in which two Cu moieties are bridged by a sulphate anion. In addition, singly charged ions formally corresponding to a complex between a protonated ligand and a CuSO4 moiety ([Cu(dGuo+H)(dGuo)n−1SO4]+, n=2–5) are observed. Finally, the effect of the counter-anion in the stabilization of the polynuclear Cu complexes is studied in a series of experiments preformed using Cu(II) nitrate and chloride salts to replace CuSO4, and the latter is found to have the highest propensity for polynuclear copper cluster generation.