Gas-phase formation of radical cations of monomers and dimers of guanosine by collision-induced dissociation of Cu(II)-guanosine complexes.

An electrosprayed water/methanol solution of guanosine and Cu(NO3)2 was observed to give rise to gas-phase copper complexed ions of [CuLn]2+, [CuL(MeOH)n]2+, and [CuG n(NO3)]+, as well as the ions [L]+, [L+H]+, [G]*+, and [G+H]+ (L=guanosine, G=guanine). The Collision-Induced Dissociation (CID) of [CuL3]2+ and [CuL(MeOH)n]2+ (n=2, 3) generates guanosine radical cations [L]+, while dimeric guanosine radical cations [L2]+ are generated in the dissociation of [CuL4]2+. Protonated guanosine [L+H]+ is one of the main products in the primary dissociation of [CuL2]2+, while the dissociation of the higher-order [CuG2]2+ produces the [G]+ radical cation. The guanosine dimer radical cation, [L2]+ presumably arises from the interaction of two guanosine molecules via proton and hydrogen bonding and is observed to dissociate into [L+H]+ and [L-H] at low energies. We propose that the first two ligands bind strongly with Cu(II) through N7 and O6 to form a [CuL2]*2+ complex with a four-coordinated planar structure and that a third ligand binds loosely with copper to form [CuL3]*2+. Additional ligation observed in the formation of [CuLn]2+ (n<or=6) ions is presumed to occur by hydrogen bonding. The ribose group of guanosine appears to play an important role in the stabilization of the doubly charged Cu-guanosine complex and in intraligand proton transfer upon CID. The molecular radical cations [L]+ observed in the ESI-MS spectrum at low declustering potentials originate primarily from [CuL(MeOH)2,3]*2+ complexes which can dissociate more easily than [CuL3]*2+.