Gas-Phase Formation of Radical Cations of Monomers and Dimers of Guanosine by Collision-Induced Dissociation of Cu(II)-Guanosine Complexes

摘要

An electrosprayed water/methanol solution of guanosine and Cu(NO3)2 was observed to give rise to gas-phase copper complexed ions of[CuL_n]~(·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 06 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[CuL_n]~(·2+)(n<=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+).