Gas-phase formation and reactions of radical cations of guanosine, deoxyguanosine and their homodimers and heterodimers

Abstract

Electrospray ionisation of methanolic solutions containing a mixture of the nucleoside deoxyguanosine, dG, incubated with Cu(NO3)2 resulted in the formation of a range of ions, including doubly charged copper nucleoside complexes [CuIIdGn]2+, with n ranging from 2 to 10. Collision-induced dissociation of these complexes proceeds via a number of different pathways that depend on the size of the cluster, n. When n=3, monomeric radical cations are formed via redox processes. When n=4, dimeric radical cations are formed. Related complexes are formed for the nucleoside guanosine, Gs, and these [CuIIGsn]2+ complexes fragment in similar fashions to their [CuIIdGn]2+ counterparts. A key finding is that the radical cations of dG and Gs have fragmentation patterns that depend on the way they are formed. Thus radical cations, dG+ and Gs+, formed directly in the electrospray ionisation source or via collision-induced dissociation of [CuIIdG3]2+ and [CuIIGs3]2+ complexes fragment in the same way, giving the radical cation of the guanine base at m/z 151 via cleavage of the N-glycosidic bond. In contrast, the collision-induced dissociation spectra of radical cations formed via the sequences [CuIIdG4]2+→dG2+→dG+ and [CuIIGs4]2+→Gs2+→Gs+ are dominated by the loss of CH2O and further loss of C2H3O2 from the sugar moiety. These different fragmentation reactions are attributed to different tautomeric structures of the radical cations. Quantum chemical calculations were carried out on possible structures of the radical cation dimer of the model 9-methylguanine. Three low energy structures were found. Two of these represent base pairs of the kind found in supramolecular motifs of guanine derivatives, and one of these possesses a novel tautomeric structure that may have important biological implications.