The effect of backbone charge on the collision-induced dissociation of oligonucleotides

Knowledge of the effects of structural changes in oligonucleotides on their dissociation reaction is important in the application of mass spectrometry to sequence determination. The effect of backbone charge on the collision-induced dissociation of multiply-charged oligonucleotides produced by electrospray was explored by examination of models in which the normal phosphodiester linkage was partially replaced with an uncharged methylphosphonate (MP) linkage. Three different MP-containing oligonucleotides were studied, designed to represent a concentration of charge on the 5'- and 3'-ends of the molecule and with an even distribution of charge along the backbone, compared with a control molecule containing only phosphodiester linkages. In all MP-containing oligonucleotides charging of over 90% of phosphate groups were observed, compared with typical charging patterns of about 60% in normal all-phosphodiester oligonucleotides. This unexpected effect is attributed to charge stabilization by interactions of charged sites with uncharged residues. Analysis of the collision-induced dissociation mass spectra showed that backbone cleavage occurred at every residue (w and a-base ion series), producing a full set of sequencing ions whether or not the linkage at that site was formally charged. It is concluded that under the multiple collision conditions of the quadrupole collision cell that backbone cleavage proceeds through two generic pathways, one involving base loss followed by cleavage of the adjacent C3'-CO bond and the other requiring neither base loss nor charged phosphate at the cleavage site. These results suggest that backbone cleavage reactions in conventional phosphodiester oligonucleotides can occur at non-ionized linkage sites, of which there are a high proportion in both electrospray- and MALDI-produced molecular ions.