Ion/ion reactions of multiply charged nucleic acid anions: electron transfer, proton transfer, and ion attachment

Abstract

Charge transfer reactions, including electron transfer, proton transfer, and cation attachment, have been investigated for ion/ion reactions involving multiply charged oligonucleotide anions. Low-mass cations, such as the oxygen cation (O 2 +) and protonated isobutylene (C 4H 9 +), have been used to manipulate charge states of high-mass oligonucleotide anions with the “trapping by proxy” approach in a quadrupole ion trap. Oxygen cations react with oligonucleotide anions by electron transfer reactions, inducing a series of w- (or d-), (a-B)-, and z-type fragment ions for polydeoxyadenylate (polydA) and polydC anions. Much less fragmentation is noted for anions of polydT and 5′-d(CGGG) 5-3′. Protonated isobutylene, C 4H 9 +, undergoes proton transfer reactions with oligonucleotide anions with significantly less fragmentation. Protonated benzoquinoline (BQ) ions react with oligonucleotides predominantly by proton transfer with minimal fragmentation, but show a small degree of attachment to polydT and G-rich oligonucleotides. The extent of fragmentation associated with ion/ion reactions involving oligonucleotides appears to be directly related to ion/ion reaction exothermicity. Electron transfer ion/ion reaction products do not appear to be significantly more or less stable than proton transfer products of the same charge state. Protonated BQ, due to its higher mass than isobutene or oxygen, obviates the need for trapping by proxy and allows the ion parking technique to be applied to oligonucleotide anions. Peptide cations react with oligonucleotide anions via proton transfer but show extensive adduct ion formation as well. Neither the N-terminal amino group nor the C-terminal carboxy group appears to be essential in adduct ion formation. Rather, the possibility for multiple non-covalent interactions between polar groups on the peptide and oligonucleotide leads to the formation of stable adducts.