Collision-Induced Dissociation of Purine Antiviral Agents: Mechanisms of Ion Formation Using Gas-Phase Hydrogen/Deuterium Exchange and Electrospray Ionization Tandem Mass Spectrometry

Abstract

Electrospray ionization (ESI) and collision-induced dissociation (CID) mass spectra were obtained for two purine nucleoside antiviral agents (acycloguanosine and vidarabine) and one purine nucleotide (vidarabine monophosphate) and for the corresponding compounds in which the labile hydrogens were replaced by deuterium gas-phase exchange. The number of labile hydrogens, x, was determined from a comparison of ESI spectra obtained with N₂ and with ND₃ as the nebulizer gas. CID mass spectra were obtained for [M + H]⁺ and [M – H]⁻ ions and the exchanged analogs, [M(D_x) + D]⁺ and [M(D_x) – D]⁻, produced by ESI using a Sciex API-III^plus mass spectrometer. Compositions of product ions were determined and mechanisms of decomposition elucidated by comparison of the CID mass spectra of the undeuterated and deuterated species. Protonated purine antiviral agents dissociate through rearrangement decompositions of base-protonated [M + H]⁺ ions by cleavage of the glycosidic bonds to give the protonated bases with a sugar moiety as the neutral fragment. Cleavage of the same bonds with charge retention on the sugar moiety gives low abundance ions, due to the low proton affinity of the sugar moiety compared with that of the purine base. CID of protonated purine bases [B + H]⁺ occurs through two major pathways: (1) elimination of NH₃ (ND₃) and (2) loss of NH₂CN (ND₂CN). Minor pathways include elimination of HNCO (DNCO), loss of CO and loss of HCN (DCN). Deprotonated acycloguanosine and vidarabine exhibit the deprotonated base [B – H]⁻ as a major fragment from glycosidic bond cleavage and charge delocalization on the base. Deprotonated vidarabine monophosphate, however, shows predominantly phosphate-related product ions. CID of deprotonated guanine shows two principal pathways: (1) elimination of NH₃ (ND₃) and (2) loss of NH₂CN (ND₂CN). Minor pathways include elimination of HNCO (DNCO), loss of CO and loss of HCN (DCN). The dissociation reactions of deprotonated adenine, however, proceed by elimination of HCN and elimination of NCHNH (NCHND). The mass spectra of the antiviral agents studied in this paper may be useful in predicting reaction pathways in other heteroaromatic ring decompositions of nucleosides and nucleotides.