COLLISION-INDUCED DISSOCIATION OF ADENINE

The collision-induced dissociation of protonated adenine, and of adenine monomethylated at positions N-1, C-2, N-3, N6, and N-7 has been studied by tandem mass spectrometry using models extensively labeled with stable isotopes. These data are used to gain understanding of the mechanisms of dissociation of protonated heterocycles in general, and as a basis for the applications of tandem mass spectrometry to structural studies of adenine derivatives and other nucleic acid constituents. Following collisional activation at 30-eV translational energy under multiple-collision conditions, protonated adenine undergoes decomposition along three independent major pathways which are of minor occurrence in electron ionization mass spectra: (1) expulsion of NH3, found to be derived in approximately equal proportion from endocyclic N-1 and exocyclic N6; (2) loss of NH2CN, derived almost exclusively from N-1-C-6-N6; and (3) formation of NH4+ directly from the protonated molecular ion, originating principally from N-1. A fourth major pathway which is also prevalent in electron ionization mass spectra involves sequential expulsion of three molecules of HCN, which occurs in the first step by highly selective loss of N-1 and C-2. Proposed mechanisms of dissociation are based on initial opening of the pyrimidine ring at N-1-C-6 or at N-1-C-2, resulting from protonation of N-1 in the primary ionization process. Activation of ions under single-collision conditions results in deposition of less internal energy, such that virtually no fragmentation occurs beyond the first step of each of the four principal pathways. Under these conditions greater selectivity in favor of loss of N-1 in expulsion of NH3 occurs, but with less effect on the other three pathways. Mass spectra of protonated methyladenine isomers and their N-15- and H-2-labeled analogues strongly support the occurrence of reaction paths analogous to those of adenine, and demonstrate the charge-localized pyrimidine ring to be the initial site of dissociation reactions.