title: Ammonia elimination from protonated nucleobases and related synthetic substrates.
abstract: The results are reported of mass-spectrometric studies of the nucleobases adenine 1h (1, R = H), guanine 2h, and cytosine 3h. The protonated nucleobases are generated by electrospray ionization of adenosine 1r (1, R = ribose), guanosine 2r, and deoxycytidine 3d (3, R = deoxyribose) and their fragmentations were studied with tandem mass spectrometry. In contrast to previous EI-MS studies of the nucleobases, NH(3) elimination does present a major path for the fragmentations of the ions [1h + H](+), [2h + H](+), and [3h + H](+). The ion [2h + H - NH(3)](+) also was generated from the acyclic precursor 5-cyanoamino-4-oxomethylene-dihydroimidazole 13h and from the thioether derivative 14h of 2h (NH(2) replaced by MeS). The analyses of the modes of initial fragmentation is supported by density functional theoretical studies. Conjugate acids 15-55 were studied to determine site preferences for the protonations of 1h, 2h, 3h, 13h, and 14h. The proton affinity of the amino group hardly ever is the substrate's best protonation site, and possible mechanisms for NH(3) elimination are discussed in which the amino group serves as the dissociative protonation site. The results provide semi-direct experimental evidence for the existence of the pyrimidine ring-opened cations that we had proposed on the basis of theoretical studies as intermediates in nitrosative nucleobase deamination.
authors.affiliation: Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211, USA.
authors.forename: Ming
authors.initials: M
authors.lastname: Qian
authors.forename: Shuo
authors.initials: S
authors.lastname: Yang
authors.forename: Hong
authors.initials: H
authors.lastname: Wu
authors.forename: Papiya
authors.initials: P
authors.lastname: Majumdar
authors.forename: Nathan
authors.initials: N
authors.lastname: Leigh
authors.forename: Rainer
authors.initials: R
authors.lastname: Glaser
mesh_terms: D000225:Adenine / Q000737:chemistry*; D000641:Ammonia / Q000737:chemistry*; D003596:Cytosine / Q000737:chemistry*; D006147:Guanine / Q000737:chemistry*; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D011522:Protons; D006147:Guanine; D000641:Ammonia; D003596:Cytosine; D000225:Adenine