title: Gas-phase tautomers of protonated 1-methylcytosine. Preparation, energetics, and dissociation mechanisms.
abstract: Tautomers of 1-methylcytosine that are protonated at N-3 (1+) and C-5 (2+) have been specifically synthesized in the gas phase and characterized by tandem mass spectrometry and quantum chemical calculations. Ion 1+ is the most stable tautomer in aqueous and methanol solution and is likely to be formed by electrospray ionization of 1-methylcytosine and transferred in the gas phase. Gas-phase protonation of 1-methylcytosine produces a mixture of 1+ and the O-2-protonated tautomer (3+), which are nearly isoenergetic. Dissociative ionization of 6-ethyl-5,6-dihydro-1-methylcytosine selectively forms isomer 2+. Upon collisional activation, ions 1+ and 3+ dissociate by loss of ammonia and [C,H,N,O], whose mechanisms have been established by deuterium labeling and ab initio calculations. The main dissociations of 2+ following collisional activation are losses of CH2=C=NH and HN=C=O. The mechanisms of these dissociations have been elucidated by deuterium labeling and theoretical calculations.
authors.affiliation: Department of Chemistry, Bagley Hall, Box 3517090, University of Washington, Seattle, WA 98195-1700, USA.
authors.forename: Chunxiang
authors.initials: C
authors.lastname: Yao
authors.forename: Maria L
authors.initials: ML
authors.lastname: Cuadrado-Peinado
authors.forename: Miroslav
authors.initials: M
authors.lastname: Polásek
authors.forename: Frantisek
authors.initials: F
authors.lastname: Turecek
mesh_terms: D000465:Algorithms; D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry; D005740:Gases / Q000737:chemistry*; D013058:Mass Spectrometry; D008956:Models, Chemical; D011522:Protons; D013237:Stereoisomerism
chemical_list: D005740:Gases; D011522:Protons; C031532:1-methylcytosine; D003596:Cytosine