Gas-phase tautomers of protonated 1-methylcytosine. Preparation, energetics, and dissociation mechanisms

✍ Scribed by Chunxiang Yao; Maria L. Cuadrado-Peinado; Miroslav Polášek; František Tureček

• Book ID: 102380655
• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 310 KB
• Volume: 40
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.923

✦ Synopsis

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 CH~2~CNH and HNCO. The mechanisms of these dissociations have been elucidated by deuterium labeling and theoretical calculations. Copyright © 2005 John Wiley & Sons, Ltd.