Mass spectrometric characterization of remotely charged amino acids and peptides
Ion–molecule reactions in a flowing afterglow are used to examine the electronic structure of 3- and 4-pyridinylnitrene-n-oxide radical anions. Reactions with nitric oxide are generally similar to those reported previously for other aromatic nitrene radical anions. In particular, phenoxide formation by nitrogen–oxygen exchange is observed with both isomers. Oxygen atom abstraction by NO is also observed with both isomers. Very significant differences in the reactivity are observed in the reactions of the two isomers with carbon disulfide. The reactivity of the 3-noxide isomer with CS2 is similar to that observed previously for nitrene radical anions, and reactions of the n-oxide moiety are not observed, similar to what is expected based on solution chemistry. The 4-n-oxide isomer, however, undergoes many reactions, including oxygen atom and oxygen ion transfer and sulfur–oxygen exchange, that involve the n-oxide oxygen. The increased reactivity of the oxygen is attributed to increased charge density at the oxygen due to pi electron donation of the nitrene anion in the para position. The dissociation pathways of gas-phase amino acids with a canonical (nonzwitterionic) α-amino acid moiety are studied by using mass spectrometry. Investigation of the canonical amino acid moiety is possible because the ionized amino acid has a charge center that is separated from the amino acid, and dissociation occurs by charge-remote fragmentation. The negatively charged amino acid is found to dissociate only by loss of NH3 upon collision-induced dissociation to form a substituted α-lactone. The collision-induced dissociation spectrum of the positively charged amino acid is complex with possibly 5 different fragmentation pathways. The results on negatively charged amino acid, para-sulfonated phenylalanine (Phe*), show that remote ionic groups can be used as mostly inert charge carriers to enable mass spectrometry to be used to investigate the gas-phase physical and chemical properties of different types of functional groups, including amino acids. The b2 ion formed upon charge remote fragmentation of dipeptides, Phe*- GlyOH and Gly-Phe*OH, is characterized using LCQ-Deca mass spectrometry. Comparison with authentic samples confirmed the lack of diketopiperazine or oxazolone on b2 ion and electronic structure calculation suggested that the b2 ion is oxazolone-enol.
Wenthold, Purdue University.
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