Mass spectrometric study of the decomposition pathways of canonical amino acids and α‐lactones in the gas phase

The dissociation pathways of a gas‐phase amino acid with a canonical (non‐zwitterionic) α‐amino acid moiety are studied by using mass spectrometry. Investigation of the canonical amino acid moiety is possible because the ionized amino acid, a sulfonated phenylalanine, has a charge center that is separated from the amino acid, and dissociation occurs by charge‐remote fragmentation. The amino acid is found to dissociate only by loss of NH₃ upon collision‐induced dissociation to form a substituted α‐lactone. The dissociation is consistent with what has been observed previously upon pyrolysis of other α‐substituted carboxylic acids. Decarboxylation, which has also been reported previously for amino acid pyrolysis, is not observed, likely because the product would be a high‐energy, ammonium ylide. The resulting α‐lactone is found to undergo dissociation by decarbonylation to give an aldehyde, and by loss of CO₂. Decarboxylation is calculated to occur through a transition state involving hydride shift coupled with lactone ring‐opening. The transition state is found to be stabilized by the negative charge, and therefore, decarboxylation is more favorable for anions. The results 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. Copyright © 2015 John Wiley & Sons, Ltd.