In fl uence of N ‐ terminal Residue Composition on the Structure of Proline-Containing b 2 + Ions

To probe the structural implications of the proline residue on its characteristic peptide fragmentation patterns, in particular its unusual cleavage at its Cterminus in formation of a b2 ion in XxxProZzz sequences, the structures of a series of proline-containing b2 + ions were studied by using action infrared multiphoton dissociation (IRMPD) spectroscopy and fragment ion hydrogen−deuterium exchange (HDX). Five different Xxx-Pro b2 + ions were studied, with glycine, alanine, isoleucine, valine, or histidine in the N-terminal position. The residues selected feature different sizes, chain lengths, and gas phase basicities to explore whether the structure of the Nterminal residue influences the Xxx-Pro b2 + ion structure. In proteins, the proline side chain-to-backbone attachment causes its peptide bonds to be in the cis conformation more than any other amino acid, although trans is still favored over cis. However, HP is the only b2 + ion studied here that forms the diketopiperazine exclusively. The GP, AP, IP, and VP b2 + ions formed from protonated tripeptide precursors predominantly featured oxazolone structures with small diketopiperazine contributions. In contrast to the b2 + ions generated from tripeptides, synthetic cyclic dipeptides VP and HP were confirmed to have exclusive diketopiperazine structures.