Effect of the Asn side chain on the dissociation of deprotonated peptides elucidated by IRMPD spectroscopy

Infrared ion spectroscopy using the free electron laser FELIX was applied to identify the structure of b-type peptide fragments generated by collision and IR multiple-photon induced dissociation from singly deprotonated peptides containing an asparagine residue, in particular AlaAsnAla (ANA) and AlaAlaAsnAla (AANA). IR spectra were recorded over the 800-1800 cm(-1) spectral range by multiple-photon dissociation (IRMPD) spectroscopy and have been compared with density functional theory (DFF) calculated spectra at the B3LYP/6-31++G(d,p) level for different isomeric ion structures for structural characterization. Results unambiguously show that the b(2) and b(3) fragment anions do not possess the common oxazolone or diketopiperazine structure, but involve cyclization of the asparagine side chain. Nucleophilic attack from the side chain amide nitrogen on the peptide backbone carbonyl carbon leads to the formation of cyclic succinimide structures. Deprotonation is shown to occur on the succinimide nitrogen, which delocalizes the negative charge over two adjacent carbonyl groups thus enhancing the gas-phase stability. (C) 2013 Elsevier B.V. All rights reserved.