Spectroscopic and Theoretical Evidence for Oxazolone Ring Formation in Collision-Induced Dissociation of Peptides

Infrared spectroscopy of the b 4 + fragment of Leu-enkephalin demonstrates that the oxazolone ring is formed during collision-induced dissociation of protonated peptides, whereas the linear acylium structure is not observed. Three distinct oxazolone structures are identified, based on the highly conformer-diagnostic CO stretching mode of the oxazolone ring, clearly showing that proton transfer from the oxazolone ring to the N-terminus takes place. Note that the presence of a cyclic peptide b 4 + isomer cannot be excluded.