Spectroscopic evidence for an oxazolone structure of the b2 fragment ion from protonated tri-alanine

Infrared multiple photon dissociation (IRMPD) spectroscopy is used to identify the structure of the b ₂⁺ ion generated from protonated tri-alanine by collision induced dissociation (CID). The IRMPD spectrum of b ₂⁺ differs markedly from that of protonated cyclo-alanine-alanine, demonstrating that the product is not a diketopiperazine. Instead, comparison of the IRMPD spectrum of b ₂⁺ to spectra predicted by density functional theory provides compelling evidence for an oxazolone structure protonated at the oxazolone N-atom.