Spectroscopic Evidence for an Oxazolone Structure of the b(2) 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. (J Am Soc Mass Spectrom 2009, 20, 334-339) (C) 2009 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry