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Infrared multiple-photon dissociation (IRMPD) spectroscopy and DFT calculations have been used to probe the most stable structures of a 3 * and a 4 * ions derived from both protonated pentaglycine (denoted G5) and pentaalanine (A5). The a 3 * and a 4 * ions derived from protonated A5 feature a CHR=N-CHR’- group at the N-terminus and an oxazolone ring at the C-terminus, as proposed previously [J. Am. Soc. Mass Spectrom. 19, 1788–1798 (2008)]. The isomeric a 4 * ion derived from A5 with a 3,5-dihydro-4H-imidazol-4-one ring structure was calculated to have a slightly better energy than the oxazolone, but the barrier to its formation is higher and there was no evidence of this ion in the IRMPD spectrum. By contrast, the a 4 * and [a 4 – H2O]+ (denoted a 4 0 ) ions from G5 gave strikingly similar IRMPD spectra and both have the 3,5-dihydro-4H-imidazol-4-one ring structure similar to that recently reported for the [GGGG + H – H2O]+ ion [Int. J. Mass Spectrom. 316–318, 268–272 (2012)]. In the absence of a solvent molecule, the pathway to the oxazolone is calculated to be lower than those to thermodynamically more stable products, the a 4 0 and the a 4 * with the 3,5-dihydro-4H-imidazol-4-one ring structure. Incorporation of one water molecule is sufficient to reduce the barrier to formation of the a 4 0 of G5 to below that for formation of the oxazolone. On the equivalent potential energy surface for protonated A5 the barrier to formation of the a 4 0 ion is 12.3 kcal mol–1 higher than that for oxazolone formation and the a 4 0 ion is not observed experimentally.

Andrii Piatkivskyi, Sandra Osburn, Kendall Jaderberg, Josipa Grzetic, Jeffrey D. Steill, Jos Oomens, Junfang Zhao, Justin Kai-Chi Lau, Udo H. Verkerk, Alan C. Hopkinson, K. W. Michael Siu, Victor Ryzhov Department of Chemistry and Biochemistry, and Center for Biochemical and Biophysical Studies, Northern Illinois University, DeKalb, IL 60115, USA FOM Institute for Plasma Physics, Rijnhuizen 14, 3439 MN Nieuwegein, The Netherlands University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands Institute for Molecules and Materials (IMM), FELIX facility, Radboud University Nijmegen, Heyendaalseweg 135, 6525AJ Nijmegen, The Netherlands Department of Chemistry and Centre for Research in Mass Spectrometry, York University, Toronto, ON, Canada Sandia National Laboratories, Livermore, CA 94550-0969, USA