Cis-trans isomerization is not rate determining for b2 ion structures: A guided ion beam and computational study of the decomposition of H+(GlyProAla)

Decomposition of the protonated tripeptide, GlyProAla (H+GPA), through collision-induced dissociation with Xe in a guided ion beam tandem mass spectrometer (GIBMS) is examined. Cross sections for the formation of [b2]+, [y2 + 2H]+, [a1]+, [b3]+, [a3]+, [a2]+, and H+(1-pyrroline) fragment ions were collected as a function of kinetic energy. Analysis of these cross sections include consideration of the effects of multiple collisions, dissociation lifetimes, reactant internal and kinetic energy distributions, competition among channels, and evolution into sequential channels. Structures and molecular parameters of reactants, transition states, intermediates, and products were identified using the B3LYP/6-311+G(d,p) level of theory, with single-point energy calculations performed at the B3LYP, B3P86, and MP2(full) levels of theory using a 6-311+G(2d,2p) basis set. Good agreement between experimental threshold energies and those calculated for rate-limiting transition states allow key reaction pathways for the formation of each product to be identified. The MP2(full) level of theory is found to agree best with the experimental results. Notably, even though H+GPA starts with a cis-peptide bond, it forms an oxazolone as its [b2]+ fragment and the absence of a diketopiperazine [b2]+ fragment is verified by other key observations.