Bond dissociation energies in glycine, alanine, and dipeptide deprotonated anions for use in analyzing collision-induced dissociation processes

Abstract

Enthalpies of product formation for both direct peptide backbone bond dissociation and fragment rearrangement pathways have been predicted at the composite correlated molecule orbital theory G3(MP2) level for the dissociation of the [M−H]− precursor anions of glycine, alanine, the dipeptides, and the amide forms of these species. The lowest energy isomers for bond cleavage and rearrangement pathways were determined by extensive sampling of the available structures for each fragment using density functional theory. The G3(MP2) results are compared to the experimental low-energy collision-induced dissociation (CID) negative ion mode mass spectra. The y1− anion, NH2CHRCO2− was the most intense product ion in the CID spectra for the dipeptide species and their amides. Among amide species, NCO− was universally observed and the enthalpies for generation of this anion are less than ±5 kcal/mol. Amino acids and their amides consistently lost neutral HNCH-R (R H or CH3). Loss of the C-terminal carbon as CO2 was only observed for diglycine and dialanine but loss of CO2 or HNCO were not observed for the amino acids or any of the amide structures. The hydrogen at the C-terminus of the amino acid and dipeptide amide molecules facilitated the observable loss of H2O in the experimental spectra. Loss of H2O was not observed for non-amide species. Several energetically favorable fragmentation pathways were not detected in the low-energy CID mass spectra for these deprotonated anions. Selected transition states were calculated, and show that a number of the observed or low-energy pathways involve multistep mechanisms.