Thermodynamics and mechanisms of protonated diglycine decomposition: a guided ion beam study

We present a full molecular description of fragmentation reactions of protonated diglycine (H+GG) by studying their collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Analysis of the kinetic energy-dependent CID cross sections provides the 0 K barriers for the sequential H2O + CO and CO + NH3 losses from H+GG as well as for the reactions involved in y1 and a1 ion formation, after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Here seven energetic barriers are measured for the fragmentation processes of H+GG, including the loss of H2O and of CO at ~140 and ~156 kJ/mol, the combined loss of (H2O + CO) and of (CO + NH3) at ~233 and ~185 kJ/mol, and formation of y1 and a1 ions at ~191 and ~212 kJ/mol, respectively, with a second channel for a1 formation opening at ~326 kJ/mol. Theoretical energies from the preceding paper are compared to our experimental energies and found to be in good agreement. This validates the mechanisms explored computationally, including unambiguous identification of the b2 ion as protonated 2-aminomethyl-5-oxazolone, thereby allowing a complete characterization of the elementary steps of H+GG decomposition. These results also demonstrate that all reactive species are available from the ground state conformation, as opposed to involving an initial broad distribution of protonated conformers. This result verifies the utility of the "mobile proton" model for understanding the fragmentation of protonated proteins.