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 H(2)O+CO and CO+NH(3) losses from H(+)GG as well as for the reactions involved in y(1) and a(1) 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 H(2)O and of CO at ~140 and ~156 kJ/mol, the combined loss of (H(2)O+CO) and of (CO+NH(3)) at ~233 and ~185 kJ/mol, and formation of y(1) and a(1) ions at ~191 and ~212 kJ/mol, respectively, with a second channel for a(1) formation opening at ~326 kJ/mol. Theoretical energies from the preceding paper are compared with our experimental energies and found to be in good agreement. This validates the mechanisms explored computationally, including unambiguous identification of the b(2) 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.