Thermodynamics and Mechanisms for Decomposition of Protonated Glycine and Its Protonated Dimer

We present a full molecular description of fragmentation reactions of protonated glycine (G) and its protonated dimer, H+G2, by studying their collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). In contrast to previous results, it is clear that H+G decomposes by loss of CO followed by H2O. Analysis of the energy-dependent CID cross sections provides the 0 K barriers for these processes as well as for the binding energy of the dimer after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion–molecule collisions. Relaxed potential energy surface scans performed at the B3LYP/6-31G(d) level are used to map the reaction surfaces and identify the transition states (TSs) and intermediate reaction species for the reactions, structures that are further optimized at the B3LYP/6-311+G(d,p) level. Single-point energies of the key optimized structures are calculated at B3LYP and MP2(full) levels using a 6-311+G(2d,2p) basis set. These theoretical results are compared to extensive calculations in the literature and to the experimental energies. The combination of both experimental work and quantum chemical calculations allows for a complete characterization of the elementary steps of H+G and H+G2 decomposition. These results make it clear that H+G is the simplest model for the ‘‘mobile proton’’, a key concept in understanding the fragmentation of protonated proteins.