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(+)G(2), 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 H(2)O. 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(+)G(2) 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.