title: Thermodynamics and mechanisms for decomposition of protonated glycine and its protonated dimer.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States.
authors.forename: P B
authors.initials: PB
authors.lastname: Armentrout
authors.forename: A L
authors.initials: AL
authors.lastname: Heaton
authors.forename: S J
authors.initials: SJ
authors.lastname: Ye
mesh_terms: D019281:Dimerization; D005998:Glycine / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D011522:Protons; D005998:Glycine