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.

Thermodynamics and Mechanisms of Protonated Diglycine Decomposition: A Guided Ion Beam Study

“…Finally, it can be seen that the cross section for reaction 8 declines at higher energies, which can be attributed to the decomposition of the H + G (y 1 ) product ion to form a 1 in the overall reaction 9b. This subsequent decomposition has been verified by the CID of H + G in a previous study [10]. In contrast to the current study, Klassen and Kebarle [2] observed only the major y 1 and a 1 channels with relative intensities that exhibit qualitatively the same behavior as that shown in Figure 1.…”

“…The ionic product from reaction 5, (CH 2 NH 2 + )(G), can also dissociate further according to reactions 6, 8, and 9a, where the former passes over a tight TS, Figure 5 of paper I, and the latter two channels involve loose TSs, Figure 1 of paper I. Finally, the y 1 (H + G) product of reaction 8 can dissociate to form the a 1 product ion in reaction 9b, as previously elucidated [10].…”

“…In the most complete study to date, Siu and coworkers examined the threshold collision-induced dissociation of protonated GGG, AGG, and GAG and extracted threshold energy information that compares well with theory [3,9]. Recently, we completed a similar study of the simplest system of this type, protonated glycine, H + G [10]. These latter studies illustrate the utility of having good quantitative thermodynamic information available to confirm theoretical studies of mechanisms.…”

“…In addition, Equation 1 naturally includes competition among parallel reactions with a full statistical treatment [26]. Previous studies have verified the efficacy of this approach in modeling reactions that compete through loose as well as loose versus tight transition states [10,14,[27][28][29][30][31].…”

“…(The combined loss of CO+NH 3 in reaction 6 can also be included in this competition; however, because this channel involves a tight TS compared with the loose TSs for reactions 8 and 9a, its presence or absence does not affect the results.) The reaction coordinate surface for this subsequent dissociation has been discussed in detail in previous work and is limited by a tight TS, H + G([N-O 2 ]-ct), although some levels of theory find that the energy of the products lies slightly above this tight TS, such that a PSL TS may also be appropriate [10]. Results obtained assuming the tight TS for reaction 9b are shown in Figure 2d, where the deviation between experimental and predicted branching ratios resulting from the sequential dissociation is evident and can be easily modeled with the sequential dissociation model of Equation 3.…”