Guided ion beam and computational studies of the decomposition of a model thiourea protein cross-linker

The dissociation of protonated methyl-d3 thiourea-4-butyric acid methyl amide (1), a model of thiourea-based protein cross-linking compounds, is examined both experimentally and computationally. Using a guided ion beam tandem mass spectrometer (GIBMS), the threshold collision-induced dissociation (TCID) of [1 + H](+) with Xe is examined as a function of collision energy. Analysis of the kinetic energy-dependent CID cross sections provides the 0 K barriers for four primary and four secondary dissociation pathways, after accounting for competition between channels, sequential dissociations, unimolecular decay rates, internal energy of reactant ions, and multiple ion-neutral collisions. Computations are used to explore the pathways for the various processes and elucidation of their rate-limiting transition states. These results indicate that dissociation is initiated by migration of the excess proton from sulfur to one of three nitrogen atoms in 1, similar to the "mobile proton" model of peptide fragmentation. The computational energies for the rate-limiting transition states are generally in good agreement with the experimentally derived threshold energies, with MP2(full)/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) results being particularly favorable. This good comparison validates the mechanisms explored theoretically and allows identification of the structures of the various product ions and neutrals.