Thermodynamics and Reaction Mechanisms for Decomposition of a Simple Protonated Tripeptide, H+GAG: a Guided Ion Beam and Computational Study.

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated glycylalanylglycine (H+GAG) with Xe using a guided ion beam tandem mass spectrometer. Kinetic energy dependent cross sections for nine ionic products were observed and analyzed to provide 0 K barriers for the six primary products: [b2]+, [y1 + 2H]+, [b3]+, CO loss, [y2 + 2H]+, and [a1]+; and three secondary products: [a2]+, [a3]+, and CH3CHNH2+, after accounting for multiple ion-molecule collisions, internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-GD3BJ/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the six primary and one secondary products (where the other two secondary products have mechanisms previously established). Geometry optimizations and single-point energy calculations were performed at several levels of theory. These theoretical energies are compared with experimental threshold energies and are found to give reasonably good agreement, with B3LYP-GD3BJ and M06-2X levels of theory performing better than other levels. The results obtained here are also compared with previous results for decomposition of H+GGG. The primary difference observed is a lowering of the threshold for the [b2]+ product ion and a concomitant suppression of the directly competing [y1 + 2H]+ product, the result of specific methylation of the [b2]+ product ion.