| This dissertation project investigates protonated and alkali-metalated cationic peptides with the use of a trisector mass spectrometer. Metastable ion (MI) decomposition and collisionally activated dissociation (CAD) are used to assess the structures, stabilities and reactivities of these species. In addition to the experiments, density functional theory calculations were performed to support some experimental findings.; The study of a series of protonated dipeptides, [XxxYyy + H]+ , proved that CO loss occurs on the a1 - y1 pathway, which ultimately creates the proton-bound dimer of an amino acid and an imine. The water loss promotes instead formation of an ion with oxazolone structure. The highest yields of [XxxYyy + H - CO] + and [XxxYyy + H - H2O]+ are observed at threshold energies. As the internal energy of the protonated dipeptides increases, these primary products are depleted by consecutive dissociations yielding mostly backbone fragments. The dissociations leading to eliminations of small neutrals generally proceed over transition states that lie higher in energy than the corresponding dissociation products, with the excess energy disposed of either in translational or rovibrational modes.; The study of mono- and dilithiated dipeptide isomers PheGly and GlyPhe unveiled remarkable findings. Metastable monolithiated PheGly and GlyPhe produce the same patterns of fragmentation through a mixed anhydride intermediate in which the sequence information is lost. In sharp contrast, the corresponding dilithiated species give distinctively different patterns of fragmentations, which makes sequencing possible.; The energetics of fragmentation of mono- and dilithiated PheGly and GlyPhe were determined by breakdown graphs, providing a wealth of information about their unimolecular chemistry. The types of fragments dominating at different energy ranges characterize the reactivity of the systems. It is observed that all species lose C6H5CH2•, leading to charged mono- and dilithiated GlyGly radicals, with an unpaired electron at the alpha-carbon of either the N- or C-terminal Gly residue.; Extensive MS3 experiments on the charged radicals showed that the two systems are distinctively different and the radical sites do not interconvert. The isomeric mono- and dilithiated radicals undergo unique dissociations, which were unequivocally elucidated by deuterium labeling. |
