Alkali metal ion binding to amino acids, peptide radicals, and polyglycol radicals: Intrinsic thermochemistry, dissociations, and ion -molecule reactions

This dissertation applies a variety of tandem mass spectrometry techniques to study the stability and unimolecular and bimolecular chemistry of alkali metal ion complexes to amino acids, amino acid esters, and polymeric radicals derived from small peptides and polyglycol oligomers. Molecular orbital theory is also used in selected cases to help interpret the experimental data and elucidate in detail the structures and stabilities of the investigated species.;The relative alkali metal ion (M)+ affinities (binding energies) between seventeen different alpha and beta-amino acids (AA) and the corresponding methyl esters (AAOMe) have been determined in the gas phase by the kinetic method based on the dissociations of AA-M+-AAOMe heterodimers (M = Li+, Na+, K+, and Cs+). These experiments and parallel ab initio calculations on the structures of the [AA + M]+ complexes indicate that most amino acids coordinate M+ via charge solvation. Proline, lysine, and arginine are notable exceptions, favoring the formation of zwitterionic complexes (salt bridges), especially with the larger metal cations. The most stable mode of charge, solvation was found to depend; on both the metal cat ion and the amino acid structure. Important determinants for the formation of zwitterionic complexes are a high proton affinity for the amino acid as well as linearity for the + - + charges.;Peptide backbone radicals with the unpaired electron on an alpha-C atom are very stable species due to the synergistic resonance stabilization of the single electron by neighboring electron-donating and electron-withdrawing groups (captodative substitution). The Na+ complexes of these radicals represent distonic ions, i.e. ions with separated radical and charge sites. Such novel ions ("charged radicals"), composed of a Na + charge and a dipeptide radical with glycine and/or alanine residues, have been prepared and studied in the gas phase for the first time. The distonic nature of these species is corroborated by density functional theory. Eight aliphatic tripeptides with alanine and/or glycine residues served as the precursors for the synthesis of Na+-metalated peptide backbone radicals. Dissociation of these ions is shown to mainly proceed via typical radical site reactions, namely 1,4- and 1,5-H rearrangements followed by simple bond cleavages. The type of radical site present and its environment significantly affect the nature of these dissociations.;The separate charge and radical sites make distonic ions ideally suitable reagents for tailored ion-molecule reactions that specifically probe the chemistry of either reactive center. The bimolecular chemistry of three different types of polyglycol radical termini (oxy, methyl, ethyl) was studied with gaseous reagents containing a double bond and/or heteroatom. Both charge-site and radical-site reactions occur, but radical-site reactions prevail.