Fragmentation Reaction of Complexes of Alkali Metal Cations with Tripeptides in Gas Phase

For exploring effects of side chain R groups on dissociations of complexes of alkali metal ions with peptides in gas phase, the complexes of Li+, Na+, K+, Rb+, Cs+ and the tripeptides Glycyl-glycyl-glycine (GGG), Glycyl-phenylalanyl-glycine(GFG) and Glycyl-glycyl-phenylalanine(GGF) were chosen to probe the fragmentation reaction process by electrospray ionization mass spectrometry. The experimental results demonstrated that Li+, Na+ or K+ and GGG, GFG or GGF can form complexes in a coordination ratio of 1 : 1 or 2 : 1, respectively. The mass spectra for competition reactions of GGG, GFG or GGF with Li+, Na+ or K+ showed peak intensity of alkali metallated-tripeptide complexes decrease in the order of Li+, Na+, K+ similarly, indicating that the binding strength of complexes descend with the ascend of the radii of alkali metal ion. Collision. induced dissociation(CID) results showed the center of mass frame collision energy E(CM)(50) values were 1.94, 1.76 and 1.63 eV for the precursor of [GGG + Na](+), [GGF + Na](+) and [GFG + Na](+), respectively. The mass spectrometric titrations determined the lgK(a1) values were 5.30, 5.25 and 5.17, respectively. Quantitative analyses of mass spectra further confirmed the stability order [GGG + Na](+) > [GGF + Na](+) > [GFG + Na](+), indicating binding strength of complexes GFG or GGF with R side chain(benzyl group) were weaker than GGG with R side chain(only hydrogen atom) due to effects of steric hindrance. The tandem mass spectrometric analysis revealed that dissociation of soliated GGG produces most abundant soliated a(2), b(n), y(n) ion series, while soliated GGF or GFG ion gives abundant soliated y(n) and b(n) ions, but rarely soliated a(n). More. over, doubly alkali meallated GGF tends to dissociate to more metallated y ion series. The breakdown graph for the complex [GGF + Na](+) showed the percentage of fragment ion [b(2) + Na + OH](+) will go up greatly upon the increasing of collision energy. According to Siu's fragmentation mechanisms of protonated tripeptide, it is inferred that soliated GFG will fragment to [b(2) + Na](+) ion with an incipient oxazolone. Then [b(2) + Na](+) will continue to fragment to [a(2) + Na](+) with a cyclic protonted 4-imidazolidone.