Fragmentation Reactions of Complexes of Alkali Metal Ions with Pentaserine, Pentaleucine and Pentalysine in Gas Phase

For exploring the effects of alkali metal ions on the dissociations of peptides with different side chains in the gas phase, the complexes of Li+, Na+, K+, Rb+ and Cs+ with pentapeptides, Ser-Ser-Ser-Ser-Ser(S5), Leu-Leu-Leu-Leu-Leu (L5) and Lys-Lys-Lys-Lys-Lys (K5), were chosen to investigate the fragmentation reaction pathways by mass spectrometry. The experimental results indicated that alkali metal ions and S5, L5, K5 can form 1: 1 and 2: 1 non-covalent complexes in the gas phase, and the binding strength between alkali metal ions and pentapeptides descend with the increasing of the radii of alkali metal ions. Quantitative analyses of mass spectra demonstrated that the binding constants for the complexes of K+ with S5, L5 and K5 are 8. 94x10(4), 2. 83x10(4) and 2. 50x10(3) L/mol, respectively, revealing that the binding strength of complexes decrease in order of S5, L5 and K5. The analyses of the fragment ions arising from the complexes of alkali metal ions with pentapeptides showed that the dissociations mainly occur in the backbones of the peptides. Among three pentapeptides, the complexes of pentaserines are the easiest to fragment, pentaleucines take second place, while pentalysines are the hardest. The dissociations also occur in the side chains, and there are significant differences among the three complexes. Moreover, the relationships among various fragment ions arising from the complex of Na+ with L5 were tested. Based on the theoretical model of Dunbar, it was proposed that in the fragmentation process, the alkali metal ion may shift to the N- or C-terminal of the pentapeptide. In 2: 1 complex, the first alkali metal ion may conjugate to the carbonyl groups of four amides, while the second one may conjugate to the oxygen atoms of carboxyl group of the pentapeptide.