Gas-Phase Interactions of Lithium Ions and Dipeptides

FAB and tandem mass spectrometry have been used to investigate the gas-phase interactions of lithium ions and dipeptides. Lithiated dipeptides decompose as metastable ions, producing two amino acid ions, those corresponding to the N-terminus ([B₁ + Li + OH]⁺) and the C-terminus ([Y₁ + L + H]⁺). The lithium ion interacts with the carboxylate anion of the C-terminus in a first step. The relative abundances of the fragment ions depend upon the site of protonation, the stability of the lithiated amino acid formed, and the stability of the mine lost. For aliphatic dipeptides, the relative abundances of the two amino acid fragment ions are approximately equal; however, if one amino acid contains a side chain of high proton affinity (e.g., His, Arg, Lys), that amino acid fragment ion (as a [M + Li]⁺ ion) dominates. Other side chains such as Phe, Met, and Ser show intermediate behavior. Upon collisional activation, the production of the [M + Li]⁺ of the C-terminus amino acid becomes more dominant, indicating this reaction is kinetically favored for higher energy ions. [M + Li]⁺ ions of His, Arg, and Lys, however, still dominate, making sequencing difficult. Structure determination, however, is still possible by using less abundant ions (e.g., [A₁ + Li-H]⁺ ions and those from side chain losses). The interactions of sodium and potassium ions with peptides are similar to that of lithium; however, the lower polarizing power of K⁺ dramatically reduces the formation of the N-terminus amino acid ion.