Abstract: To obtain more structural information of polypeptides, glycine pentapeptide (simplified as GGGGG or G5) was chosen as a model to investigate the impact of alkali metal ions on the dissociation of GGGGG in the gas phase. Stoichiometric G5 and alkali metal salt solutions, including Li ⁺ , Na⁺ , K⁺ , Rb⁺ , were mixed, respectively, and then the solutions were left to stand at room temperature for 10 h to reach equilibrium. The mass spectrometric results indicated that the alkali metal ions and G5 could form 1:1 or 2: 1 non-covalent complexes in solution. The energy of the collision induced dissociation (CID) was 25 eV. The gas phase CID results demonstrate that in the 1:1 complexes, the extent of fragmentation decreases according to the order: Li⁺, Na⁺, K⁺, Rb⁺. Moreover, the unusual c, z ions were observed in the Rb⁺ complex. In the 2:1 non-covalent complexes, the extent of fragmentation increases according to the order: Li⁺ , Na⁺ , K⁺, Rb⁺. The gas phase dissociation of the Na⁺, K⁺, Rb⁺ 2:1 complexes are easier than their 1:1 complexes. Except for Li ⁺ , the activation abilities of the double metal ions to G5 are stronger than that of the single metal ion to G5, which can induce more dissection sites in the glycine pentapeptide and lead to the formation of more kinds of fragment ions.

Key words: Mass spectrometry, Collision-induced dissociation, Alkali metal ion, Pentapeptide, Gas phase dissociation