Isomeric differentiation and quantification of α,β-amino acid-containing tripeptides by the kinetic method: alkali metal-bound dimeric cluster ions

摘要

The kinetic method is applied to differentiate and quantify mixtures of isomeric tripeptides by generating and mass-selecting alkali metal ion-bound dimeric clusters and examining their competitive dissociations in an ion trap mass spectrometer. This methodology readily distinguishes the pairs of isomers examined here: (α-A)GG/(β-A)GG, G(α-A)G/G(β-A)G, and GG(α-A)/GG(β-A). The isomeric selectivity increases with decreasing size of the metal ion, viz. from Cs to Rb to K to Na to Li. When alanine is at the N-terminus, as in the case of (α-A)GG/(β-A)GG, the isomeric selectivity can exceed 10~3. The corresponding proton-bound dimers behave similarly to the Li clusters. Structural features that favor zwitterionic versus charge-solvated forms of the alkali metal-bound clusters are reflected in the b_n and y_n fragment ion abundances recorded by tandem mass spectrometry, and the propensities to form the charge-solvated or zwitterionic structures play a key role in promoting isomeric differentiation. The zwitterionic forms favor intramolecular interactions in the cluster and hence isomeric distinction. There is no discrimination in the formation of the alkali metal-bound dimers, so isomeric quantification is based entirely on dissociation kinetics. Previous kinetic method-based isomeric analyses have used the trimeric clusters and shown linear correlations between composition of the mixture of isomers and the logarithm of the branching ratio for competitive fragmentation. A similar relationship is found for the dimeric clusters examined here. As used here, the kinetic method provides a possible way for future quantitative analysis of mixtures of larger peptides such as those generated in combinatorial synthesis of peptides and peptide mimics.