Tandem mass spectrometric analysis of alkali metal cation–tripeptide clusters

Clusters of tripeptides bound to alkali metal cations are generated through electrospray ionization, and collision-induced dissociation tandem mass spectrometry is carried out. Fragmentation products of peptides bound to small ions are b- and y-type fragments with the ion attached, whereas fragmentation of peptides bound to larger ions leads to complete dissociation of the ion. Density functional theory calculations suggest that the ion–peptide binding energy decreases with both the increasing ion radius and the average distance between the ion and the carbonyl O atoms. There is no apparent relation between the binding energy and the partial ion charge.