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. (C) 2014 Elsevier B.V. All rights reserved.