Formation and Reactions of Clusters in the Gas Phase: Small Peptides and Carboxylic Acids

Abstract

The cluster formation of 17 small dipeptides with different primary structures and vanillic acid was investigated by means of a neutral laser desorption and supersonic beam expansion followed by multi-photon ionization time of flight mass spectrometry. The structures of these clusters have been characterized by mass spectrometric methods as well as by density functional theory calculations. It shows that the structure of the cluster from a dipeptide and vanillic acid is described by a hydrogen bond between the phenolic group of the vanillic acid and the N-terminal amino function of the dipeptide. The intensity of the cluster ion and the main fragmentation product, the protonated peptide ion, can be linked to the proton affinity of the peptide. Furthermore, the fragmentation reactions of the protonated peptide are accompanied by extensive hydrogen rearrangements yielding both a and y fragments. The intensities of these fragments follow the proton affinity of the dipeptide.