Electrospray ionization of copper-glycine solutions

We have characterized the electrospray ionization (ESI) spectra of mixtures of the 3d transition metal ion Cu(2+) with both a model amino acid (glycine) and a simple peptide system (glycylglycine). We found what appear to be two major classes of ions. We suggest that these apparent classes may offer some further insight into the nature of the ion formation process in ESI. Specifically, we argue that one group of ions is a consequence of gas-phase reactions of the intramolecular charge transfer type. We also argue that the second group of ions has a close association with species that have been identified in the bulk phase, and which therefore may arise in ESI by a direct desorption process.