Matrix-assisted laser desorption/ionization mass spectrometry of noncovalent protein-transition metal ion complexes

Transition metal ion complexes with proteins and peptides are important in many areas of analytical and biological chemistry. We used positive and negative ion MALDI-MS to detect complexes with Cu and Ni ions, and show that the specific and non-specific transition metal ion-peptide complexes can be distinguished by the use of different analytical protocols. The pH dependent stability of these complexes is also reflected in the MALDI data. We further show that triple complexes of peptides or protein with chelated metal ions can be detected efficiently and rapidly by MALDI mass spectrometry. Such triple complexes play an important role in metal chelate affinity chromatography, where histidine containing biopolymers in particular are thought to bind metal-ligand complexes, depending on the oxidation state of the metal and the number of unoccupied coordination sites of the ligand.