MATRIX-ASSISTED LASER DESORPTION/IONIZATION OF NONCOVALENTLY BOUND COMPOUNDS

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was used to obtain spectra of peptide-metal ion complexes formed by a zinc finger peptide of the transcription factor IIIA (Cys(2)-His(2)) type and zinc and cobalt ions, as well as peptide-enzyme complexes. Peptides and proteins analyzed by mass spectrometry are generally dissolved in 0.1% aqueous TFA (pH < 2.0). At this pH, peptides and proteins denature. We therefore reasoned that MALDI mass spectrometry might be able to detect noncovalently bound compounds if conditions were used to prepare the samples that allowed macromolecular assemblies to retain tertiary structure. Samples were dissolved in 1 M ammonium bicarbonate, and a saturated matrix solution was prepared using ethanol-ammonium bicarbonate (1:1) solution or ethanol-ammonium citrate (1:1) solution. All preparations of zinc finger peptides were done in a glovebox under nitrogen to prevent oxidation of the metal binding cysteine residues. Using this approach, we have been able to demonstrate that MALDI mass spectrometry can be used to study both noncovalent metal binding complexes and noncovalent peptide-enzyme complexes.