Gas phase studies of the interactions of Fe2+ with cysteine-containing peptides.

Gas-phase complexes of cysteine-containing peptides and Fe2+ were produced by fast atom bombardment and studied by tandem mass spectrometry. Specific and strong interactions of the iron and sulfur from the thiol group of the cysteine side chain are preserved in the gas phase and are the basis for highly specific fragmentation to give abundant [a(n) - 2H + Fe]+ ions, where n is position of the cysteine residue from the N-terminus of peptide. Metal/peptide complexes containing more than one Cys residue were also investigated; they display similar chemistry upon collisionally activated decompositions, indicating that the Fe2+ ion primarily binds at cysteine sites.