Ferrichrome: Surprising stability of a cyclic peptide-Fe(III) complex revealed by mass spectrometry

Ferrichrome, a fungal siderophore that is also utilized by some bacterial species, was studied with liquid secondary ion mass spectrometry (LSIMS) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. A strong ionic signal corresponding to a Fe(III) complex was observed with LSIMS in the positive ion mode. Switching the polarity of the mass spectrometer did not necessarily result in reduction of ferric ion, although certain conditions led to appearance of a Fe(II) complex signal as well. The results of the structural studies of the metal ion-cyclic peptide complex with collisionally induced dissociation allowed unambiguous identification of the chelation sites. The action of the siderophore on Fe(III) was studied by in vitro chelation of ferric ion (from ferric citrate) by the iron-free ferrichrome. Effective chelation of ferric ion was compared to actions of the iron-free ferrichrome on other metal ions. Unlike LSIMS, desorption with MALDI did not form selectively molecular ions of intact ferrichrome: the spectra contained abundant peaks corresponding to the cyclic peptide itself and its nonspecific association with alkali metal ions.