Differences between positive and negative ion stabilities of metal–sulfur cluster proteins: an electrospray ionization fourier transform ion cyclotron resonance study

Abstract The stability of iron–sulfur proteins during electrospray ionization in both positive ion and negative ion modes was investigated using Fourier transform ion cyclotron resonance mass spectrometry. Positive ion and negative ion mode mass spectra of iron and zinc rubredoxin from Clostridium pasteurianum and ferredoxins from Pyrococcus furiosus containing [3Fe–4S], [4Fe–4S], [3FeNi–4S], and [3FeMn–4S] clusters are compared. The results demonstrate that all clusters are stable as negative ions, whereas only the [4M–4S], (M = Fe, Mn, Ni) clusters are stable as positive ions. This is the first direct evidence of the existence of the [3FeMn–4S]-containing cluster from Pyrococcus furiosus. The formal oxidation state of each metal–sulfur cluster is determined by the mass-to-charge measurement, and is found to be the same for both positive and negative ions.