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

The stability of iron-sulfur proteins during electrospray ionization in bot h positive ion and negative ion modes was investigated using Fourier transf orm ion cyclotron resonance mass spectrometry. Positive ion and negative io n mode mass spectra of iron and zinc rubredoxin from Clostridium pasteurian um and ferredoxins from Pyrococcus furiosus containing [3Fe-4S], [4Fe-4S], [3FeNi-4S], and [3FeMn-4S] clusters are compared. The results demonstrate t hat 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 direc t evidence of the existence of the [3FeMn-4S]-containing cluster from Pyroc occus 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 f or both positive and negative ions. (C) 2001 Elsevier Science B.V.