First observation by mass spectrometry of a 3+ oxidation state for a [4Fe-4S] metalloprotein: an ESI-FTICR mass spectrometry study of the high potential iron-sulfur protein from Chromatium vinosum.

Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FTICR) is used to measure the molecular weight of the high potential iron-sulfur protein (HiPIP) from Chromatium vinosum (C. vinosum) and its corresponding apoprotein. By accurate mass measurement of the metalloprotein, the oxidation state of the [4Fe-4S] metal center is assigned as 3+. This is the highest oxidation state yet observed by mass spectrometry for a [4Fe-4S] cluster, which usually appears in the 2+ oxidation state. In order to make this assignment correctly, the mass spectrum of the apoprotein was acquired, and a 1 Da difference was found between the molecular mass of the apoprotein and its published amino acid sequence. The mass spectra of the trypsin and cyanogen bromide digests of the alkylated apoprotein were obtained, and the data suggests that the C-terminal glycine residue is amidated.