Multifield Saturation Magnetization Measurements of Oxidized and Reduced Ribonucleotide Reductase from Escherichia coli

A diiron(II) center doubly bridged by carboxylate groups (see sketch) is present in reduced ribonucleotide reductase according to magnetization measurements: the strong anti-ferromagnetic coupling of the oxidized diiron(III) site consistent with an oxo bridge is no longer observed. Protonation of the oxo bridge during the reduction is postulated. The loss of the resulting aqua bridge then leads to the structural unit shown here.