Some structural features of cluster-coordinating cysteines of Clostridium pasteurianum ferredoxin are revealed by 2D TOCSY 1H NMR on the oxidized protein

Different sets of geminal J coupling constants for the eight β-CH2 protons in the iron-coordinating cysteines in Clostridium pasteurianum ferredoxin were detected by 2D TOCSY 1H NMR experiments on the oxidized protein. Four resonances were characterized by quite similar high values of J, two more resonances had a J value about half of the former ones, while the last two had extremely low J values. These findings suggest that the cysteines required for cubane symmetry around the iron atoms are constrained into different geometries. The simplified model used for fine tuning of τ(m) in these TOCSY experiments is also presented and discussed.