Individual reduction potentials of the iron ions in Fe2S2 and high-potential Fe4S4 ferredoxins

The DelPhi program package has been used to confirm that the span in reduction potentials among high-potential Fe4S4 ferredoxins must be mainly ascribed to the net protein charges arising from acidic and basic residues. Subsequently, the order of the individual reduction potentials of the iron ions in Fe2S2 ferredoxins as found from NMR spectroscopy was explained mainly on the basis of different solvation contributions to the electrostatic potential. The individual reduction potentials of the iron ions in high-potential Fe4S4 ferredoxins, again available from NMR spectroscopy, are only qualitatively reproduced. It is proposed that the protein triggers a distortion in the cluster which would be a further contribution to the electrochemical inequivalence of the individual iron ions.