Tetrahedral Iron in the Active Center of Plant Ferredoxins and Beef Adrenodoxin

The coordination structure of the iron-sulfur complex in spinach ferredoxin and adrenodoxin is investigated by optical spectroscopy. The circular-dichroism and absorption spectra of these two-iron iron-sulfur proteins reveal weak electronic transitions in the near-infrared wavelength range, 0.8-2.5 mum (12,500-4000 cm(-1)). On the basis of the low absorption intensities and large anisotropy factors, d --> d transitions of the iron can be identified in the reduced proteins at about 4000 cm(-1) and 6000 cm(-1). The low energy of these one-center ligand-field transitions, together with the similarity to the ligand-field spectrum of the one-iron protein rubredoxin, leads to the conclusion that the reduced two-iron iron-sulfur proteins also contain a high-spin ferrous ion in a distorted tetrahedral site.