1. Rubredoxin isolated from the green photosynthetic bacterium Chloropseudomonas ethylica was similar in composition to those from anaerobic fermentative bacteria. Amino acid analysis indicated a minimum molecular weight of 6352 with one iron atom per molecule. 2. The circular-dichroism and electron-paramagnetic-resonance spectra of Ch. ethylica rubredoxin showed many similarities to those of Clostridium pasteurianum, but suggested that there may be subtle differences in the protein conformation about the iron atom. 3. Mössbauer-effect measurements on rubredoxin from Cl. pasteurianum and Ch. ethylica showed that in the oxidized state the iron (high-spin Fe3+) has a hyperfine field of 370±3kG, whereas in the reduced state (high-spin Fe2+) the hyperfine field tensor is anisotropic with a component perpendicular to the symmetry axis of the ion of about −200kG. For the reduced protein the sign of the electric-field gradient is negative, i.e. the ground state of the Fe2+is a [unk] orbital. There is a large non-cubic ligand-field splitting (Δ/k=900°K), and a small spin-orbit splitting (D~+4.4cm-1) of the Fe2+levels. 4. The contributions of core polarization to the hyperfine field in the Fe3+and Fe2+ions are estimated to be −370 and −300kG respectively. 5. The significance of these results in interpretation of the Mössbauer spectra of other iron–sulphur proteins is discussed.

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