The ferredoxin from parsley has been purified to a high degree. The protein contains 2.o iron atoms per molecule, has an iron to labile sulfide ratio of unity and consumes 9 moles of mercurial, leading to calculated molecular weights of 1o8oo, 1o75o and 1o96o, respectively. These compare well with the results of amino acid analysis which gave a molecular weight of 1o66o. The dithionite-reduced material showed an EPR spectrum with the experimental g values : 1.899 , 1.959, 2.o61. C1 was found to have a specific broadening effect upon this spectrum. Selenium has been substituted for the labile sulfur of the native protein to yield a biologically active homolog. A new method for this reaction has been developed. The optical and circular dichroism spectra of the oxidized and reduced forms of the selenimn protein have been recorded and are compared with the corresponding state of the native protein. The following isotopic derivatives have been prepared and studied by EPR spectroscopy: ~2SS~Fe (native), ~2SSWe, a°Se~6Fe, 77Sea*Fe and 8°SeSWe. The 3~S~TFe derivative shows a broadened EPR spectrum but no hyperfine structure is resolved, this is presumably due to the broad line of the native material. The 8°Se~Fe spectrum tended toward axial symmetry with apparent g values of 1.937, 1.965, 2.o62. This derivative also showed a substantially narrower line width in the z-region. Hyperfine structure is resolved in the z-region of the 77SeSaFe homolog, and the EPR spectrum has been analyzed to determine the number of [ = 1/2 hyperfine centers contributing to the line widths. Hyperfine structure was not resolved in the s°SeaWe species; analysis of the line shape of the low field resonance suggests that the broadening observed is consistent with two iron (I = 1/2) centers with similar effective hyperfine splitting constants.
