The number of iron atoms in the paramagnetic center (G =1.94) of reduced putidaredoxin, a nonheme iron protein.

A number of oxidation-reduction proteins, containing iron and an acid-labile form of sulfide, have been shown to exhibit low-temperature electron paramagnetic resonance (EPR) signals near g = 1.94, on reduction.l-4 These proteins include oxidases of the xanthine type, ferredoxins, and nonheme-iron and iron flavoproteins from mitochondria and bacteria. By substitution with iron5 and sulfur6' 7 isotopes having nonzero nuclear spin, both elements were conclusively shown to participate in the paramagnetic center. The nuclear magnetic moments may couple to the moment of the unpaired electron of the center, producing small positive and negative incremental local fields which act to split or broaden the EPR spectrum compared to the unsubstituted cases. The effect depends, in the simplest case of isotropic hyperfine interaction, on three variables, viz., the enrichment in isotope of nonzero nuclear spin (Fe57 of S33 in the present case), the effective local field contribution or hyperfine splitting constant of each atom, and the number of atoms of each isotope involved with a single unpaired electron. If the enrichment can be estimated, trial spectra can be calculated for various hyperfine splittings and numbers of atoms. In favorable cases comparison to the observed spectrum will reveal the values of the variables that best fit hypothesis to experiment. In this way one can determine the number and kinds of atoms interacting with the unpaired electron under the assumption of predominantly isotropic hy