Electron Paramagnetic Resonance of Beef-Heart Ferricytochrome c

We have observed an anisotropic EPR spectrum in a frozen solution of the low‐spin $\mathrm{(S\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2}\mathrm{;)}$ hemoprotein ferricytochrome c. These results are interpreted in terms of the theory for the low‐spin ferric ion in an octahedral crystal field which is subject to an (axial) distortion along the axis perpendicular to the heme plane and a (rhombic) distortion in the plane of the heme, and a spin‐orbit constant which is less than that found in the free ion. This model also leads to calculated values for the effective magnetic moment which agree well with experiment.