Electron Paramagnetic Resonance of Beef-Heart Ferricytochrome c

We have observed an anisotropic EPR spectrum in a frozen solution of the low-spin (S = 12;) 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.