EPR SPECTROSCOPY OF IRON-SULFUR PROTEINS

This chapter describes electron paramagnetic resonance (EPR) spectroscopy of iron–sulfur proteins. The first EPR experiments on biological systems were carried out along the same lines. Many spectra were taken as a function of the orientation of the magnetic field with respect to the axes of the hemoglobin crystals frozen in their mother liquor. The orientation of the g-tensor principal axes with respect to crystal axis systems was determined. Later, with the X-ray structures available, the g-tensor axes were found to approximately correspond to the heme plane and heme normal. Detailed information on the orientation of the haem plane may be combined with X-ray measurements to calculate the polypeptide chain directions and similar factors. A biological metal site formally does not have any symmetry at all, if only because it is ligated by a polypeptide made up of levorotatory amino acid residues. This is especially true when the protein ligation forms a part of the first coordination sphere, as in iron–sulfur proteins. © 1992, Academic Press, Inc.