Hyperfine structure from the four pyrrole nitrogens of heme.

Hemin and hematin have been incorporated into single crystals of the condensed-ring planar hydrocarbon perylene, the approximate molecular ratio of heme to aromatic being 10(-3). X-ray diffraction and electron paramagnetic resonance (EPR) measurements indicate that these mixed crystals maintain their integrity when subjected to cryogenic temperatures and then returned to room temperature. EPR X-band orientation studies of single crystals at liquid helium temperature have revealed the following two important features: (1) presence of ligand hyperfine structure, which can be used to quantitate the distribution of unpaired metal electron on the ligands and (2) removal of the immediate fourfold symmetry of the ligand field about the metal, as evidenced by the rhombic character of the high-spin ferric system. The ligand hyperfine splittings have been interpreted as arising chiefly from an isotropic Fermi contact interaction. The observed splittings then indicate that 2.7 per cent of an unpaired spin is in each nitrogen 2s orbital. The g-values have been interpreted in terms of the spin-Hamiltonian: [Formula: see text] which gives g(x,y) = 6.00 +/- 24E/D and g(z) = 2.00 - 34(E/D)(2). For various hemin and hematin sites these formulae for the g-values are nicely obeyed when E/D is in the range 0.015 to 0.03.