Determination of haem electronic structure in cytochrome b5 and metcyanomyoglobin

The paramagnetic shifts of 13C nuclei positioned alpha to the haems in the A and B forms of rat cytochrome b5 and in metcyanomyoglobin have been analysed in terms of molecular orbitals based on D4h symmetry with a rhombic perturbation. The contribution to the 13C shifts from pseudocontact interactions is calculated from parameters obtained for a metal-centred dipolar shift tensor by fitting 1H shifts. The effect of electron delocalisation onto the vinyl groups of these haems b is separated with reference to the shifts of the vinyl beta carbons. In each case, it was found that the orientation of the magnetic axes in the plane of the haem is rotated away from the iron-nitrogen vectors in the opposite sense to the rotation of the rhombic perturbation and the molecular orbitals. The orientation of the orbitals is closely aligned with the normal to the single His ligand in metcyanomyoglobin, and with the average of the two normals in the bis-His cytochrome b5. It is concluded that the in-plane anisotropy of haems b is dominated by the orientation of the axial ligands in a similar manner to that in haems c and that the approximations used are weakened, but not invalidated, by the presence of partially conjugated vinyl groups.