Determination of haem electronic structure in His-Met cytochromes c by 13C-NMR. The effect of the axial ligands.

The assignment of 13C resonances of nuclei alpha to the haem in horse ferricytochrome c is completed and the Fermi contact shifts are evaluated at 30 degrees C and 50 degrees C using empirical magnetic susceptibility tensors to correct for dipolar interactions. The Fermi contact shifts are fitted to a model of molecular orbitals of eg symmetry, which are subject to a rhombic perturbation. A similar analysis is performed using published data for Pseudomonas aeruginosa cytochrome c551. The relationship between the orientation of the effective g tensor and that of the rhombic perturbation in these proteins is shown to agree with theoretical predictions. A comparison between the orientation of the rhombic perturbations and the crystal structures of horse cytochrome c and P. aeruginosa cytochrome c551 reveals that the orientation of the histidine and methionine axial ligands dominates the rhombic perturbation and that the two ligands have approximately equal influence. The magnitude of the perturbation shows that the orientation of the axial ligands has little effect on the haem redox potential. However, the relationship that is established between the magnetic susceptibility tensor, the partially filled haem molecular orbitals, and the orientation of the haem ligands offers a new source of precise structural information.