Ligand orientation and haem electronic structure in ferricytochrome c'' from Methylophilus methylotrophus studied by C-13 NMR

Cytochrome c '' from Methylophilus methylotrophus contains a single haem with bis-histidine coordination which couples electron and proton transfer by reversible detachment of one of the axial ligands on reduction. C-13 NMR of the haem substituents is now used to determine the orientations of the two histidine ligands of the ferricytochrome. The relative orientation of the ligands is found to be nearly perpendicular (an angle of 85+/-2 degrees between the histidine planes was obtained), which is consistent with the near-axial g-tensor determined by EPR, Although the absolute orientation of the axial ligands is not well defined in the presence of near-axial symmetry, C-13 NMR is found to be a useful complement to EPR for obtaining quantitative information for systems of this type.