LIGAND ORIENTATION AND HEME ELECTRONIC-STRUCTURE IN FERRICYTOCHROME-C'' FROM METHYLOPHILUS-METHYLOTROPHUS STUDIED BY C-13 NMR

Cytochrome c '' from Methylophilus methylotrophus contains a single ha em with bis-histidine coordination which couples electron and proton t ransfer by reversible detachment of one of the axial ligands on reduct ion. C-13 NMR of the haem substituents is now used to determine the or ientations of the two histidine ligands of the ferricytochrome. The re lative 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 def ined in the presence of near-axial symmetry, C-13 NMR is found to be a useful complement to EPR for obtaining quantitative information for s ystems of this type.