Pitfalls in assigning heme axial coordination by EPR. c-Type cytochromes with atypical Met-His ligation.

Different monohemic c-type cytochromes were analyzed by visible, EPR and 1H NMR spectroscopies. While the visible and NMR data show unambiguously that the heme iron has a Met-His heme axial coordination, the EPR data indicate an axial ligand field typical of that for a bis-histidinyl ligation. The validity of the widely used EPR methods for the determination of the heme iron axial coordination, based on the crystal field parameters (tetragonality and rhombicity), is questioned.