Mössbauer and EPR study of a cytochrome b model

Mössbauer and EPR measurements were performed with MOP1, a novel cytochrome b model compound with two heme binding sites. The measurements show for the first site well-resolved normal B-hemichrome EPR signals with characteristic g-values of 2.95, 2.27 and 1.50 which are a feature of the parallel planes orientation of axial histidines. The second site shows a poorly resolved EPR signal with a broad peak at g ≈ 3.5, although both sites are occupied by the same amount of heme as found in the Mössbauer spectra. This site is correlated with the axial symmetry of the g-tensor and with the perpendicular alignment of the histidine planes. The Mössbauer spectra are simulated with due account for the spin-spin relaxation taking place within one molecular unit. It shows that the Fe(III) ion in the second site is in a previously not characterised spin-mixed state and not in a pure low-spin state, such as a HALS state. The redox potential of this site is about 65 mV more positive than that of the first site.