Mossbauer and EPR study of a cytochrome b model

Mossbauer 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 approximate to 3.5, although both sites are occupied by the same amount of heme as found in the Mossbauer spectra. This site is correlated with the axial symmetry of the g-tensor and with the perpendicular alignment of the histidine planes. The Mossbauer 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.