Multi-frequency and high-field EPR study of manganese(III) protoporphyrin IX reconstituted myoglobin with an S = 2 integer electron spin

We investigate the electronic state of Mn(III) center with an integer electron spin S = 2 in the manganese(III) protoporphyrin IX reconstituted myoglobin, Mn(III)Mb, by means of multi-frequency electron paramagnetic resonance (MFEPR) spectroscopy. Using a bimodal cavity resonator, X-band EPR signal from Mn(III) center in the Mn(III)Mb was observed near zero-field region. The temperature dependence of this signal indicates a negative axial zero-field splitting value, D < 0. The EPR analysis shows that this signal is attributed to the transition between the closely spaced Ms = ±2 energy levels for the z-axis, corresponding to the heme normal. To determine the zero-field splitting (ZFS) parameters, EPR experiments on the Mn(III)Mb were performed at various temperatures for some frequencies between 30 GHz and 130 GHz and magnetic fields up to 14 T. We observed several EPR spectra which are analyzed with a spin Hamiltonian for S = 2, yielding highly accurate ZFS parameters; D = −3.79 cm−1 and |E| = 0.08 cm−1 for an isotropic g = 2.0. These ZFS parameters are compared with those in some Mn(III) complexes and Mn(III) superoxide dismutase (SOD), and effects on these parameters by the coordination and the symmetry of the ligands are discussed. To the best of our knowledge, these EPR spectra in the Mn(III)Mb are the very first MFEPR spectra at frequencies higher than Q-band in a metalloprotein with an integer spin.