Analysis of powder and single-crystal electron paramagnetic resonance spectra for manganese(II) protoporphyrin IX myoglobin at various microwave frequencies

Powder and single-crystal electron paramagnetic resonance spectra for manganese(II protoporphyrin IX myoglobin (Mn²⁺Mb) were measured at S-, X-, K-, and Q-band microwave frequencies to determine principal g values, hyperfine coupling constants, and zero-field splitting parameters. The effective EPR parameters for the lowest Kramers doublet were found to be g_{{norm of matrix}} = 1.99, g_{{norm of matrix}} = 5.90, A_{{norm of matrix}} = 8.64 mT, and A_⊥ = 8.24 mT. Zerofield splitting between the lowest Kramers doublet (M_{s = ± 1 2}) and the second Kramers doublet (M_s = ± 3 2) was estimated to be 1.39 cm^-1. The effective g tensor is approximately axially symmetric. Hyperfine coupling with the ⁵⁵Mn nucleus was nearly isotropic, but also reflects axial symmetry. The orientation of manganese(II) protoporphyrin plane in single crystals of Mn ²⁺Mb is nearly identical to that for high-spin iron(III) myoglobin single crystals. The spin-Hamiltonian parameters for Mn ²⁺Mb are fully consistent with single-crystal or powder spectra obtained at all four microwave frequencies.