Abstract
Powder and single-crystal electron paramagnetic resonance spectra for manganese(II protoporphyrin IX myoglobin (Mn 2+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 ‖ = 1.99, g ‖ = 5.90, A ‖ = 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.
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