Single Crystal X- and Q-Band EPR Spectroscopy of a Binuclear Mn2(III,IV) Complex Relevant to the Oxygen-Evolving Complex of Photosystem II

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Abstract
The anisotropic g and hyperfine tensors of the Mn di-μ-oxo complex, [Mn2(III,IV)O2(phen)4](PF6)3·CH3CN, were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn2(III,IV)O2(phen)4](PF6)3·CH3CN EPR spectra showed distinct resolved 55Mn hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn2(III,IV) di-μ-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystal EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn2(III,IV)O2(phen)4 as shown in the crystallographic data by Stebler et al. (Inorg. Chem.1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: gx = 1.9887, gy = 1.9957, gz = 1.9775, and hyperfine coupling constants are AIIIx = |171| G, AIIIy = |176| G, AIIIz = |129| G, AIVx = |77| G, AIVy = |74| G, AIVz = |80| G.

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