The dimanganese(II,II) catalase from Thermus thermophilus, MnCat(IIJI), arginase from rat liver, Arg(II,II), and several dimanganese(II,II) compounds, LMn2XY2, which are functional catalase mimics, all possess a pair of coupled Mn(II) ions in their catalytic sites. For each of these, we have measured by EPR spectroscopy the relative energies separating the three lowest electronic states (singlet, triplet, and quintet), described a general method for extracting the individual spectra for these states by multicomponent analysis, and determined the Mn-Mn separation. The triplet-singlet and quintet-singlet energy gaps were modeled well by fitting the temperature dependence of the EPR intensities to a Boltzmann expression for a pair of Mn(II) ions coupled by isotropic Heisenberg spin exchange (-2JS1S2). This dependence indicates diamagnetic ground states with ∆E10 (cm-1) = \2J\ = 4 and 11.2 cm-1 for Arg- (II.II) (+borate) and MnCat(II,II)(phosphate), respectively. This large difference in \2J\ reflects either a difference in the bridging ligands or, possibly, a weaker ligand field (larger ionization potential) for the Mn(II) ions in arginase. In n-butanol/CH2Cl2 the triplet-singlet energy gaps for [LMn2(CH3C02)](C104)2 (1), [LMn2(CH3CO2)3] (2), and [LMn2Cl3] (3), where HL = N,N,N’,N'-tetrakis(2-methylenebenzimidazole)- l,3-diaminopropan-2-ol, are 23-24 cm-1. Comparison of the Heisenberg exchange interaction constants for more than 30 dimanganese(II,II) complexes suggests a possible bridging structure of (µ-OH)(µ-carboxylate) 1-2 for MnCat(II,II), while the 3-fold weaker coupling in Arg(II,II) suggests µ-aqua in place of µ-hydroxide. EPR spectra of both the triplet and quintet electronic states were extracted and found to exhibit zero-field splittings (ZFS) and resolved 55Mn hyperfine splittings indicating spin-coupled Mn2- (II.II) species. The major ZFS interaction could be attributed to the magnetic dipole-dipole interaction between the Mn(II) ions. A linear correlation is observed between the crystallographically determined Mn-Mn distance and the ZFS of the quintet state (D2) for five dimanganese pairs for which both data sets are available. Using this correlation, the Mn-Mn distance in Arg(II,II) is predicted to be 3.36-3.57 A for the native enzyme (multiple forms) and 3.59 Å for MnCat(II,II)(phosphate). Addition of the inhibitor borate to Arg(II,II) simplifies the ZFS, indicative of conversion to a single species with mean Mn-Mn separation of 3.50 Å. The second metal ion in dinuclear complexes possessing a shared bridging ligand has been shown to attenuate the strength of the µ-ligand field potential, as monitored by the strength of the single ion ZFS. A weakened µ-ligand field potential may play a role, for example, in promoting ionization of a proton from a manganese-bound water molecule in arginase, with the resulting hydroxide ligand being the nucleophile needed for hydrolysis of substrate.
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