Determination of the Metal Ion Separation and Energies of the Three Lowest Electronic States of Dimanganese(II,II) Complexes and Enzymes: Catalase and Liver Arginase

Abstract

The dimanganese (II,II) catalase from Thermus thermophilus, MnCat(II,II), 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 delta E10 (cm-1) = magnitude of 2J = 4 and 11.2 cm-1 for Arg-(II,II)(+borate) and MnCat(II,II)(phosphate), respectively. This large difference in magnitude of 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(CH3CO2)](C1O4)2 (1), [LMn2(CH3CO2)3] (2), and [LMn2Cl3] (3), where HL = N,N,N',N'-tetrakis(2-methylenebenzimidazole)-1,3-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 (mu-OH)(mu-carboxylate)1-2 for MnCat(II,II), while the 3-fold weaker coupling in Arg(II,II) suggests mu-aqua in place of mu-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 A 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 A. The second metal ion in dinuclear complexes possessing a shared bridging ligand has been shown to attenuate the strength of the mu-ligand field potential, as monitored by the strength of the single ion ZFS.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectroscopic Evidence for Ca2+Involvement in the Assembly of the Mn4Ca Cluster in the Photosynthetic Water-Oxidizing Complex

“…Both types of EPR signals are easily observable and differ spectrally from Mn 2+ and Mn 3+ based on extensive literature of model complexes (45)(46)(47). Thus, we conclude that no photooxidizable Mn 2+ -Mn 2+ dimer structures are preformed in the dark prior to freezing at -20°C, neither in the absence or presence of 10 mM Ca 2+ .…”

Mn(II) Binding by the Anthracis Repressor from Bacillus anthracis

“…As shown in Figure 4, our low-temperature cw EPR spectra lack the spectral signatures associated with spin-exchange coupling and therefore are not consistent with formation of a binuclear manganese cluster in AntR. Spin-exchange coupling is strongly dependent on the metal-metal distance, the chemistry of the bridging atoms, and their coordination geometry (38). The simplest explanation for the absence of spinexchange coupling between the two bound Mn(II) ions in AntR is that the two metals are separated by more than ∼4 Å.…”

The Active Form of the R2F Protein of Class Ib Ribonucleotide Reductase from Corynebacterium ammoniagenesIs a Diferric Protein

“…Protein R2F purified from Mn-supplemented cells, Mn-R2F, displays a complex multiline spectrum, superimposed on a broad signal often found in dimanganese proteins (22). A small amount of free radical signal is also seen, but the concentration is lower in this sample compared with the C. ammoniagenes R2F sample without manganese addition (Fig.…”

“…4A) is half of the value (9.5 mT) observed for mononuclear Mn 2ϩ and is typical of a coupled dimanganese center in the Mn 2ϩ state. Dimanganese Mn(II) is also found in arginase and manganese catalase (22,(25)(26)(27)(28) and the hyperfine pattern observed in manganese-reconstituted C. ammoniagenes R2F resembles arginase, where the manganese center (29) is bridged by two carboxylates and one oxygen (water or hydroxyl).…”