Binuclear Mn Complexes Research Articles

Metallo-inhibition of Mnx, a bacterial manganese multicopper oxidase complex

The manganese oxidase complex, Mnx, from Bacillus sp. PL-12 contains a multicopper oxidase (MCO) and oxidizes dissolved Mn(II) to form insoluble manganese oxide (MnO2) mineral. Previous kinetic and spectroscopic analyses have shown that the enzyme's mechanism proceeds through an activation step that facilitates formation of a series of binuclear Mn complexes in the oxidation states II, III, and IV on the path to MnO2 formation. We now demonstrate that the enzyme is inhibited by first-row transition metals in the order of the Irving-Williams series. Zn(II) strongly (Ki ~ 1.5 μM) inhibits both activation and turnover steps, as well as the rate of Mn(II) binding. The combined Zn(II) and Mn(II) concentration dependence establishes that the inhibition is non-competitive. This result is supported by electron paramagnetic resonance (EPR) spectroscopy, which reveals unaltered Mnx-bound Mn(II) EPR signals, both mono- and binuclear, in the presence of Zn(II). We infer that inhibitory metals bind at a site separate from the substrate sites and block the conformation change required to activate the enzyme, a case of allosteric inhibition. The likely biological role of this inhibitory site is discussed in the context of Bacillus spore physiology. While Cu(II) inhibits Mnx strongly, in accord with the Irving-Williams series, it increases Mnx activation at low concentrations, suggesting that weakly bound Cu, in addition to the four canonical MCO-Cu, may support enzyme activity, perhaps as an electron transfer agent.

Synthesis of three new binuclear Mn complexes: characterization and DNA binding and cleavage properties

A series of novel binuclear Mn complexes, [Mn2(L1)2]·2DMF (1), [Mn2O(L2)2](ClO4)2·CH3OH (2), and [Mn2(L3)2Cl2]·2DMF (3), were synthesized and structurally characterized by single-crystal X-ray diffraction. Moreover, their DNA-binding and cleavage properties were investigated. Characterization of the complexes by electron absorption and fluorescence spectroscopy confirmed the binding to calf-thymus DNA (CT-DNA). The apparent binding constants (Kapp) for 1, 2, and 3 were determined as 7.06 × 105, 1.10 × 105, and 3.77 × 105 M−1, respectively, indicating intermediate-strength binding between the complexes and CT-DNA. Moreover, 2 exhibited static fluorescence quenching, while 1 and 3 displayed dynamic fluorescence quenching. Agarose gel electrophoresis studies of the complexes revealed that only 2 exhibits chemical nuclease activity in the absence of a redox agent. When H2O2 was added as the redox agent, 3 was still unable to cleave DNA; however, 1 showed chemical nuclease activity and the activity of 2 was enhanced. The cleavage mechanism exerted by the complexes on plasmid DNA is likely to involve oxidative cleavage processes with hydroxyl radicals (·OH) and singlet oxygen (1O2) as the active species.

The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the CO2 Reduction Catalysis.

Herein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn(CO)3 complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO2 reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, 1, a binuclear Mn complex with methoxyphenol unit instead, 2, and the mononuclear analogue with a phenol unit, 3. Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H2 formation forming [Mn2(H-1L1)(CO)6Br] and [Mn(H-1L3)(CO)3], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn-H species. 2 dimerizes during reduction, forming [Mn2(L2)(CO)6]2, but 1 and 3 do not. Reduction of 1, 2, and 3 is accompanied by bromide loss, and the final species represent [Mn2(H-1L1)(CO)6]3-, [Mn2(L2)(CO)6]2-, and [Mn(H-1L3)(CO)3]2-, respectively. 1 and 2 are active catalysts in the electrochemical CO2 reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H+/2e- reduced products of CO2 in dmf/water mixtures. For 2, CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1. Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn-H species and subsequent CO2 insertion as well as the reaction rate of the reduced metal center with CO2 forming CO are similar. The overpotential with regard to the standard redox potential of CO2 to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s-1 are higher with 1 than with 2, that is, the OH group is beneficial for catalysis due to the internal proton transfer.

Electrogenic reactions in Mn-depleted photosystem II core particles in the presence of synthetic binuclear Mn complexes

An electrometrical technique was used to investigate electron transfer between synthetic binuclear manganese (Mn) complexes, designated M − 2 and M − 3, and the redox-active neutral tyrosine radical (YZ•) in proteoliposomes containing Mn-depleted photosystem II (PS II) core particles in response to single laser flashes. In the absence of Mn-containing compounds, the observed flash-induced membrane potential (ΔΨ) decay was mainly due to charge recombination between the reduced primary quinone acceptor QA− and the oxidized YZ•. More significant slowing down of the ΔΨ decay in the presence of lower concentrations of M − 2 and M − 3 associated with electron donation from Mn in the Mn-binding site to YZ• indicates that these synthetic compounds are more effective electron donors than MnCl2. The exponential fitting of the kinetics of additional electrogenic components of ΔΨ rise in the presence of Mn-containing compounds revealed the following relative amplitudes (A) and lifetimes (τ): for MnCl2 - A∼ 3.5, τ∼150 μs, for M − 2 - A∼5%, τ∼1.4 ms, and for M − 3 - A∼5.5%, τ∼150 μs. This suggests that the efficiency of the manganese complexes in electron donation depends on the chemical nature of ligands. The experiments with EDTA-treated samples indicated that the ligands for M − 2 and M − 3 are required for their tight binding with the PS II reaction center. The obtained results demonstrate the importance of understanding the molecular mechanism(s) of flash-induced electrogenic reduction of the tyrosine radical YZ• by synthetic Mn complexes capable of splitting water into oxygen and reducing equivalents.

Boosting the electron spin coherence in binuclear Mn complexes by multiple microwave pulses

We investigate a possibility to enhance the coherence time of electron spins in magnetic molecular complexes by application of the Carr-Purcell-Meiboom-Gill (CPMG) multiple microwave pulse sequence. Our theoretical analysis shows that the CPMG sequence can efficiently suppress the spin decoherence channel arising due to spectral diffusion induced by a random modulation of the hyperfine interaction which is an important source of the spin dephasing in molecular magnets. We confirm this by employing the CPMG protocol in pulse electron spin resonance experiments on model binuclear 1,2-diphosphacyclopentadienyl manganese complexes. We show that, compared to the standardly used two-pulse primary spin-echo technique, the CPMG experiment can boost the phase memory time up to one order of magnitude, bringing it to above 10 $\ensuremath{\mu}$s at low temperatures. This finding may be important for the implementation of quantum computation protocols on molecular magnets. We discuss a possible interesting analogy with the Zeno's paradox in quantum theory (the Zeno quantum effect), which could be implicit in the CPMG experiment.

Reconstitution of the water-oxidizing complex in manganese-depleted photosystem II preparations using synthetic binuclear Mn(II) and Mn(IV) complexes: production of hydrogen peroxide.

Reconstitution of Mn-depleted PSII particles with synthetic binuclear Mn complexes (one Mn(II)(2) complex and one Mn(IV)(2) complex) was examined. In both cases the electron-transfer rates in the reconstituted systems were found to be up to 75-82% of that measured in native PSII but the oxygen evolution activity remained lower (<5-40%). However, hydrogen peroxide was also produced by the reconstituted samples. These samples therefore represent a new type of reconstituted PSII that generates hydrogen peroxide as the final product in reconstituted PSII centers.

Intermediates in Assembly by Photoactivation after Thermally Accelerated Disassembly of the Manganese Complex of Photosynthetic Water Oxidation

The Mn4Ca complex bound to photosystem II (PSII) is the active site of photosynthetic water oxidation. Its assembly involves binding and light-driven oxidation of manganese, a process denoted as photoactivation. The disassembly of the Mn complex is a thermally activated process involving distinct intermediates. Starting from intermediate states of the disassembly, which was initiated by a temperature jump to 47 degrees C, we photoactivated PSII membrane particles and monitored the activity recovery by O2 polarography and delayed chlorophyll fluorescence measurements. Oxidation state and structural features of the formed intermediates of the Mn complex were assayed by X-ray absorption spectroscopy at the Mn K-edge. The photoactivation time courses, which exhibit a lag phase characteristic of intermediate formation only when starting with the apo-PSII, suggest that within approximately 5 min of photoactivation of apo-PSII, a binuclear Mn complex is formed. It is proposed that a MnIII2(di-mu-oxo) complex is a key intermediate both in the disassembly and in the assembly reaction paths.

Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes

The efficiency of a trinuclear and two binuclear manganese complexes in reconstituting electron transport and O 2 evolution activity in Mn-depleted Photosystem II preparations is analyzed. The trinuclear Mn-complex is more efficient than two binuclear Mn-complexes in restoring oxygen evolution, but it is less effective as an electron donor than binuclear Mn-complexes. It is inferred from our results that recovery of electron transport and O 2 evolution with polynuclear Mn-complexes is affected with different factors. Moreover, the trinuclear Mn-complex is extremely sensitive to the addition of CaCl 2. It is suggested that there is an interaction between Ca 2+ and carboxyl within the trinuclear Mn-complex during photoactivation and this interaction benefits the ligation of Mn atom to the apo-WOC and form an active WOC. Binuclear Mn(III)Mn(III) complex shows slightly higher efficiency than binuclear Mn(III)Mn(IV) complex in restoration of O 2 evolution activity. The efficiency of three Mn-complexes in the reconstitution of WOC is in an order: trinuclear Mn 3(III) > binuclear Mn(III)Mn(III) > binuclear Mn(III)Mn(IV).

Synthesis, structure, and magnetism of bimetallic manganese or nickel complexes of a bridging verdazyl radical.

Two binuclear metal-radical complexes, formed by the reaction of M(hfac)(2) x 2H(2)O (M = Mn or Ni; hfac = hexafluoroacetylacetonate) with the 1,5-dimethyl-3-(4,6-dimethylpyrimidin-2-yl)-6-oxoverdazyl radical (3), were synthesized. The binuclear Mn complex 5 (i.e., 3[Mn(hfac)(2)](2)) crystallizes in the monoclinic space group C2/c: C(30)H(17)N(6)O(9)F(24)Mn(2), a = 29.947(3), b = 17.143(3), c = 16.276(3) A, beta = 123.748(3)*, Z = 4. The compound consists of two pseudo-octahedral Mn(II) ions, both bearing two hfac ancillary ligands, bridged by the bis(bidentate) radical 3. The temperature dependence of the magnetic susceptibility of 5 reveals moderate antiferromagnetic exchange between each of the Mn(II) ions and the verdazyl radical (J = -48 cm(-1)). The S = 9/2 ground spin state of the complex was corroborated by low-temperature magnetization versus field measurements. In contrast, the magnetic susceptibility versus temperature behavior of 6 (whose molecular structure is presumed to be analogous to that of 5) indicates that the two Ni(II) ions are strongly ferromagnetically coupled to the verdazyl radical (J = +220 cm(-1)). The magnetization versus field behavior of 5 is consistent with an S = 5/2 ground-state species.

EPR polarization studies on Mn catalase from Lactobacillus plantarum.

The binuclear manganese active site of Mn catalase catalyzes redox disproportionation of hydrogen peroxide, forming dioxygen and water. We report here multifrequency EPR and microwave polarization studies of the catalytically active homovalent Mn2+ complex of Lactobacillus plantarum Mn catalase, resolving spectra from each of the thermally accessible multiplet states of the coupled complex by multivariate methods. The experimental spectra have been simulated using computational approaches for the binuclear cluster to predict both intensity and polarization for arbitrary values of the ground state parameters. These two spectroscopic properties define the nature of the ground state wavefunctions and so serve as a sensitive and quantitative measure of the inter-ion interactions in the reduced complex. Interpretation of the spectra in terms of a pair Hamiltonian that includes Heisenberg exchange, dipolar, single site zero field splitting, and Zeeman perturbations leads to the most complete ground state description of the active site metal centers. The results of this spectroscopic analysis support a picture of two high spin ions weakly coupled by exchange interactions (J = 40 cm-1) with relatively small dipole-dipole coupling and single site zero field splittings for the ligand-free reduced enzyme. The coupling between fluoride binding and protonation of the complex has been demonstrated by proton uptake studies. The binding of two fluoride ions in the active site dramatically changes the pair spectra, reflecting a substantially reduced J-coupling (J = 10.5 cm-1) that must be a consequence of perturbation of the bridging ligands. Anion binding to the binuclear Mn complex appears to result in poisoning of the active site by protons, possibly associated with insertion of fluoride into bridging positions of the dimanganese core.

ChemInform Abstract: Synthesis and Characterization of Binuclear Manganese Complexes: Redox Models for the Water Oxidation Cofactor of Photosystem II.

AbstractNew binuclear Mn complexes are prepared which exhibit a two‐electron‐redox chemistry that is compatible with the reversible oxidation of water to oxygen.

Manganese proteins isolated from spinach thylakoid membranes and their role in O2 evolution. II. A binuclear manganese-containing 34 kilodalton protein, a probable component of the water dehydrogenase enzyme.

Extraction conditions have been found which result in the retention of manganese to the 33-34 kDa protein, first isolated as an apoprotein by Kuwabara and Murata (Kuwabara, T. and Murata, N. (1979) Biochim. Biophys Acta 581, 228-236). By maintaining an oxidizing-solution potential, with hydrophilic and lipophilic redox buffers during protein extraction of spinach grana-thylakoid membranes, the 33-34 kDa protein is observed to bind a maximum of 2 Mn/protein which are not released by extended dialysis versus buffer. This manganese is a part of the pool of 4 Mn/Photosystem II normally associated with the oxygen-evolving complex. The mechanism for retention of Mn to the protein during isolation appears to be by suppression of chemical reduction of natively bound, high-valent Mn to the labile Mn(II) oxidation state. This protein is also present in stoichiometric levels in highly active, O2-evolving, detergent-extracted PS-II particles which contain 4-5 Mn/PS II. Conditions which result in the loss of Mn and O2 evolution activity from functional membranes, such as incubation in 1.5 mM NH2OH or in ascorbate plus dithionite, also release Mn from the protein. The protein exists as a monomer of 33 kDa by gel filtration and 34 kDa by gel electrophoresis, with an isoelectric point of 5.1 +/- 0.1. The protein exhibits an EPR spectrum only below 12 K which extends over at least 2000 G centered at g = 2 consisting of non-uniformly separated hyperfine transitions with average splitting of 45-55 G. The magnitude of this splitting is nominally one-half the splitting observed in monomeric manganese complexes having O or N donor ligands. This is apparently due to electronic coupling of the two 55Mn nuclei in a presumed binuclear site. Either a ferromagnetically coupled binuclear Mn2(III,III) site or an antiferromagnetically coupled mixed-valence Mn2(II,III) site are considered as possible oxidation states to account for the EPR spectrum. Qualitatively similar hyperfine structure splittings are observed in ferromagnetically coupled binuclear Mn complexes having even-spin ground states. The extreme temperature dependence suggests the population of low-lying excited states such as are present in weakly coupled dimers and higher clusters of Mn ions, or, possibly, from efficient spin relaxation such as occurs in the Mn(III) oxidation state. Either 1.5 mM NH2OH or incubation with reducing agents abolishes the low temperature EPR signal and releases two Mn(II) ions to solution. This is consistent with the presence of Mn(III) in the isolated protein.(ABSTRACT TRUNCATED AT 400 WORDS)