The [Mn₂(2-OHsalpn)₂]^2-,1-,0 System:  An Efficient Functional Model for the Reactivity and Inactivation of the Manganese Catalases

Abstract

The complexes [Mn(2-OH(X-sal)pn)] 2 n-(where X ) 5-OCH 3 , H, 5-Cl, 3,5-diCl, or 5-NO 2 and where n ) 0 or 2) are shown to be excellent hydrogen peroxide disproportionation catalysts in acetonitrile. When carried out in an open vessel, the reaction can occur for over 5000 turnovers without an indication of catalyst decomposition. The disproportionation reaction cycles between the [Mn III (2-OH(X-sal)pn)] 2 and the [Mn II (2-OH(X-sal)pn)] 2 2oxidation levels. All derivatives show saturation kinetics with the highest k cat (21.9 ( 0.2 s -1 ) observed for the [Mn III (2-OH(5-Clsal)pn)] 2 dimer and the optimal k cat /K M (990 ( 60 s -1 ‚M -1 ) observed for the [Mn III (2-OHsalpn)] 2 . The first step of the reaction is proposed to be the binding of peroxide to the [Mn III (2-OH(X-sal)pn)] 2 through an alkoxide shift to form a ternary intermediate {[Mn III (2-OH(X-sal)pn)] 2 (H 2 O 2 )}. We propose that the turnoverlimiting step is the oxidation of peroxide from this intermediate. The binding efficiency of the peroxide is dependent on the phenyl-ring substitution with the derivatives donating the most electrons having the highest affinity for the substrate. Studies with isotopically labeled H 2 O 2 indicate that protons are important in the turnover-limiting step of the reaction and that the O-O bond is not cleaved during peroxide oxidation. In a closed vessel, the product dioxygen will oxidize [Mn II (2-OH(5-NO 2 sal)pn)] 2 2to [Mn II/III (2-OH(5-NO 2 sal)pn) 2 ] -, and this species can then be stoichiometrically oxidized by hydrogen peroxide to give [Mn III/IV (2-OH(5-NO 2 sal)pn) 2 (µ 2 -O) 2 ] -. This diµ 2 -oxobridged species is catalytically incompetent; however, addition of hydroxylamine hydrochloride restores catalytic activity. The relationship of this catalytic disproportionation of hydrogen peroxide and inactivation of the catalyst will be used to define a model for similar reactions observed for the Lactobacillus plantarum Mn catalase.Similar to the heme catalases, the manganese catalases disproportionate hydrogen peroxide according to eq 1.These manganese enzymes have been isolated from three different bacteria: Lactobacillus plantarum, 2,3 Thermus thermophilus, 4 and Thermoleophilium album. 5 Through activity studies, 5-7 spectroscopy, 8-11 and X-ray crystallographic structural analysis, 12-14 all three enzymes appear to be very similar.The dinuclear manganese center probably cycles between the Mn II 2 T Mn III 2 oxidation levels during the normal catalytic cycle with at least two proposed mechanisms for the catalytic disproportionation of hydrogen peroxide by the Mn catalases having been presented. 7 Evidence for this cycle has been shown by the identical steady-state kinetics for the oxidized (Mn III 2 ) and reduced (Mn II 2 ) forms of T. thermophilus 6 and by observation of changes in XANES spectra during turnover of the L. plantarum enzyme. 15 Both the L. plantarum 7 and T. thermophilus 6 enzymes exhibit substrate saturation kinetics and show no inhibition by hydrogen peroxide even at high substrate c...

Carboxylate Ligands Drastically Enhance the Rates of Oxo Exchange and Hydrogen Peroxide Disproportionation by Oxo Manganese Compounds of Potential Biological Significance

“…[14][15][16] Numerous reports on the "catalase-like reaction" have been published for very diverse manganese compounds. Nevertheless, the most detailed mechanistic studies were performed on complexes of binucleating ligands involving either alkoxides [17][18][19] or phenolates, [20][21][22][23] which stabilize the dimanganese unit by providing it with an internal bridge. Depending upon the Mn ligands, all oxidation states of the pair from Mn ii…”

Synthesis, Structure and Characterisation of a New Trinuclear Di‐μ‐phenolato‐μ‐carboxylato MnIIIMnIIMnIII Complex with a Bulky Pentadentate Ligand: Chemical Access to Mononuclear MnIV–OH Entities

“…[25,28,[34][35][36][37][38] Indeed, these complexes possess the ability to form high-valent Mn-oxo species, [39,40] the sixth position being occupied by various ligands. [41,42] In particular, the use of pyridine as the sixth ligand has been proposed to assist the formation of Mn Voxo species from the [(salen)Mn III ] + complex, using tertbutyl hydroperoxide as the oxidant.…”

Highly Efficient Disproportionation of Dihydrogen Peroxide: Synthesis, Structure, and Catalase Activity of Manganese Complexes of the Salicylimidate Ligand

“…[26] For the [Mn(2-OH-salpn)] 2 (k cat ϭ 4-22 s 2-hydroxy-1,3-diaminopropane], the rate increases tremendously when the alkoxo-bridged dimer is replaced by a µ-oxo-bridged manganese dimer. [10] NaOH and N-methylimidazole were studied as bases in the catalase reaction with the manganese-mesalim and -etsalim complexes. Both bases increased the total conversion as well as the rate of the catalase reaction.…”

“…[10,13,14] In this paper, we report the synthesis, full characterization and catalase activity of two manganese complexes, [Mn 2 (etsalim) 4 (2), in which Hmesalim and Hetsalim are methyl and ethyl salicylimidate, respectively (shown in Figure 1). We also report the catalase studies on the complex [Mn(mesalim) 2 Cl] (3), the crystal structure of which has been recently reported.…”