Class Ib Ribonucleotide Reductases: Activation of a Peroxido-MnIIMnIII to Generate a Reactive Oxo-MnIIIMnIV Oxidant.

In the postulated catalytic cycle of class Ib Mn2 ribonucleotide reductases (RNRs), a MnII2 core is suggested to react with superoxide (O2·-) to generate peroxido-MnIIMnIII and oxo-MnIIIMnIV entities prior to proton-coupled electron transfer (PCET) oxidation of tyrosine. There is limited experimental support for this mechanism. We demonstrate that [MnII2(BPMP)(OAc)2](ClO4) (1, HBPMP = 2,6-bis[(bis(2 pyridylmethyl)amino)methyl]-4-methylphenol) was converted to peroxido-MnIIMnIII (2) in the presence of superoxide anion that converted to (µ-O)(µ-OH)MnIIIMnIV (3) via the addition of an H+-donor (p-TsOH) or (µ-O)2MnIIIMnIV (4) upon warming to room temperature. The physical properties of 3 and 4 were probed using UV-vis, EPR, X-ray absorption, and IR spectroscopies and mass spectrometry. Compounds 3 and 4 were capable of phenol oxidation to yield a phenoxyl radical via a concerted PCET oxidation, supporting the proposed mechanism of tyrosyl radical cofactor generation in RNRs. The synthetic models demonstrate that the postulated O2/Mn2/tyrosine activation mechanism in class Ib Mn2 RNRs is plausible and provides spectral insights into intermediates currently elusive in the native enzyme.

A MnII MnIII -Peroxide Complex Capable of Aldehyde Deformylation.

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn2 RNRs superoxide (O2.- ) was postulated to react with the MnII2 core to yield a MnII MnIII -peroxide moiety. The reactivity of complex 1 ([MnII2 (O2 CCH3 )2 (BPMP)](ClO4 ), where HBPMP=2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol) towards O2.- was investigated at -90 °C, generating a metastable species, 2. The electronic absorption spectrum of 2 displayed features (λmax =440, 590 nm) characteristic of a MnII MnIII -peroxide species, representing just the second example of such. Electron paramagnetic resonance and X-ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a MnII MnIII -peroxide complex. Unlike all other previously reported Mn2 -peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O2 -activation in Class Ib Mn2 RNRs, and the highly reactive intermediates in their catalytic cycle.

Catechol oxidase activity of comparable dimanganese and dicopper complexes.

Synthetic Cu complexes have been widely investigated as model systems for catechol oxidase enzymes. The catechol oxidase reactivity of Mn complexes has been less explored, and the effect of metal substitution in catecholase mimics has not been explored. A series of Mn and Cu complexes supported by the same poly-benzimidazole ligand framework have been synthesised and investigated in catecholase activity in acetonitrile medium using 3,5-di-tert-butylcatechol (3,5-DTBC) as a substrate. The Cu complexes proved to be good catechol oxidase mimics with moderate kcat values (∼45 h-1). The kinetic parameters for Mn complexes exhibited lower kcat values (∼8-40 h-1) when compared to the Cu complexes. Our findings demonstrate that later transition metals supported by relatively electron rich ligands yield the highest kcat values for catechol oxidation.

Mimicking Class†I b Mn2 -Ribonucleotide Reductase: A MnII2 Complex and Its Reaction with Superoxide.

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn2 ) active site in class I b RNRs that requires superoxide anion (O2.- ), rather than dioxygen (O2 ), to access a high-valent Mn2 oxidant. Complex 1 ([Mn2 (O2 CCH3 )(N-Et-HPTB)](ClO4 )2 , N-Et-HPTB=N,N,N',N'-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane) was synthesised in high yield (90 %). 1 was reacted with O2.- at -40 °C resulting in the formation of a metastable species (2). 2 displayed electronic absorption features (λmax =460, 610 nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a MnII MnIII entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of MnII2 in 1 to MnII MnIII for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a MnII MnIII -peroxide complex. 2 was capable of oxidizing ferrocene and weak O-H bonds upon activation with proton donors. Our findings provide support for the postulated mechanism of O2.- activation at class I b Mn2 RNRs.

Nucleophilic reactivity of a copper(II)-superoxide complex.

Metal-bound superoxide intermediates are often implicated as electrophilic oxidants in dioxygen-activating metalloenzymes. In the nonheme iron α-ketoglutarate dependent oxygenases and pterin-dependent hydroxylases, however, Fe(III)-superoxide intermediates are postulated to react by nucleophilic attack on electrophilic carbon atoms. By reacting a Cu(II)-superoxide complex (1) with acyl chloride substrates, we have found that a metal-superoxide complex can be a very reactive nucleophile. Furthermore, 1 was found to be an efficient nucleophilic deformylating reagent, capable of Baeyer-Villiger oxidation of a number of aldehyde substrates. The observed nucleophilic chemistry represents a new domain for metal-superoxide reactivity. Our observations provide support for the postulated role of metal-superoxide intermediates in nonheme iron α-ketoglutarate dependent and pterin-dependent enzymes.