Class Ib Ribonucleotide Reductases: Activation of a Peroxido-Mn^IIMn^III to Generate a Reactive Oxo-Mn^IIIMn^IV Oxidant

Abstract

In the postulated catalytic cycle of class Ib Mn₂ ribonucleotide reductases (RNRs), a Mn^II₂ core is suggested to react with superoxide (O₂^·-) to generate peroxido-Mn^IIMn^III and oxo-Mn^IIIMn^IV entities prior to proton-coupled electron transfer (PCET) oxidation of tyrosine. There is limited experimental support for this mechanism. We demonstrate that [Mn^II₂(BPMP)(OAc)₂](ClO₄) (1, HBPMP = 2,6-bis[(bis(2 pyridylmethyl)amino)methyl]-4-methylphenol) was converted to peroxido-Mn^IIMn^III (2) in the presence of superoxide anion that converted to (μ-O)(μ-OH)Mn^IIIMn^IV (3) via the addition of an H⁺-donor (p-TsOH) or (μ-O)₂Mn^IIIMn^IV (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.