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

In the postulated catalytic cycle of class Ib Mn-2 ribonucleotide reductases (RNRs), a Mn-2(II) core is suggested to react with superoxide (O-2(-)) to generate peroxido-(MnMnIII)-Mn-II and oxo-(MnMnIV)-Mn-III entities prior to proton-coupled electron transfer (PCET) oxidation of tyrosine. There is limited experimental support for this mechanism. We demonstrate that [Mn-2(II)(BPMP)(OAc)(2)](ClO4) (1, HBPMP = 2,6-bis[(bis(2 pyridylmethyl)amino)methyl]-4-methylphenol) was converted to peroxido-(MnMnIII)-Mn-II (2) in the presence of superoxide anion that converted to (mu-O)(mu-OH)(MnMnIV)-Mn-III (3) via the addition of an H+-donor (p-TsOH) or (mu-O)(2)(MnMnIV)-Mn-III (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 O-2/Mn-2/tyrosine activation mechanism in class Ib Mn-2 RNRs is plausible and provides spectral insights into intermediates currently elusive in the native enzyme.