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(O2CCH3)(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 MnIIMnIII entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of MnII2 in 1 to MnIIMnIII for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a MnIIMnIII-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.
Bibliographical noteFunding Information:
This publication has emanated from research supported by the Irish Research Council (IRC) under Grant Number GOIPG/2014/942 to A. M. Research in the McDonald lab is supported in part by the European Union (ERC-2015-STG-678202) and research grants from Science Foundation Ireland (SFI/12/RC/2278, SFI/15/RS-URF/3307). Support for research in the Que lab is provided by the US National Institutes of Health (GM38767). XAS data collection at SSRL BL 9-3 was made possible by support from the National Institutes of Health (P30-EB-009998 and P41-GM-103393) and Department of Energy (Contract DE-AC02-76SF00515 to SLAC National Accelerator Laboratory from the Office of Science, Office of Basic Energy Sciences). We thank the DJEI/DES/SFI/HEA Irish Centre for High-End Computing (ICHEC) for the provision of computational facilities and support. We are grateful to Dr. Paolo Pirovano and Ms. Subhasree Kal for assistance with EPR measurements.
- bioinorganic chemistry
- dimanganese complexes
- dioxygen/superoxide activation
- oxidation reactions
- ribonucleotide reductases