EPR Spectroscopy on Mononuclear Molybdenum-Containing Enzymes

Carbon Dioxide Utilisation—The Formate Route

“…The involvement of the sulfido group as the hydride acceptor during FDH catalysis was demonstrated by electron paramagnetic resonance (EPR) spectroscopic studies that showed that, in formate-reduced FDH, the formate Ca hydrogen atom is transferred to an acceptor group located within magnetic contact to the molybdenum atom of FDHs from different sources (E. coli [166], D. desulfuricans [136] or Cupriavidus necator (previously known as Ralstonia eutropha) [184]). The observation of a strongly coupled, solvent-exchangeable and substrate-derived proton, with a hyperfine constant of 20-30 MHz, is consistent with the hydrogen atom being transferred to a ligand in the first coordination sphere of the molybdenum atom upon its reduction [196,197,200,201,208]. Similar hyperfine constant values were determined in model complexes [209] and also in real enzymes, as in xanthine oxidase, where the strong coupled hydrogen is originated from the xanthine C8 hydrogen atom (the position that is hydroxylated by that enzyme (see Footnote 6) [196-198, 200-202, 208].…”

“…EPR spectroscopy: Further experimental evidence for the stable molybdenum/tungsten hexa-coordination came from EPR spectroscopy that clearly showed that the selenocysteine/cysteine must remain bound to the Mo 5+ centre of formate-reduced enzyme [208]. When the EPR spectrum is obtained from 77 Se-enriched enzyme, a very strong and anisotropic interaction with selenium is observed (A 1,2,3 ( 77 Se) = 13.2, 75, 240 MHz) [166].…”

Toward the Mechanistic Understanding of Enzymatic CO2 Reduction

“…The sulfido ligand is introduced by a dedicated chaperone and is essential for activity. − Despite considerable studies, the mechanism used by FDHs to oxidize formate or reduce CO 2 is still unclear. Several proposals have been made, and the topic has raised considerable debate. ,,− Critical issues still under discussion are (i) whether the Sec/Cys ligand dissociates from the metal during catalysis and is replaced by formate, ,,− or if metal coordination is maintained during catalysis; ,, and (ii) whether oxidation of formate involves hydride transfer to the metal or sulfido group, ,,, or C α proton abstraction by a protein residue. , A number of experimental and computational tools have been explored to try to elucidate this mechanism, namely X-ray crystallography, ,− X-ray absorption spectroscopy, ,, EPR, ,− theoretical calculations, ,, and inhibition studies. ,, However, the results are not always conclusive due to the difficulty in confidently assigning the observations to relevant catalytic states, compounded by the intrinsic instability of the proteins, which may be present in inactivated states. There is, thus, an urgent need for detailed structural information on FDHs in different catalytic/redox states.…”

Electronic Structure and Excited State Dynamics of the NIR-II Emissive Molybdenum(III) Analog to the Molecular Ruby

“…62 Detailed investigations of the zero-field splitting (ZFS) of mononuclear molybdenum complexes are limited to few reports. [65][66][67][68] Studies of luminescent molybdenum(III) complexes are equally rare, with only 10 examples covered in three publications, all of which feature at least three (pseudo)halide ligands. 24,[69][70][71][72] Due to the low interelectronic repulsion in the extended 4d orbitals, the reported emission is red-shifted compared to typical chromium(III) complexes and located >1100 nm.…”