Fig. 1: The three catalytic C cluster states Cred1, Cint and Cred2.
From: Metalloradical-driven enzymatic CO2 reduction by a dynamic Ni–Fe cluster
a, EPR spectra of the three electronic states of the C cluster in crystal suspensions, recorded at 10 K and 0.25 mW. The intense signal from Ti(III) (dashed lines) dominates the centre of the spectra (Extended Data Fig. 2). The rhombic signals from Cred1 (red) and Cred2 (blue) were simulated, while species indicative of the Ni-deficient C cluster state CredΔNi and Bred are highlighted in purple and dark cyan, respectively. The absence of any paramagnetic C cluster signal is indicative for the diamagnetic Cint state (orange) in the mddle EPR spectrum. The asterisk labels a deviation from the simulation, which could originate from an additional minor species or relate to the slight angle dependence of the EPR spectra (Extended Data Fig. 3). b–d, Crystal structures of the EPR-identified states determined from crystals treated in the same way as those used for EPR spectroscopy. For each state, the σA-weighted 2Fobserved − Fcalculated (2Fo − Fc) maps (Fobserved, observed structure factor amplitudes; Fcalculated, calculated structure factor amplitudes) in blue mesh, contour level = 1.0σ, and blue isosurface, contour level = 2.0σ) of Ni–Fe1 with the bound OH or CO2 ligand are shown separately to the right of the refined crystal structure. Two alternative positions of Fe1 are shown as ‘i’ for the ‘in’ conformation (denoted Fe1,i) and ‘o’ for the ‘out’ conformation (denoted Fe1,o), with the alternative conformation of His261 coordinating Fe not shown for clarity. The occupancies of the OH ligands in b and c and of CO2 in d were refined to 0.67, 0.52 and 0.66, respectively. Each state was chemically poised at stable redox potentials (Cred1 = −409 mV, Cint = −530 mV and Cred2 = −530 mV versus the standard hydrogen electrode (SHE)) using Ti(III)–EDTA (see Methods).
