Extended Data Fig. 8: EPR experimental spectra and simulations of CODH crystal suspensions recorded at 10 K, reduced with Eu(II)-DTPA.

(a) Eu(II)-reduced pH 6.0 sample and (b) Eu(II)-reduced pH 6.0 + CO sample, both recorded at 0.063 mW microwave power. (c) Eu(II)-reduced pH 8.0 sample and (d) Eu(II)-reduced pH 8.0 + CO₂ sample, both recorded at 0.25 mW microwave power. The black solid line represents the experimental spectrum and the grey dotted line the simulated spectrum. The simulated species have the following colour code: B_red (dark cyan), C_redΔNi (violet), C_red2 (blue), C_red2’ (light blue) and Ni-signal (brown). The difference between the experimental and the simulated data likely corresponds to the observed minor orientation dependence of the EPR spectra (Extended Data Fig. 3). The Ni-signal (g = 2.35, 2.25, 2.21; g_av = 2.27) is presumably related to a weak population of Ni from a degraded C-cluster or non-physiological nickel complex. A similar type of signal with a g_av significantly larger than 2 (g_av of C_red1 and C_red2 signals are clearly below 2, see Table S3) was observed before for the CODH from Moorella thermoacetica (assigned to a C_red3 or C* state⁵²) or the CODH from Rhodospirillum rubrum⁵³. For these states a catalytic relevance has not yet been shown. The B_red signal was used as an internal standard for quantification, since the B-cluster is fully occupied in the crystal structure and should be fully reduced at the applied redox potential.

Source data