Fig. 1. EPR spectra of the [4Fe–4S]¹⁺ clusters in AhbD. The spectra were recorded at 10 K for (a) wt AhbD, (c) AhbD C321A/C324A and (d) AhbD C19A/C23A. The trace in (b) represents the sum of (c) and (d). Experimental parameters: field modulation 8 Gauss. Microwave frequency = 9.6428 GHz. Power 0.2 mW.