Fig. 2. Spectroscopic properties of the as-prepared and reduced forms of [4Fe4S]–FdM; (Top): UV/Vis spectra, [4Fe4S]²⁺–FdM (20 μM black line) and [4Fe4S]⁺–FdM (20 μM, red line), (inset): apo-FdM (128 μM); (Bottom) X-band EPR spectra of [4Fe4S]²⁺–FdM (120 μM, black solid line), [4Fe4S]⁺–FdM (120 μM, red solid line) and a reduced solution of iron and sodium sulfide in the presence of β-mercaptoethanol (dashed line). All samples prepared in HEPES buffer (50 mM, pH 8.0). EPR spectra recorded at 10 K; microwave frequency 9.28 GHz; modulation amplitude: 10 G; microwave power: 1 mW.