FIGURE 4.

EPR spectra of Qrc complex. A, shown is the [3Fe-4S]⁺¹ signal present in the oxidized sample. B, shown are low-spin heme signals present in the oxidized sample (1), theoretical simulation obtained by the sum of four components (three low-spin hemes and one [3Fe-4S]⁺¹ center) in a 3:2:1:1 ratio (hemes: g_a = 3.16, 1.9, 1.3; g_b = 2.93, 2.21, 1.5; g_c = 2.825, 2.28, 1.6; [3Fe-4S]⁺¹ center, g = 2.01, 2.01, 1.2) (2), and after reduction with ascorbic acid 0.2 m (3). C, shown is the spectrum after dithionite reduction with some indicative g values shown for reference (1) and after reduction with 0.5 μm [NiFeSe] hydrogenase/0.5 μm TpIc₃ in the presence of H₂ (2). Spectra were recorded under the following conditions: microwave frequency, 9.38 GHz; microwave power, 20.1 milliwatts (A) and 2.4 milliwatts (B and C); modulation frequency, 100 kHz; modulation amplitude, 1 millitesla; temperature, 10 K.