FIGURE 9.

Mössbauer and EPR spectra of reconstituted N_StrepChlR transcription activator. A, Mössbauer spectra of ⁵⁷Fe-labeled reconstituted N_StrepChlR (630 μm homodimeric N_StrepChlR). The spectra were recorded at 4.2 K in magnetic fields of 53 mT (top, vertical bars) and 6 T (bottom, vertical bars) applied parallel to the γ beam. Simulations with parameters quoted in the text corresponding to the [4Fe-4S]²⁺ and [2Fe-2S]²⁺ clusters are shown in blue and green, respectively, whereas the black line corresponds to their summation. The green and black arrows indicate typical features associated with [2Fe-2S]²⁺ and adventitiously bound iron. B, continuous wave EPR spectra of anoxically reconstituted N_StrepChlR (red line), anoxically reconstituted N_StrepChlR after reduction with 5 mm sodium dithionite (DT) (blue line), and anoxically reconstituted N_StrepChlR exposed to O₂ for 30 min (black trace). Experimental conditions were as follows: microwave power, 0.63 milliwatt; modulation amplitude, 0.5 mT; conversion time, 0.082 s; temperature, 10 K; microwave frequency, 9.480 GHz; sample concentration, 63 μm dimeric N_StrepChlR.