Figure 4.

RFQ EPR spectra of FDP_red (blue spectrum) and 200 ms after mixing FDP_red with ∼0.5 equiv of NO at 4 °C (green spectrum). Simulated spectra are shown as black traces. The black trace overlaying the green spectrum is the sum of “simulated Fe^II{FeNO}⁷” and “simulated dipole-coupled” between a minor portion (<3%) of Fe^II{FeNO}⁷ and FMN_sq at a distance of 4.5 Å. The g = 1.92 indicates a small amount of free NO. A radical species (<1%) at g = 2.0 attributed to FMN_sq has been clipped in both experimental spectra. Conditions immediately after mixing: 250 μM FDP_red and 130 μM NO in 50 mM MOPS, pH 7.3. Instrumental parameters: microwaves, 9.645 GHz, 0.26 mW; temperature, 4 K.