FIGURE 6.

A, Mo^V-Fe/S EPR spectrum (black) and simulation (red) of FdsABG collected at 150 K with modulation amplitude = 3 Gauss and microwave power = 4 mW. The sample was prepared under anaerobic conditions by reduction of 50 μm of FdsABG in 200 mm Tris-HCl, pH 8.5, in the presence of 10 mm KNO₃, with 0.25 mm sodium formate on ice and mixing for 5 s before freezing. The Mo^V component represents ∼10% of total spin density. B, simulation of the Fe/S1 contribution to the spectrum in A. C, simulation of the Mo^V contribution to the spectrum in A. D, Mo^V-Fe/S EPR spectrum (black) and simulation (red) of FdsABG collected at 150 K with modulation amplitude = 2 Gauss and microwave power = 4 mW. The sample in A above was thawed in a room temperature water bath and incubated for an additional 55 s before refreezing. The Mo^V component represents ∼19% of total spin density. E, simulation of the Fe/S1 contribution to the spectrum in D. F, simulation of the Mo^V contribution to the spectrum in D. sim, simulation.