EPR spectroscopic characterization of complex I. EPR spectra of complex I isolated from the parental (a) and the deletion strain (b). The spectra shown in (A) were recorded at 40 K and 2 mW microwave power, the spectra shown in (B) were recorded at 13 K and 5 mW microwave power. The signals of the Fe/S clusters N1a, N1b, N2, N3 and N4 are indicated. The signal of cluster N1a is nearly saturated at 13 K and 5 mW. Other EPR conditions were: microwave frequency, 9.44 GHz; modulation amplitude, 0.6 mT; time constant, 0.124 s; scan rate, 17.9 mT/min.