X-band EPR spectra of WT nitrogenase turnover samples trapped under 1 atm of N2 (with stirring to facilitate transfer of H2 formed during turnover into the headspace15), and under low P(N2) in H2O buffer (without stirring) shown in comparison with spectrum of E4(4H) state trapped during turnover of α-70Val→Ile MoFe protein of the same concentration (see M&M). EPR conditions: temperature, 12 K; microwave frequency, 9.36 GHz; microwave power, 10 mW; modulation amplitude, 9 G; time constant, 160 ms; field sweep speed, 20 G/s.