A novel S = 3/2 EPR signal associated with native Fe‐proteins of nitrogenase

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Abstract
In addition to their g = 1.94 EPR signal, nitrogenase Fe‐proteins from Azotobacter vinelandii, Azotobacter chroococcum and Klebsiella pneumoniae exhibit a weak EPR signal with g ≅5. Temperature dependence of the signal was consistent with an S = system with negative zero‐field splitting, d = −5 ± 0.7 cm−1. The m s, = ± ground state doublet gives rise to a transition with g eff = 5.90 and the transition within the excited m s = ± doublet has a split g eff = 4.8, 3.4. Quantitation gave 0.6 to 0.8 spin mol−1 which summed with the spin intensity of the S = g = 1.94 line to roughly 1 . MgATP and MgADP decreased the intensity of the s = signal with no concomitant changes in intensity of the s = signal.