Assignment of protonated -homocitrate in extracted FeMo-cofactor of nitrogenase vibrational circular dichroism spectroscopies.

Protonation of FeMo-cofactor is important for the process of substrate hydrogenation. Its structure has been clarified as Δ-MoFeSC(-homocit)(cys)(Hhis) for the efforts of nearly 30 years, while it remains controversial whether FeMo-cofactor is protonated or deprotonated with chelated ≡C-O(H) homocitrate. We have used protonated molybdenum(V) lactates and its enantiomer as model compounds for -homocitrate in FeMo-cofactor of nitrogenase. Vibrational circular dichroism (VCD) spectrum of at 1051 cm is attributed to ≡C-O vibration, and molybdenum(VI) -lactate at 1086 cm is assigned as ≡C-O vibration. These vibrations set up labels for the protonation state of coordinated -hydroxycarboxylates. The characteristic VCD band of NMF-extracted FeMo-cofactor is assigned to ν(C-O), which is based on the comparison of molybdenum(VI) -homocitrate. Density Functional Theory calculations are in consistent with these assignments. To the best of our knowledge, this is the first time that protonated -homocitrate in FeMo-cofactor is confirmed by VCD spectra.