Assignment of protonated R-homocitrate in extracted FeMo-cofactor of nitrogenase via vibrational circular dichroism spectroscopy

Abstract

Protonation of FeMo-cofactor is important for the process of substrate hydrogenation. Its structure has been clarified as Δ-Mo*Fe₇S₉C(<i>R</i>-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 <b>1</b> and its enantiomer as model compounds for <i>R</i>-homocitrate in FeMo-cofactor of nitrogenase. Vibrational circular dichroism (VCD) spectrum of <b>1</b> at 1051 cm^-1 is attributed to ≡C-O_H vibration, and molybdenum(VI) <i>R</i>-lactate at 1086 cm^-1 is assigned as ≡C-O _{<i>α</i>-alkoxy} vibration. These vibrations set up labels for the protonation state of coordinated <i>α</i>-hydroxycarboxylates. The characteristic VCD band of NMF-extracted FeMo-cofactor is assigned to ν(C-O_H), which is based on the comparison of molybdenum(VI) <i>R</i>-homocitrate. Density Functional Theory calculations are in consistent with these assignments. To the best of our knowledge, this is the first time that protonated <i>R</i>-homocitrate in FeMo-cofactor is confirmed by VCD spectra.