Protonation of FeMo-cofactor (FeMo-co) is important for the process of substrate hydrogenation. Its structure has been clarified as Δ-Mo*Fe₇S₉C(R-homocit*)(cys)(Hhis) after the efforts of nearly 30 years, but it remains controversial whether FeMo-co is protonated or deprotonated with chelated ≡C − O(H) homocitrate. We have used protonated molybdenum(V) lactate 1 and its enantiomer as model compounds for R-homocitrate in FeMo-co of nitrogenase. Vibrational circular dichroism (VCD) spectrum of 1 at 1051 cm⁻¹ is attributed to ≡C − O_H vibration, and molybdenum(VI) R-lactate at 1086 cm⁻¹ is assigned as ≡C − O_α-alkoxy vibration. These vibrations set up labels for the protonation state of coordinated α-hydroxycarboxylates. The characteristic VCD band of NMF-extracted FeMo-co is assigned to ν(C − O_H), which is based on the comparison of molybdenum(VI) R-homocitrate. Density functional theory calculations are consistent with these assignments. To the best of our knowledge, this is the first time that protonated R-homocitrate in FeMo-co is confirmed by VCD spectra.

FeMo-辅因子 (FeMo-co) 的质子化对于底物氢化过程很重要。经过近30年的努力，其结构已明确为Δ-Mo*Fe ₇ S ₉ C( R -homocit*)(cys)(Hhis)，但FeMo-co到底是质子化还是去质子化的螯合物 ≡ C − O(H) 高柠檬酸盐。我们使用质子化的乳酸钼 (V) 1及其对映异构体作为固氮酶 FeMo-co 中R -homocitrate 的模型化合物。1在 1051 cm ^-1处的振动圆二色性 (VCD) 光谱归因于 ≡C - O _H振动，以及1086 cm ^-1处的钼 (VI) R -乳酸指定为≡C − O _{α -烷氧基}振动。这些振动为配位的α -羟基羧酸盐的质子化状态设置了标签。NMF 提取的 FeMo-co 的特征 VCD 带分配给 ν(C − O _H )，这是基于钼 (VI) R -高柠檬酸盐的比较。密度泛函理论计算与这些任务一致。据我们所知，这是第一次通过 VCD 光谱证实 FeMo-co中质子化的R-高柠檬酸盐。