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

ABSTRACT: Protonation of FeMo-cofactor is important for the process of substrate hydrogenation. Its structure has been clarified as Δ-Mo*Fe₇S₉C(R-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 1 and its enantiomer as model compounds for R-homocitrate in FeMo-cofactor 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-cofactor is assigned to ν(C-O_H), which is based on the comparison of molybdenum(VI) R-homocitrate. Density Functional Theory calculations are in consistent with these assignments. To the best of our knowledge, this is the first time that protonated R-homocitrate in FeMo-cofactor is confirmed by VCD spectra.

SUBMITTER: Deng L

PROVIDER: S-EPMC8323615 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:A similar pair of protonated and deprotonated mononuclear oxidovanadium glycolates [VO(Hglyc)(phen)(H2O)]Cl·2H2O (1) and [VO(glyc)(bpy)(H2O)] (2) and a mixed-(de)protonated oxidovanadium triglycolate (NH4)2[VO(Hglyc)2(glyc)]·H2O (3) were isolated and examined. The ?C-O(H) (?C-OH or ?C-O) groups coordinated to vanadium were spectroscopically and structurally identified. The glycolate in 1 features a bidentate chelation through protonated ?-hydroxy and ?-carboxy groups, whereas the glycolate in 2 coordinates through deprotonated ?-alkoxy and ?-carboxy groups. The glycolates in 3 coordinate to vanadium through ?-alkoxy or ?-hydroxy and ?-carboxy groups and thus have both protonated ?C-OH and deprotonated ?C-O bonds simultaneously. Structural investigations revealed that the longer protonated V-O?-hydroxy bonds [2.234(2) Å and 2.244(2) Å] in 1 and 3 are close to those of FeV-cofactor (FeV-co) 2.17 Å1 (FeMo-co 2.17 Å2), while deprotonated V-O?-alkoxy bonds [2, 1.930(2); 3, 1.927(2) Å] were obviously shorter. This shows a similar elongated trend as the Mo-O distances in the previously reported deprotonated vs protonated molybdenum lactates (Wang, S. Y. et al. Dalton Trans. 2018, 47, 7412-7421) and these vanadium and molybdenum complexes have the same local V/Mo-homocitrate structures as those of FeV/Mo-cos of nitrogenases. The IR spectra of these oxidovanadium and the previously synthesized molybdenum complexes including different substituted ?C-O(H) model compounds show red-shifts for ?C-OH vs ?C-O alternation, which further assign the two IR bands of extracted FeMo-co at 1084 and 1031 cm-1 to ?C-O and ?C-OH vibrations, respectively. Although the structural data or IR spectra for some of the previously synthesized Mo/V complexes and extracted FeMo-co were measured earlier, this is the first time that the ?C-O(H) coordinated peaks are assigned. The overall structural and IR results well suggest the coexistence of homocitrates coordinated with ?-alkoxy (deprotonated) and ?-hydroxy (protonated) groups in the extracted FeMo-co.

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

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