Molybdenum citrate towards the protonation of FeMo-co in nitrogenase

The structure of FeMo-cofactor (FeMo-co) in nitrogenase has been clarified as MoFe7S9C[R-(H)homocit](Hhis)(cys) (H(4)homocit = homocitrate, Hhis = histidine, Hcys = cysteine), where homocitrate chelates to Mo(III) via alpha-alkoxy/alpha-hydroxy and a-carboxy groups. Recent model comparisons and theoretical calculations suggested a hydroxy coordinated homocitrate in FeMo-co, which may play an important role in storing and transferring hydrogen source for the reduction of substrate. Herein, two molybdenum citrates [(Mo2O)-O-IV(Hcit)(2)(tpy)(2)]center dot 3H(2)O (1) (H(4)cit = citrate, tpy = alpha,alpha,alpha-terpyridine) and (H(2)tpy)(2)[(Mo2O5)-O-VI(Hcit)(2)]center dot 7.5H(2)O (2) have been obtained via hydrothermal reactions under the reduction of hydrazine hydrochloride in acidic condition. 1 and 2 were fully characterized by elemental analyses, IR, UV-vis, EPR and C-13 NMR spectra, bond valence calculations and X-ray single crystal diffractions. Structural analysis indicates that 1 is a binuclear complex, which seven coordination sites are occupied by one mu 2-O atom, three nitrogen atoms of terpyridine, three oxygen atoms from citrate. Citrate chelates to Mo via alpha-alkoxy, alpha-carboxy and beta-carboxy groups, the uncoordinated beta-carboxylic acid group interacts with water molecules through strong hydrogen bonds. 2 is a common binuclear Mo(VI) complex counterbalanced by protonated terpyridine cations. The coordination of citrates in 2 is similar to that of 1. C-13 NMR spectrum indicates that 2 partially dissociates in solution. We have also analyzed Mo-O distances of 1 and 2, as well as previously reported molybdenum alpha-hydroxycarboxylates. It is found that trans-effect of Mo=O group, protonation of alpha-alkoxy group and oxidation state of Mo are the three major factors that affect the distances between Mo and coordinated atoms from alpha-hydroxycarboxylates. Mo-O distances are elongated about 0.05-0.13 angstrom from trans-effect, while this effect for alpha-carboxy group is stronger than that of alpha-alkoxy group. Mo-O (alpha-hydroxy) distance is about 0.11 angstrom longer than that of Mo-O (alpha-alkoxy) due to protonation effect. Mo-O (alpha-alkoxy/alpha-carboxy) distances show negative correlation with oxidation state of Mo. Linear fit gives a Mo-III-O (alpha-carboxy) distance that is close to those of wild-type FeMo-co and citrate-substituted cofactor of variant Mo-nitrogenase, while the calculated Mo-III-O (alpha-alkoxy) distance is much shorter. The difference is well in agreement with the protonation effect. This result gives a quantitative conclusion for the protonation mode of homocitrate in FeMo-co. A new protonated model is also deduced for citrate-substituted cofactor of variant Mo-nitrogenase. It seems more structural data of model compounds should be included for statistical analysis, especially those complexes in low valences.

Automated summary

The structure of FeMo-cofactor in nitrogenase has been elucidated, suggesting a potential role of hydroxy-coordinated homocitrate in hydrogen storage and transfer. Two molybdenum citrates were synthesized as models, revealing insights into Mo-O distances and the effects of Mo trans-effect, group protonation, and oxidation state on the coordination structure.