Molybdenum citrate towards the protonation of FeMo-co in nitrogenase

Abstract

<p indent="0mm">The structure of FeMo-cofactor (FeMo-co) in nitrogenase has been clarified as MoFe₇S₉C[<italic>R</italic>-(H)homocit](Hhis)(cys) (H₄homocit = homocitrate, Hhis = histidine, Hcys = cysteine), where homocitrate chelates to Mo(III) via α-alkoxy/α-hydroxy and α-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 [Mo^IV₂O(Hcit)₂(tpy)₂]·3H₂O (<bold>1</bold>) (H₄cit = citrate, tpy = α,α,α-terpyridine) and (H₂tpy)₂[Mo^VI₂O₅(Hcit)₂]·7.5H₂O (<bold>2</bold>) have been obtained via hydrothermal reactions under the reduction of hydrazine hydrochloride in acidic condition. <bold>1 </bold>and <bold>2</bold> were fully characterized by elemental analyses, IR, UV-vis, EPR and ¹³C NMR spectra, bond valence calculations and X-ray single crystal diffractions. Structural analysis indicates that <bold>1</bold> is a binuclear complex, which seven coordination sites are occupied by one μ₂-O atom, three nitrogen atoms of terpyridine, three oxygen atoms from citrate. Citrate chelates to Mo via α-alkoxy, α-carboxy and β-carboxy groups, the uncoordinated β-carboxylic acid group interacts with water molecules through strong hydrogen bonds. <bold>2</bold> is a common binuclear Mo(VI) complex counterbalanced by protonated terpyridine cations. The coordination of citrates in <bold>2</bold> is similar to that of <bold>1</bold>. ¹³C NMR spectrum indicates that <bold>2</bold> partially dissociates in solution. We have also analyzed Mo–O distances of <bold>1</bold> and <bold>2</bold>, as well as previously reported molybdenum α-hydroxycarboxylates. It is found that <italic>trans</italic>-effect of Mo=O group, protonation of α-alkoxy group and oxidation state of Mo are the three major factors that affect the distances between Mo and coordinated atoms from α-hydroxycarboxylates. Mo–O distances are elongated about 0.05−0.13 Å from <italic>trans</italic>-effect, while this effect for α-carboxy group is stronger than that of α-alkoxy group. Mo–O (α-hydroxy) distance is about 0.11 Å longer than that of Mo–O (α-alkoxy) due to protonation effect. Mo–O (α-alkoxy/α-carboxy) distances show negative correlation with oxidation state of Mo. Linear fit gives a Mo^III–O (α-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 (α-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.