Synthesis and characterization of homochiral polymeric S-malato molybdate(VI): toward the potentially stereospecific formation and absolute configuration of iron-molybdenum cofactor in nitrogenase

Abstract

Reaction of sodium or potassium molybdate and excess malic acid in a wide range of pH values (pH 4.0–7.0) resulted in the isolation of two cis-dioxo-bis(malato)-Mo(VI) complexes, viz. Na 3[MoO 2H( S-mal) 2] and K 3[MoO 2H( S-mal) 2]·H 2O (H 3mal=malic acid). The sodium complex is also characterized by an X-ray structure analysis, showing that the mononuclear Mo units are linked together via very strong symmetric CO 2···H··· O 2C-hydrogen bond [2.432(5) Å], forming a polymeric chain. The molybdenum atoms are quasi-octahedrally coordinated by two cis-oxo groups and two bidentate malate ligands via its alkoxy and α-carboxyl groups, while the β-carboxylic and carboxylate groups remain uncomplexed, as the coordination of vicinal carboxylate and alkoxide of homocitrate in FeMo cofactor of nitrogenase. The absolute configuration of the metal center in this S-malato complex is assigned as Λ and the homochirality within the chain is established as a homochiral form ···Λ S–Λ S–Λ S–Λ S···. It is proposed that the chiral configuration of the metal center in wild-type FeMo-co biosynthesis might be induced by the early coordination of the chiral R-homocitric acid, while a mixture of raceme might be obtained in the biosynthesis of NifV − FeMo-cofactor. The absolute configuration of wild-type FeMo-cofactor is assigned as Δ R.