Corroborative cobalt and zinc model compounds of α-amino-β-carboxymuconic-ε-semialdehyde decarboxylase (ACMSD)

We have synthesised and characterised a series of new Co^II complexes (1–4, 6, 7) and one new Zn^II complex (5) employing N₃- and N₃O-donor ligands [biap: N,N-bis(2-ethyl-5-methyl-imidazol-4-ylmethyl)amino-propane, KBPZG: potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate, KBPZA: potassium N,N-bis(3,5-dimethylpyrazolylmethyl) alaninate, KBⁱPrPZG: potassium N,N-bis(3,5-di-iso-propylpyrazolylmethyl) glycinate, and KB(^tBuM)PZG: potassium N,N-bis(3-methyl-5-tert-butyl-pyrazolylmethyl)glycinate] as structural models of the metalloenzyme α-amino-β-carboxymuconic-ε-semialdehyde decarboxylase (ACMSD). These complexes were characterised by several techniques including X-ray crystallographic analysis, X-band EPR, and mass spectrometry (ESI-MS). The crystal structures of 1, 2, 6, 7 revealed that they exist as mononuclear Co(II) complexes with trigonal-bipyramidal geometry in the solid state. Compounds 3 and 5 form infinite polymeric chains of Co^II or Zn^II complexes, respectively, linked by the pendant carboxylate arms of the BPZG⁻ ligand. By comparing the degree of distortion in the penta-coordinate complexes, defined by the Addison-parameter τ, with the value determined for the five-coordinate centres found in the active site of ACMSD, it could be seen that complexes 5 and 7 are very good matches for the geometry of the zinc(II) centre in monomer A of the native enzyme. All complexes could be seen as model compounds for the active site of the enzyme ACMSD, where the Co(II) complexes reflected the structural flexibility found in case of two histidine (His177 and His228) residues found in the active site of the enzyme.