Impressive promiscuous biomimetic models of ascorbate, amine, and catechol oxidases

Abstract

Copper metalloenzymes ascorbate oxidase (AOase), amine oxidase (AmOase), and catechol oxidase (COase) possess copper(II) sites of coordination, which are trimeric, homodimeric, and dimeric, respectively. Two newly mononuclear copper(II) complexes, namely, [Cu(L)(bpy)](ClO₄) (1) and [Cu(L)(phen)](ClO₄) (2) where HL = Schiff base, have been synthesized. UV–visible, EPR and single-crystal X-ray diffraction examinations were used to validate the geometry in solution and solid state. For complex 1, the metal exhibits a coordination sphere between square pyramidal and trigonal bipyramidal geometry (τ, 0.49). A positive Cu^II/^I redox potential indicates a stable switching between Cu^II and Cu^I redox states. Despite the monomeric origin, both homogeneous catalysts (1 or 2) in MeOH were found to favor three distinct chemical transformations, namely, ascorbic acid (H₂A) to dehydroascorbic acid (DA), benzylamine (Ph-CH₂-NH₂) to benzaldehyde (Ph-CHO), and 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) [k_cat: AOase, 9.6 (1) or 2.0 × 10⁶ h⁻¹(2); AmOase, 13.4 (1) or 9.4 × 10⁶ h⁻¹ (2); COase, 2.0 (1) or 1.9 × 10³ h⁻¹ (2)]. They exhibit higher levels of AOase activity as indicated by their k_cat values compared to the AOase enzyme. The k_cat values for COase activity in buffer solution [5.93 (1) or 2.95 × 10⁵ h⁻¹ (2)] are one order lower than those of the enzymes. This is because of the labile nature of the coordinated donor, the flexibility of the ligand, the simplicity of the catalyst-substrate interaction, and the positive Cu^II/^I redox potential. Interestingly, more efficient catalysis is promoted by 1 and 2 concerning that of other mono- and dicopper(II) complexes.