Inhibition of the Catecholase Activity of Biomimetic Dinuclear Copper Complexes by Kojic Acid

The inhibition of the catechol oxidase activity
exhibited by three dinuclear copper(II) complexes,
derived from different diaminotetrabenzimidazole ligands,
by kojic acid [5-hydroxy-2-(hydroxymethyl)-g-pyrone]
has been studied. The catalytic mechanism of
the catecholase reaction proceeds in two steps and for
both of these inhibition by kojic acid is of competitive
type. The inhibitor binds strongly to the dicopper(II)
complex in the first step and to the dicopper-dioxygen
adduct in the second step, preventing in both cases
the binding of the catechol substrate. Binding studies
of kojic acid to the dinuclear copper(II) complexes
and a series of mononuclear analogs, carried out spectrophotometrically
and by NMR, enable us to propose
that the inhibitor acts as a bridging ligand between
the metal centers in the dicopper(II) catalysts.