The enzyme tyrosinase is crucial for the biosynthesis of the pigment melanin. Starting from the amino acid tyrosine it catalyzes the selective hydroxylation to the ortho-catechol L-DOPA and furthermore the dehydrogenation to L-dopaquinone. The ortho-selectivity derives from the protein structure, especially the catalytic center. Two copper ions, each coordinated by three histidine residues, can incorporate and therefore activate one oxygen molecule. The result is a μ-η2:η2-peroxide dicopper(II) core with catalytic activity. Bis(pyrazolyl)methane copper complexes mimic the enzymes active site not only structurally, they even show selective catalytic hydroxylation activity with phenolic substrates. So far, only few catalytic models have been developed. Reactions with substituted phenolates result in Michaelis-Menten-like kinetics and reveal information about the mechanism of the hydroxylation step. Our bis(pyrazolyl)methane ligands consist of three N-donors (Fig. 1, left), of which two are pyrazolyl moieties and one is either a pyridinyl or an imidazolyl moiety. By varying the third N-donor the stability of the peroxide complex, the velocity of the hydroxylation step and the catalytic activity are changed. Peroxide dicopper(II) complexes with bis(pyrazolyl)methane ligands are characterized by ESI, Raman, XAS and UV/Vis (Fig. 1, right) and are furthermore analyzed by theoretical calculations.

Tyrosinase model systems with efficient catalytic activity

255th National Meeting (2018)

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