Design and catalytic studies of structural and functional models of the catechol oxidase enzyme

The catechol oxidase activity of three copper/bicompartmental salen derivatives has been studied. One mononuclear, [CuL] (1), one homometallic, [Cu2L(NO3)(2)] (2), and one heterometallic, [CuMnL(NO3)(2)] (3) complexes were obtained using the ligand H2L = N,N '-bis(3-methoxysalicylidene)-1,3-propanediamine through different synthetic methods (electrochemical, chemical and solid state reaction). The structural data indicate that the metal ion disposition models the active site of type-3 copper enzymes, such as catechol oxidase. In this way, their ability to act as functional models of the enzyme has been spectrophotometrically determined by monitorization of the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butyl-o-benzoquinone (3,5-DTBQ). All the complexes show significant catalytic activity with ratio constants (k(obs)) lying in the range (223-294) x 10(-4) min(-1). A thorough kinetic study was carried out for complexes 2 and 3, since they show structural similarities with the catechol oxidase enzyme. The greatest catalytic activity was found for the homonuclear dicopper compound (2) with a turnover value (k(cat)) of (3.89 +/- 0.05) x 10(6) h(-1), which it is the higher reported to date, comparable to the enzyme itself (8.25 x 10(6) h(-1)). Graphic abstract