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, [Cu 2 L(NO 3 ) 2 ] ( 2 ), and one heterometallic, [CuMnL(NO 3 ) 2 ] ( 3 ) complexes were obtained using the ligand H 2 L = 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) × 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) × 10 6 h −1 , which it is the higher reported to date, comparable to the enzyme itself (8.25 × 10 6 h −1 ). Graphic abstract