Altering the activity of catechol oxidase model compounds by electronic influence on the copper core

The catecholase activity of the dicopper(II) complexes [Cu-2(L-1)mu-OCH3)(NCCH3)(2)](PF6)(2)(H2OCH3CN)-H-.-C-. (1), [Cu-2(L-2)(mu-OH)(MeOH)(NCCH3)](BF4)(2) (2), [Cu-2(L-3)(mu-OMe)(NCCH3)(2)](BF4)(2)(.)2CH(3)CN(.)H(2)O (3), [CU2(L-2)(mu-OAc)(2)](BF4H2O)-H-. (4), (CU2(L-4)(mu-OAc)(2)]ClO4 (5) and [Cu-2(L-5)(mu-OMe)(NCCH3)(3)(OH2)](ClO4)(2)(2CH3OH)-C-.-CH3CN (6) consisting of varying para-substituted phenol ligands HL1 = 4-trifluoromethyl-2,6-bis((4-methylpiperazin-1-yl)methyl)phenol, HL2 = 4-bromo-2,6bis((4-methyl-1,4-diazepati-1-yl)methyl)phenol, HL3 = 4-bromo-2((4-methyl-1,4-diazepati-1-yl)methyl)-6-((4-methylpiperazin-1-yl)- methyl)phenol, HL4 2,6-bis((4-methylpiperazin-l-yl)methyl)-4-nitrophenol and HL5 4-tert-butyl-2,6-bis((4-methylpiperazin-l-yl)methyl)phenol was studied. The main difference within the six complexes lies in the individual copper-copper separation that is enforced by the chelating side arms of the phenolate ligand entity and more importantly in the exogenous bridging solvent, hydroxide, methanolate or acetate ions. The distance between the copper cores varies from 2.94 angstrom in 1 to 3.29 angstrom in 5. The catalytic activity of the complexes 1-6 towards the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butylquinone characteristic absorption band at about 400 nm over time saturated with O-2. The complexes are able to oxidize the substrate 3.5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity (k(cat) between 92 h(-1) and 189 h(-1)), with an order of decreasing activity 6 > 5 > 1, 2, 4 : 3. A kinetic treatment of the data based on the Michaelis-Menten approach was applied. A correlation of the catecholase activities with the variation of the para-substituents as well as other effects resulting from the copper core distances is discussed. [Cu-2(L-5)(mu-OMe)(NCCH3)(3)(OH)(2)](CIO4)(2)(.)2CH(3)OH(.)CH(3)CN (6) exhibited the highest activity of the six complexes as a result of its high turnover rate.