Catechol Oxidase Activity of a Series of New Dinuclear Copper(II) Complexes with 3,5-DTBC and TCC as Substrates: Syntheses, X-ray Crystal Structures, Spectroscopic Characterization of the Adducts and Kinetic Studies

Abstract

A series of dinuclear copper(II) complexes has been synthesized with the aim to investigate their applicability as potential structure and function models for the active site of catechol oxidase enzyme. They have been characterized by routine physicochemical techniques as well as by X-ray single-crystal structure analysis: [Cu 2(H 2L2 (2))(OH)(H 2O)(NO 3)](NO 3) 3.2H 2O ( 1), [Cu(HL1 (4))(H 2O)(NO 3)] 2(NO 3) 2.2H 2O ( 2), [Cu(L1 (1))(H 2O)(NO 3)] 2 ( 3), [Cu 2(L2 (3))(OH)(H 2O) 2](NO 3) 2, ( 4) and [Cu 2(L2 (1))(N 3) 3] ( 5) [L1 = 2-formyl-4-methyl-6R-iminomethyl-phenolato and L2 = 2,6-bis(R-iminomethyl)-4-methyl-phenolato; for L1 (1) and L2 (1), R = N-propylmorpholine; for L2 (2), R = N-ethylpiperazine; for L2 (3), R = N-ethylpyrrolidine, and for L1 (4), R = N-ethylmorpholine]. Dinuclear 1 and 4 possess two "end-off" compartmental ligands with exogenous mu-hydroxido and endogenous mu-phenoxido groups leading to intermetallic distances of 2.9794(15) and 2.9435(9) A, respectively; 2 and 3 are formed by two tridentate compartmental ligands where the copper centers are connected by endogenous phenoxido bridges with Cu-Cu separations of 3.0213(13) and 3.0152(15) A, respectively; 5 is built by an end-off compartmental ligand having exogenous mu-azido and endogenous mu-phenoxido groups with a Cu-Cu distance of 3.133(2) A (mean of two independent molecules). The catecholase activity of all of the complexes has been investigated in acetonitrile and methanol medium by UV-vis spectrophotometric study using 3,5-di- tert-butylcatechol (3,5-DTBC) and tetrachlorocatechol (TCC) as substrates. In acetonitrile medium, the conversion of 3,5-DTBC to 3,5-di- tert-butylbenzoquinone (3,5-DTBQ) catalyzed by 1- 5 is observed to proceed via the formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically for the first time. In methanol medium no such enzyme-substrate adduct has been detected, and the 3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by 1- 5 very efficiently. The substrate TCC forms an adduct with 2- 5 without performing further oxidation to TCQ due to the high reduction potential of TCC (in comparison with 3,5-DTBC). But most interestingly, 1 is observed to be effective even in TCC oxidation, a process never reported earlier. Kinetic experiments have been performed to determine initial rate of reactions (3,5-DTBC as substrate, in methanol medium) and the activity sequence is 1 > 5 > 2 = 4 > 3. A treatment on the basis of Michaelis-Menten model has been applied for kinetic study, suggesting that all five complexes exhibit very high turnover number, especially 1, which exhibits turnover number or K cat of 3.24 x 10 (4) (h (-1)), which is approximately 3.5 times higher than the most efficient catalyst reported to date for catecholase activity in methanol medium.

Tuning the coordination properties of multi-histidine peptides by using a tripodal scaffold: solution chemical study and catechol oxidase mimicking

“…Catechol oxidases are type 3 copper enzymes, 66,67 therefore mostly dinuclear complexes were used for the biomimetic studies. [68][69][70][71][72][73][74] Nevertheless, several mononuclear [75][76][77] or trinuclear 78 complexes were also reported to possess important catecholase-like activity. To screen the enzyme mimetic properties of our oligonuclear copper(II) complexes, their catecholase activity was investigated by using the widely studied 3,5-di-tert-butylcatechol (H 2 DTBC) as model substrate.…”

Catecholase-Like Activity Studies of Tc-99m Radiopharmaceuticals

“…Synthesized enzymes can be used for industrial purposes and medical diagnosis of human brain diseases [5]. Catecholase was isolated for the first time in 1973 [6]. The first crystal structure of purified catecholase from Ipomoea batatas (sweet potato) was described in 1998 [7].…”

“…Catecholase is one of the most well-known representative proteins of one of the seven classified protein groups which are known as type-3 active site proteins [6,8]. Catecholase oxidizes the catechols as defense mechanisms against insects and pathogenic agents in injured tissues in plants [8,9], following:…”

Synthesis, X-ray structural and thermal analysis of a novel copper compound containing dissymmetric independent 5- and 6-coordinate CuL(H2O)n (n=2,3) units and hydrogen bond bridges

“…[4][5][6][7][8][9][10][11][12] Although different structures are often adopted, it can reasonably be assumed that these properties are partly due to the availability of sites of coordinative unsaturation at the metal centre and the presence of amino acid residues. In particular, a defining property of Cu(II) ions is that, unless a tridentate ligand is flexible enough to complement its distorted coordination sphere, 13 Cu(II) will preferably bind only one ligand.…”