Relation between the Catalytic Efficiency of the Synthetic Analogues of Catechol Oxidase with Their Electrochemical Property in the Free State and Substrate-Bound State

Abstract

A library of 15 dicopper complexes as synthetic analogues of catechol oxidase has been synthesized with the aim to determine the relationship between the electrochemical behavior of the dicopper(II) species in the absence as well as in the presence of 3,5-di-tert-butylcatechol (3,5-DTBC) as model substrate and the catalytic activity, kcat, in DMSO medium. The complexes have been characterized by routine physicochemical techniques as well as by X-ray single-crystal structure analysis in some cases. Fifteen "end-off" compartmental ligands have been designed as 1 + 2 Schiff-base condensation product of 2,6-diformyl-4-R-phenol (R = Me, (t)Bu, and Cl) and five different amines, N-(2-aminoethyl)piperazine, N-(2-aminoethyl)pyrrolidine, N-(2-aminoethyl)morpholine, N-(3-aminopropyl)morpholine, and N-(2-aminoethyl)piperidine. Interestingly, in case of the combination of 2,6-diformyl-4-methylphenol and N-(2-aminoethyl)morpholine/N-(3-aminopropyl)morpholine/N-(2-aminoethyl)piperidine 1 + 1 condensation becomes the reality and the ligands are denoted as L2(1-3). On reaction of copper(II) nitrate with L2(1-3) in situ complexes 3, 12, and 13 are formed having general formula Cu2(L2(1-3))2(NO3)2. The remaining 12 ligands obtained as 1 + 2 condensation products are denoted as L1(1-12), which produce complexes having general formula Cu2(L1(1-12))(NO3)2. Catecholase activity of all 15 complexes has been investigated in DMSO medium using 3,5-DTBC as model substrate. Treatment on the basis of Michaelis-Menten model has been applied for kinetic study, and thereby turnover number, kcat, values have been evaluated. Cyclic voltametric (CV) and differential pulse voltametric (DPV) studies of the complexes in the presence as well as in the absence of 3,5-DTBC have been thoroughly investigated in DMSO medium. From those studies it is evident that oxidation of 3,5-DTBC catalyzed by dicopper(II) complexes proceed via two steps: first, semibenzoquinone followed by benzoquinone with concomitant reduction of Cu(II) to Cu(I). Our study reveals that apparently there is nearly no linear relationship between kcat and E° values of the complexes. However, a detailed density functional theory (DFT) calculation sheds light on this subject. A very good correlation prevails in terms of the energetics associated with the Cu(II) to Cu(I) reduction process and kcat values, as revealed from the combined theoretical and experimental approach.

Azido bridge mediated catecholase activity, electrochemistry and magnetic behavior of a dinuclear copper(II) complex of a phenol based “end-off” compartmental ligand

“…At more negative potentials, at À1.12 V and À1.78 V, ligand based reductions were observed. In the Zn(II) complex of the above ligand, formulated as Zn 2 LCl 3 , with chloro and phenoxo bridges connecting the Zn(II) ions, two irreversible ligand centered reductions were observed at À0.9 and À1.5 V versus Ag/AgCl reference electrode [2]. This justifies our assignment of a quasi-reversible reduction at À0.55 V to a metal centered rather than a ligand centered reduction process.…”

“…This justifies our assignment of a quasi-reversible reduction at À0.55 V to a metal centered rather than a ligand centered reduction process. It may be noted that in the related hydroxo bridged complex the Cu(II)/Cu(I) reductions could not be detected, and an irreversible reductive wave at À0.75 V in cyclic voltammetry was assigned to Cu(I)/Cu(0) process by comparison with thirteen similar complexes [2]. This was probably due to the sluggish rate of heterogeneous electron transfer for the Cu (II)/Cu(I) reduction, which broadens the reductive wave so much to render it undetectable.…”

“…where the phenol is a 4-chloroor 4-tert-butyl derivative, respectively, and metals are bridged by a hydroxyl OH group rather than the azide [2]. It is worth of note the crystal packing showing nitrate anions H-bond connected by the coordinated water molecules so that to form a polymeric chain elongated along axis a (Fig.…”

“…Dinuclear copper complexes with Cu-Cu separation of $3 Å are interesting as corroborative model of the active site of catechol oxidase to elucidate the functional mechanism of the native enzyme and to develop bio-inspired catalytic systems [1][2][3][4][5][6][7][8][9]. When that separation is associated with bridging azido ligand the species becomes of particularly interesting for gaining deeper insight into magneto-structural correlation, a study very much essential for developing new functional molecule based materials [10][11][12][13][14].…”

Investigation of 3d-transition metal acetates in the oxidation of substituted dioxolene and phenols

“…Among the various catalytic conversions carried by enzymes we chose the ones similar to that carried by catechol oxidase Researchers around the world have put significant effort to model CO and PHS through design of metal complexes of complicated ligand systems [20][21][22][23] and provided insight about the mechanistic pathway [21,22,[24][25][26] of these enzymes. The metal salts, especially the acetates which are used as precursor in syntheses of many such biomimetic complexes are scarcely probed for their efficiency in such catalysis [27][28][29][30].…”