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

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, tBu, 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 L21–3. On reaction of copper­(II) nitrate with
L21–3 in situ complexes 3, 12, and 13 are formed having general formula Cu2(L21–3)2(NO3)2. The remaining 12 ligands obtained as 1 + 2 condensation products
are denoted as L11–12, which produce complexes having
general formula Cu2(L11–12)­(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 CuII to CuI. 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 CuII to CuI reduction process and kcat values, as
revealed from the combined theoretical and experimental approach.

本研究合成了一组15种双铜络合物，作为儿茶酚氧化酶的合成类似物，旨在探究双铜（II）物种在无3,5-二叔丁基儿茶酚（3,5-DTBC）存在以及存在该模型底物条件下的电化学行为，与在DMSO介质中的催化活性kcat之间的关系。这些络合物通过常规的物理化学技术进行了表征，在部分情况下还通过X射线单晶结构分析进行了确定。设计了一种由2,6-二甲酰-4-R-苯酚（R = Me, tBu, 和 Cl）与五种不同的胺（N-(2-氨基乙基)哌嗪、N-(2-氨基乙基)吡咯烷、N-(2-氨基乙基)吗啉、N-(3-氨基丙基)吗啉和N-(2-氨基乙基)哌啶）的1+2席夫碱缩合产物组成的15种“端部”配体。在2,6-二甲酰-4-甲基苯酚与N-(2-氨基乙基)吗啉/N-(3-氨基丙基)吗啉/N-(2-氨基乙基)哌啶的1+1缩合反应中，生成了L21–3型配体。当硝酸铜与L21–3型配体原位反应时，形成了具有通式Cu2(L21–3)2(NO3)2的络合物3、12和13。其余12种作为1+2缩合产物的配体被称为L11–12，它们产生的络合物具有通式Cu2(L11–12)(NO3)2。在DMSO介质中，以3,5-DTBC作为模型底物，对所有15种络合物的儿茶酚酶活性进行了研究。采用Michaelis-Menten模型进行了动力学研究，从而评估了转换数kcat值。在DMSO介质中，对络合物在存在以及不存在3,5-DTBC条件下的循环伏安法（CV）和差分脉冲伏安法（DPV）进行了深入研究。从这些研究中可以明显看出，由双铜（II）络合物催化的3,5-DTBC氧化反应分为两步进行：首先，半苯醌的形成，随后是苯醌的形成，同时CuII被还原为CuI。本研究揭示，络合物的kcat值与E°值之间几乎没有线性关系。然而，通过详细的密度泛函理论（DFT）计算，揭示了这一现象。结合理论与实验方法，CuII到CuI还原过程的能量学及其kcat值之间存在着非常好的相关性。