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To obtain insights into the role of the carboxylate group of Glu73 in the active site of quercetin 2,4-dioxygenase (2,4-QD), the copper(II) complexes [CuIIL n(AcO)] [ 1 (L n = L1), 2 (L n = L2), 3 (L n = L3), and 4 (L n = L4)] supported by a series of carboxylate-containing ligands [ L1H = 2-{[bis(pyridin-2-ylmethyl)amino]methyl}benzoic acid; L2H = 3-{[bis(pyridin-2-ylmethyl)amino]methyl}benzoic acid,; L3H = 2-({bis[2-(pyridin-2-yl)ethyl]amino}methyl)benzoic acid; L4H = 3-({bis[2-(pyridin-2-yl)ethyl]amino}methyl)benzoic acid] as well as the ternary CuII flavonolate (fla-) complexes [CuIIL n(fla)] [ 5 (L n = L1), 6 (L n = L2), 7 (L n = L3), and 8 (L n = L4)] were synthesized and characterized as structural and functional models for the active site of 2,4-QD. The ternary complexes [CuIIL n(fla)] showed different reactivities in the dioxygenation of bound flavonolate to benzoic acid, salicylic acid, and N, N-dimethylbenzamide at 75-90 °C (single-turnover reaction) in the order 5 >> 7 > 8 ≈ 6. A similar reactivity tendency was found in the catalytic dioxygenation of the substrate flavonol (multiturnover reaction) by the binary complexes. The different reactivities of the copper complexes could be attributed to the different Lewis acidities of the copper(II) ions induced by the different coordination environments of the ligands. The results will provide important insights into the pivotal catalytic role of the carboxylate group of Glu73 in 2,4-QD. [ABSTRACT FROM AUTHOR]

In order to get insights into the metal ion effects and the carboxylate effects on enzymatic activity, a series of the carboxylate ligand supported transition metal complexes [MIIL(OAc)] (M = Mn (1), Fe (2), Co (3), Ni (4), Cu (5) and Zn (6); LH = 2-{[bis-(pyridin-2-ylmethyl)amino]methyl}-4-methoxy benzoic acid) were synthesized and characterized as structural and functional models for the active sites of various MII-substituted resting quercetin 2,3-dioxygenases (2,3-QD). Their structures, spectroscopic features, redox properties, as well as the catalytic reactivity toward the substrate flavonol and O2 have been investigated in detail. The model complexes show higher enzymatic reactivities in the catalytic dioxygenation (oxidative ring opening) of the substrate flavonol at lower temperatures (55–100 °C), presumably caused by the carboxylate group in the supporting model ligand, which could lower the redox potential of the bound substrate flavonolate by electron donation. The catalytic reactivity of [MIIL(OAc)] exhibits notable differences and it is in a metal ion dependent order of Co (3) ＞ Ni (4) ＞ Zn (6) ＞ Fe (2) ＞ Mn (1) ＞ Cu (5). The differences in the reactivities among them could be ascribed to the redox potential of the bound substrate flavonolate, which was drastically influenced by the metal ions via tuning the electron density of flavonolate, providing important insights into the metal ion effects and the carboxylate effects on the enzymatic activity of various MII-substituted 2,3-QD. Our model complexes [MIIL(OAc)] are the first examples of a series of structural and functional models of various MII-substituted resting 2,3-QD. [ABSTRACT FROM AUTHOR]

The cover picture shows dioxygen activation by CuII complexes supported by carboxylate‐containing ligands as structural and functional models of quercetin 2,4‐dioxygenase (2,4‐QD), highlighting the role of the carboxylate group in the structure and reactivity. The green fields, the blue sky, and the tree in the background symbolize molecular oxygen as a green and environmentally benign oxidant. Details are discussed in the article by Y.‐J. Sun, S. Itoh et al. on page 1845 ff (DOI: 10.1002/ejic.201601371). For more on the story behind the cover research, see the Cover Profile (DOI: 10.1002/ejic.201700245). [ABSTRACT FROM AUTHOR]