Synthesis, Characterization And Reactivity Of Model Complexes For The Active Site Of Flavonol2,4-dioxygenase

Dioxygen activation for the purpose of biomimetic oxidations has received considerable attention, owing to the possible application in catalysts for bioremediation of aromatic waste recently. Flavonol2,4-dioxygenase activates dioxygen to catalyze the oxygenative ring opening reaction of the O-heterocycle of flavonol to the corresponding depside and carbon monoxide. In order to get insights into the catalytic role of metal ion effects, electronic and steric effects of the ligands, and the carboxylate effects of Glu73on the structure, reactivity and catalytic mechanism of FDO, we designed, synthesized and characterized a series of complexes as the structural and functional ES models for the active site of FDO. Their structures, spectroscopic properties, electrochemical properties as well as reactivity towards molecule dioxygen have been investigated.(1) A novel model ligand L11H (2-{[bis(6-bromo-pyridin-2-ylmethyl)amino]methyl} benzoic acid) has been designed, synthesized and characterized by1H NMR, API/MS, IR.(2) Twelve new M(â…¡) ternary model complexes [Mâ…¡Ln(fla)](M:Fe, Cu, Co, Ni, Zn, Mn, n:11,8) of L11H and L8H (5-bromo-2-{[bis(2-pyridinylmethyl)amino]methyl} benzoic acid) have been designed, synthesized as the structural and functional ES model complexes for the active site of FDO, and characterized by X-ray diffraction, IR, API/MS, UV-Vis and CV.(3) The reactivities of the model complexes towards dioxygen have been investigated by UV-Vis monitoring. Our conclusions are as follows:â‘ Metal ion effects:with the same ligand, the reactivities of different metal ion model complexes are quite different, and in the order of Fe>Ni> Cu>Zn>Mn>Co for [Mâ…¡L11(fla)] and Fe> Cu> Co> Ni> Zn> Mn for [Mâ…¡L8(fla)], respectivetely.â‘¡Electronic effects of the model ligands:with the same metal ions, the reactivities of the model complexes with different ligands show notable differences.Electronic effects of the phenyl ring:the reactivities are in the order of L10(o-OMe)> L9(o-Me)> L1(-H)> L8(m-Br)> L7(m-NO2), which indicate that electron withdrawing group of the phenyl ring of the ligand sidearm can decrease the reaction rate, and the oxygenation reaction is an electrophilic reaction.Electronic effects of the pyridine ring:the reactivivity is in the order of L13(-CH3)> L1(-H)> L11(-Br), which indicates that electronic withdrawing group of the pyridine ring of the ligand sidearm can decrease the reaction rate, and the oxygenation reaction is an electrophilic reaction. â‘¢Steric effects of the model ligands:with the same metal ions, the reactivities of the model complexes are in the order of L1(-H)> L8(Ph-m-Br)> L11(Py-o-Br), which indicate that the steric hindrance of the pyridine ring of the ligands plays a major role.â‘£Carboxylate effects of the model ligand:The reactivities of model Fe(â…¡) and Cu(â…¡)-complexes in which the supporting ligands with a carboxylate group are much higher than the reported model complexes without any carboxylate group, which indicate the carboxyl group of Glu73is a catalytic site of FDO indeed.