Iron Systems, Part 2: Selected Systems from Al-B-Fe to C-Co-Fe

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These compounds are important dietary components and have attracted considerable attention owing to their antioxidizing properties.


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Flavonoids are polyphenolic compounds that are widely distributed in vascular plants, and form active constituents of a number of herbal and traditional medicines. In the soil environment, fungal and bacterial flavonol 2,4-dioxygenases quercetinases catalyze the oxidative degradation of flavonols to a depside phenolic carboxylic acid esters with concomitant evolution of carbon monoxide.

Flavonol 2,4-dioxygenase was first recognized more than four decades ago in species of Aspergillus grown on rutin, and quercetinases from Aspergillus flavus Oka et al. The diffraction studies showed that the enzyme forms homodimers, and each unit is mononuclear, with a type 2 copper center. Interestingly, an X-ray structure of Aspergillus japonicus anaerobically complexed with the natural substrate quercetin indicated that flavonols bind to the copper ion in a monodentate fashion.

With the availability of the sequence and structural parameters for Aspergillus japonicus flavonol 2,4-dioxygenase, homologous enzymes were sought from other species.

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Both enzymes belong to the cupin superfamily, in which the cupin domain compraises two conserved motifs. These two motifs have been found to ligate a number of divalent metal ions e. Recent studies have been described the protein YxaG as an iron-containing flavonol 2,4-dioxygenase, but direct evidence for the natural cofactor is still missing Gopal et al. Metal-substituted flavonol 2,4-dioxygenases were generated by expressing the enzyme in Escherichia coli grown on minimal media in the presence of various divalent metals.

The turnover number of the Mn II -containing enzyme was found to be in the order of 25 s -1 , nearly fold higher than that of the Fe II -containing enzyme and similar in magnitude to that of the Cu II -containing flavonol 2,4-dioxygenase from Aspergillus japonicus. On the basis of earlier kinetic and spectroscopic data it can be said that Mn II might be the preferred cofactor for this enzyme and that the catalytic enzyme mechanism is different from that of the Aspergillus species.

After formation of the flavonoxyl-manganese-superoxide intermediate, the reaction could proceed via two pathways Fig. In the first pathway a , the superoxide intermediate reacts with the flavonoxyl radical to form a lactone intermediate and a hydroxide ion via a Criegee intermediate.

A Baeyer-Villiger rearrangement with alkyl migration would then generate the final products.

Monitoring the State-of-Charge in All-Iron Aqueous Redox Flow Batteries

This is identical to the mechanism proposed for extradiol catechol dioxygenases. In the second pathway b , a 2,4-endoperoxide intermediate is formed and decomposes into the depside and carbon monoxide, similar to the mechanism proposed for the Aspergillus flavonol 2,4-dioxygenase. Interestingly, recent investigations of flavonol 2,4-dioxygenase from Streptomyces sp.

FLA expressed in E. In this case the nonredox role of the metal center was proposed Merkens et al. Studies on structural and functional models are important in order to elucidate the mechanism of the enzyme reaction. Extensive studies report on the coordination chemistry of flavonols with various metal ions. The stability of the metal flavonolates above can be explained by the chelation and formation of a stable five-membered ring in the flavonolate complexes. It can be assumed that the coordination mode of the substrate in the enzymatic and model systems could give rise to differences in the degradation rates.

Apart from the coordination mode of flavonolate ligands, it is important to know how the flavonolate ligand is activated for the reaction with molecular oxygen. From our earlier results obtained both with redox and non-redox metal-containing systems, the conclusion could be drawn that the oxygenolysis of the flavonolate ion in aprotic solvents takes place via an 2,4-endoperoxide intermediate Kaizer et al. Since there is no manganese- or iron-containing systems in the literature, in this book we report details for synthesis and characterization of some manganese and iron III flavonolate complexes as synthetic models for the YxaG dioxygenase, and their direct and carboxylate-enhanced dioxygenation compared to the copper-containing models, respectively.

We will show that bulky carboxylates as coligands dramatically enhance the reaction rate, which can be explained by two different mechanisms, caused by the formation of more reactive monodentate flavonolate complexes. Synthetic manganese and iron complexes have been synthesized and characterized by IR, UV-vis spectroscopy and X-ray crystallography Fig. This can be interpreted by the formation of a stable five-membered chelate that is formed upon the coordination of the 3-OH and 4-CO oxygen atoms of flavonol. For example Mn fla 2 py 2 , exhibits band I at nm.

The molecular structure and atom numbering scheme for complex Mn fla 2 py 2 can be seen in Fig.

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The manganese has a slightly distorted tetragonal bipyramidal geometry, which possesses high symmetry with trans coordination of the flavonolate ligands in the basal plane and the two pyridines in apical positions. Spectroscopic and structural data for synthetic ES type complexes: a Pap et al. The manganese-oxygen bond distances are in the range of 2. The average Mn—O distance in Mn fla 2 py 2 2. The O2—C1 distance is shorter while the O3—C9 distance is longer than those in the uncoordinated flavonol [1.

Introduction

Due to coordination to the manganese ion there are also changes in the bond lengths of the pyranone ring. The O1—C2 [1. The iron-oxygen bond distances are in the range of 1. This is well consistent with the structure of the complex where iron is surrounded by ligands resulting a considerable asymmetric charge distribution reflected by the obtained quadrupole splitting value. The molecular structure of Fe O-bs salen as well as selected bond lengths and angles is shown in Fig.

The molecule is monomeric in the solid state.

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The overall geometry around the six-coordinated iron ion is described as a distorted octahedral geometry. Flavonolate as a chelating ligand forms stable complexes with copper, manganese and iron ions. The GLC-MS analysis of the residue of the hydrolyzed complexes, after treatment with ethereal diazomethane, showed the presence of the O-benzoylsalicylic acid methylester.

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