Quercetin is a bioactive compound that is widely used in botanical medicine and traditional Chinese medicine due to its potent antioxidant activity. In recent years, antioxidant activities of quercetin have been studied extensively, including its effects on glutathione (GSH), enzymatic activity, signal transduction pathways, and reactive oxygen species (ROS) caused by environmental and toxicological factors. Chemical studies on quercetin have mainly focused on the antioxidant activity of its metal ion complexes and complex ions. In this review, we highlight the recent advances in the antioxidant activities, chemical research, and medicinal application of quercetin.

Antioxidant Activities of Quercetin and Its Complexes for Medicinal Application.

Flavonoids constitute a large group of polyphenolic phytochemicals with antioxidant properties which are overwhelmingly exerted through direct free radical scavenging. Flavonoids also exhibit antioxidant properties through chelating with tran- sition metals, primarily Fe(II), Fe(III) and Cu(II), which participate in reactions ge- nerating free radicals. Metal-flavonoid chelates are considerably more potent free radical scavengers than the parent flavonoids and play a prominent role in protec- ting from oxidative stress. To unravel the origin of their potent biological action ex- tensive physico-chemical studies were undertaken to reveal the chemical structure, chelation sites, assess the impact of the metal/ligand ratio on the structure of the com- plexes and the capacity of flavonoids to bind metal ions. In spite of such extensive efforts, data on the composition, structure and complex-formation properties are in- complete and sometimes even contradictory. The aim of this paper is to give a per- sonal account on the development of the field through a retrospective evaluation of our own research which covers approximately 40 complexes of flavonoids from dif- ferent flavonoids subclasses (rutin, quercetin, 3-hydroxyflavone, morin and hespe- ridin) with several metal ions or groups and suggest directions for future research. Special emphasis will be given to the site of the central ion, the composition of the complexes, the role of pH in complex formation, the stability of metal-flavonoid com- plexes and their potential application for analytical purposes.

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Investigation of metal-flavonoid chelates and the determination of flavonoids via metal-flavonoid complexing reactions*

Biological activity of metal ions complexes of chromones, coumarins and flavones

Synthesis, characterization and antioxidant activity copper-quercetin complex.

Flavonoids are widely occurring polyphenol compounds of plant origin with multiple biological and chemical activities. Due to the presence of carbonyl and hydroxyl groups they can coordinate metal ions and form complexes. Metal complexes of flavonoids have many interesting properties: they are colored, often fluorescent, anti- or pro-oxidant, antimicrobial, antiproliferative and biologically active in many other ways. There are many papers covering specific aspects of activity of flavonoid metal complexes, e.g. their antioxidant properties, enzyme-mimicking behavior, therapeutic potential or use in chemical analysis. However, for a researcher interested in this theme, it would be useful to find an extensive review on more than one selected area. Our aim was to cover a wide spectrum of possible activities and potential applications of flavonoids coordinated to metal ions in order to give our readers a broad view on the topic of this class of compounds, their activity and potential applications. While a significant amount of information on the chemical properties and biological activity of flavonoid metal complexes can be found in the literature, an in-depth understanding of structure–property relationships is still lacking. In an attempt to address this issue, a comprehensive discussion of the available data is presented.

Ning Tang

Papers

Synthesis, characterization and DNA-binding properties of La(III) complex of chrysin.

Synthesis, characterization, antioxidative and antitumor activities of solid quercetin rare earth(III) complexes