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

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.