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

Abstract

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 transition metals, primarily Fe(II), Fe(III) and Cu(II), which participate in reactions generating free radicals. Metal-flavonoid chelates are considerably more potent free radical scavengers than the parent flavonoids and play a prominent role in protecting from oxidative stress. To unravel the origin of their potent biological action extensive 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 complexes 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 incomplete and sometimes even contradictory. The aim of this paper is to give a personal account on the development of the field through a retrospective evaluation of our own research which covers approximately 40 complexes of flavonoids from different flavonoids subclasses (rutin, quercetin, 3-hydroxyflavone, morin and hesperidin) 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 complexes and their potential application for analytical purposes. .

Flavonoidi predstavljaju veliku grupu polifenolnih jedinjenja biljnog porekla, koji imaju antioksidantna svojstva zbog direktnog "hvatanja" slobodnih radikala. Antioksidantno dejstvo flavonoidi takođe ostvaruju stvaranjem helata sa jonima prelaznih metala, prvenstveno sa jonima Fe(II), Fe(III) i Cu(II), koji sami učestvuju u reakciji stvaranja slobodnih radikala. Helati metal-flavonoid su mnogo efikasniji "hvatači" slobodnih radikala od matičnih flavonoida i mogu zaštititi potencijalne mete u organizmu od oksidativnog stresa. Da bi se razjasnio mehanizam biološkog delovanja kompleksa metal-flavonoid, ispitivane su fizičko-hemijske karakteristike kompleksa: mesto vezivanja jona metala, zavisnost strukture kompleksa od odnosa metal/ligand, afinitet flavonoida za vezivanje metalnog jona, itd. Međutim, podaci o sastavu, strukturi i osobinama kompleksa su nekompletni i ponekad kontradiktorni. Stoga je namera ovog rada da kroz retrospektivu naših rezultata damo lični doprinos u ispitivanju metal-flavonoid kompleksa. U prikazanom revijalnom radu sakupljeni su svi naši rezultati o približno 40 kompleksa obrazovanih između pet flavonoida (rutin, kvercetin, 3-hidroksiflavon, morin i hesperidin) i većeg broja metalnih jona ili metalnih grupa. Istaknuti su podaci o mestu vezivanja centralnog jona u kompleksu, stehiometrijskom odnosu metal-ligand, uticaju pH na formiranje kompleksa i konstantama stabilnosti kompleksa. Takođe, u ovom radu prikazali smo i primenu metal-flavonoid kompleksa u analitičke svrhe. .