Flavonol-serum albumin complexation. Two-electron oxidation of flavonols and their complexes with serum albumin

Quercetin (3,3',4',5,7-pentahydroxyflavone) and quercetin derivatives (3-me thylquercetin, isoquercitrin, rutin) are strong polyphenolic antioxidants a bundant in plants and in the human diet. Recent investigations have shown t hat significant concentrations of albumin-bound quercetin conjugates are pr esent in the plasma of humans fed a quercetin-rich diet. In this work, binding of quercetin and quercetin glycosides to bovine serum albumin (BSA) is quantitatively investigated by fluorescence spectroscopy. The strong fluorescence enhancement of quercetin upon binding points to th e fact that a significant fraction of quercetin adopts a pyrylium-like stru cture in the complex. On the other hand, the observation of a very efficien t quenching of tryptophan fluorescence by quercetin is consistent with a bi nding occurring in the IIA domain. Flavonoid-derived quinones may be formed upon quenching of reactive oxygen species by flavonoids (antioxidant activity). In this work, the quinones ar e conveniently formed upon periodate oxidation of the selected flavonoids i n methanol and in aqueous buffers with and without BSA. A kinetic investiga tion by UV-visible spectroscopy shows that albumin-bound flavonoids are oxi dized as quickly as free flavonoids. Interestingly, the quercetin quinone, which is merely detectable in the absence of BSA because of fast solvent ad dition, is efficiently stabilized in the complex by charge transfer interac tions (pH 9). No evidence for quercetin-BSA conjugates could be found, thus showing that water addition (and subsequent degradation) remains the sole significant pathway of quinone transformation in the complex.