Isolation of quercetin's salts and studies of their physicochemical properties and antioxidant relationships.

Hydroxyflavones in alkaline solutions show high free radical scavenging activities. Quercetin, one of these hydroxyflavones may be submitted to chemical reactions yielding a mixture of mono-, di- and tri-sodium salts. These salts were recovered after solubilization and stepwise precipitation in methylalcohol/ethylacetate solvents. The different salts were analyzed using sodium emission spectrophotometry and nuclear magnetic resonance to determine the number of acid hydrogens at pH10 and the position of these acid hydrogens. Our study demonstrates that among the three salts of quercetin, the di-sodium compound is endowed with the more efficient scavenging properties in a phosphate buffer at physiological pH7.4. Physicochemical parameters and free hydroxyl radical scavenging activity relationships were also determined, allowing to explain the mechanisms whereby hydroxyl groups exert their radical scavenging activities.