Dependence of DPPH radical scavenging activity of dietary flavonoid quercetin on reaction environment.

Many of nowadays diseases are due to the oxidative stress resulting from imbalance between formation and neutralization of free radicals. Intake of dietary antioxidants is therefore crucial to maintain good health. As the safety of synthetic antioxidants has raised several questions the search for natural efficient antioxidants from chemicals produced by plants has gained increased popularity in recent years. Quercetin is an abundant dietary flavonoid with well-known radical scavenging properties being often used as a reference compound in many antioxidant tests. Its reaction with 1,1- diphenyl-2-picrylhydrazyl (artificial DPPH radical) is rapid and stoichiometric; however the published inhibitory constants vary in a very wide range, from 95 nM to 226 µM. The analysis indicates the dependence of antiradical capacity of quercetin on the composition of solvent systems where the reactions are performed and also on the surrounding temperature showing somewhat higher scavenging ability at body temperature compared to room conditions. At the same time, this activity is generally independent on the initial DPPH concentration. These data highlight the importance to consider the reaction environment and conditions when predicting the redox behavior of quercetin in a certain cellular context. Moreover, due to the changes in cellular environment accompanying with different pathogeneses the redox action of polyphenols can essentially vary leading even to the situations where the well-known antioxidant quercetin may reveal prooxidant properties.