[36] Flavonoids as antioxidants: Determination of radical-scavenging efficiencies

This chapter discusses the radical-scavenging efficiencies of flavonoids as antioxidants. To test whether a certain substance acts as radical scavenger, the clearest evidence comes from the determination of reaction rate constants with a set of different specifically produced radicals. In the tests discussed in the chapter, hydroxyl, azide, superoxide, linoleic acid peroxyl, tert-butoxyl, and sulfite radicals are used. The spectral observation of radical reactions with flavonoids is assisted by the strong absorption characteristics of both the parent compounds and their respective aroxyl radicals. Owing to the low solubility of flavonoids, conditions for pseudo-first order reactions with radical species can rarely be achieved for flavonoid aglycones. Therefore, kinetic modeling calculations are employed. The radical chemistry of flavonoids not only is of interest from a kinetic or mechanistic point of view but also offers considerable insight into structural relationships of highly evolved plant components. First, the consistently high rate constants for attack by different types of radicals demonstrate the effective radicals scavenging capabilities of most flavonoids. Second, owing to extensive electron delocalization as a prerequisite for radical stabilization, multiple mesomeric structures exist for aroxyl radical species of flavonoids.