Antioxidant Assays Based on Free Radical Neutralization

Many antioxidant assays are based on their ability to neutralize or quench free radicals. The two free radicals that have been most commonly used for assessing antioxidant activity are 1,1-diphenyl-2-picrylhydrazyl (DPPH) (43,44), and 2,2'-azinobis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS) (45,46) (Figure 10.3).

The DPPH free radical is a stable radical with one electron delocalized over the molecule (Figure 10.3). This delocalization gives a deep purple color with absorption maxima at 517 nm in an ethanol solution (43,44). When an antioxidant capable of donating a hydrogen reacts with the DPPH radical it gives rise to a nonradical reduced form of DPPH which has a yellow color. The decrease in the absorption is measured spectrophoto-metrically and is compared with an ethanol control to calculate the DPPH free radical scavenging activity (43,44).

This method is a very quick and simple method for the measurement of antioxidant activity. The antioxidant capacity in this assay depends on the chemical structure of the antioxidant (43,44). The reduction in the DPPH radical is dependent on the number of hydroxyl groups present in the antioxidant. This method, therefore, gives an indication of the structural dependence of the antioxidant functionality of many biological antioxidants (43,44).

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