Use of reference compounds in antioxidant activity assessment.

The choice of reference compounds is examined as a "critical control point" of antioxidant activity assessment. Gallic, caffeic, sinapic, uric, and ascorbic acids, isoeugenol, and Trolox were tested using different redox (FRAP, Folin-Ciocalteu) and radical scavenging (DPPH*, ABTS*+, CBA, ORAC) assays. The ability to chelate transition metals was assessed to support some of the findings. Analytes were also tested in liposomes. On the basis of the findings, we do not recommend uric acid (due to solubility constrains) and ascorbic acid (due to fast degradation kinetics) as references. The behavior of the rest of the compounds could not always be attributed to typical structural characteristics. Selection of suitable reference compounds for in vitro antioxidant activity assays is not an easy task to achieve. The choice of reference compounds has to remain at the convenience of the researchers, with regard to the aim of the study.