A review of phenolic compounds from medicinal plants and nutraceuticals, and their characterization by different antioxidant assays

Medicinal plants have been a primary source of bioactive compounds that are finally being developed as commercial drugs. Nowadays, many phytochemicals are associated with health benefits and they are used as antioxidants, antimicrobials, anticancer and antiinflammatory agents. Phenolic compounds are the main secondary metabolites and are highly emphasized due to their well-documented bioactivities. These compounds may be classified into different groups as a function of the number of phenol rings that they contain and on the basis of structural elements that bind these rings to one another. Numerous studies indicated a positive association between health-promotion and well-being and the consumption of a polyphenolic rich diet. Generally, the health benefit effects of phenolic compounds depend on their antioxidant effects. Various methods are used to investigate the antioxidant properties of plant extracts or phenolic compounds. Especially concerning radical scavenging (DPPH, ABTS, etc.) and reducing power (CUPRAC, FRAP, etc.), phosphomolybdenum assays are amongst the most common antioxidant assays. This chapter aims to critically review several aspects concerning the occurrence of phenolics in different medicinal plant matrices and nutraceuticals, and their characterization by HPLC coupled with different antioxidant assays. Moreover, a special aim of this work would be to create a glimpse concerning modern approaches used to assess the antioxidant capacity of phenolics by HPLC online coupling techniques.