Xican Li
Chinese herbal medicines (CHMs) are used in traditional Chinese medicines (TCM) clinical practice under the guidance of the holism theory. Their clinical usages are basically related to their antioxidant effects; therefore, there have been accumulated a great deal of antioxidant studies in the past 40 years. The review systematically analyzes 769 available references, to find that the antioxidant capacity of CHMs overwhelmingly arises from phenolics (including polyene-conjugated phenolics) rather than saponins or polysaccharides. The antioxidant mechanisms of phenolics, however, mainly include electron-transfer (ET), proton-transfer (PT), hydrogen atom transfer (HAT), ET-PT, PT-ET, and radical adduct formation (RAF). The RAF can occur at different binding sites to form different bonding types to cause various final products (including phenolic-radical adducts, and phenolic-phenolic dimers) and be affected by structural factors to show certain stereochemical characters. Further structure-activity relationships analysis indicates that the antioxidant effect of phenolics is seemingly attributed to the existence of specific antioxidant moieties (e.g., catechol); however, the high levels of these so-called "specific antioxidant moieties" can be attributed to the stability of the oxidized products from their reactions with free radicals and, ultimately, to the degree of conjugation of the whole antioxidant molecule.