The antioxidant active ingredients of plant extracts mainly include alkaloids, saponins, vitamins, polyphenols, polypeptides and polysaccharides. Most of these ingredients are extracted from grains, herbs, vegetables and fruits. This article describes several major antioxidative components in plant extracts.

1.Alkaloids
Alkaloids are a class of organic compounds that have complex nitrogen-containing cyclic structures and significant physiological activities, most of which are distributed in higher plants, especially dicotyledons such as Ranunculaceae, Papaveraceae, Solanaceae, Musk Branch, legume, etc. The structural factors affecting the antioxidant activity of alkaloids are mainly stereostructure and electrical properties. The more “naked” the nitrogen atom in the heterocycle is, the more favorable it is to fully react with the active oxygen, the better the antioxidant effect. Electronic groups or structural factors that make the nitrogen atom rich in electrons can also increase its antioxidant activity. The alkaloids with antioxidation include brucine, aloperin, tetrahydroberberine, higenamine, magnoline, ligustrazine, berberine, glaucine, jatrorrhizine, anonaine and so on.

2.Saponins
Saponins are an important class of active substances in herbal medicine. According to the chemical structure of aglycones, saponins can be classified into steroidal saponins and triterpenoid saponins. Steroidal saponins are mostly found in liliaceae and dioscorea plants, and triterpenoid saponins are found in araliaceae and umbelliferae plants. Recent studies have shown that most saponins have significant antioxidant effects. The total saponins contained in Gynostemma pentaphyllum, Rhodiola, scutellaria, Qiye, Bupleurum, Momordica, Polygonum cuspidatum, Mangosteen and Camellia oleifera also have strong antioxidant activity.

3.Vitamins
Vitamins are the most important antioxidants in the human body. Vitamins in plants include VE, VC and carotenoids. VE is a general term for various tocopherols and is the most important fat-soluble antioxidant. VC has an enol-type hydroxyl group capable of dissociating hydrogen ions and is the most important water-soluble antioxidant. It can remove active oxygen free radicals by gradually providing electrons and can promote the regeneration of VE. There are more than 600 carotenoids in nature. All carotenoids have an isoprenoid structure with 11 double bonds. Studies have found that carotenoids such as lutein, zeaxanthin, lycopene and astaxanthin have significant antioxidant functions.

4.Polyphenols
Plant polyphenolic antioxidants can be divided into three major categories: tannins, flavonoids and phenolic acids.

4.1 Tannins

Tannins are widely distributed in plants and usually refer to plant polyphenols with a relative molecular mass of 500-3000. According to the difference in molecular structure and hydrolysis, it can be divided into hydrolyzed tannins (such as gallnut tannins, ellagitannins), condensed tannins (such as proanthocyanidins, oligomeric proanthocyanidins), complex tannins (such as camellia B, guavacin A). Factors affecting the antioxidant activity of tannins include the unit binding mode, whether the hydroxyl group is free, and the quantity of hexahydroxydibenzoyl (HHDP), gallic acid, dehydrohexahydroxydibenzoyl (DHHDP) groups. When the tannin binding unit (such as catechin) is bound by a hydrolyzable ester bond or a glycosidic bond, the antioxidant capacity of the molecule is enhanced, and when it is combined with a carbon-carbon bond to form a condensation type, the molecular antioxidant capacity is greatly reduced. In the binding unit, the greater the number of HHDP, gall, DHHDP groups, the greater the activity of tannin.

4.2 Flavonoids

Flavonoids are the most abundant species in polyphenols, and almost all tissues of all plants contain such natural products. Flavonoid compounds generally refer to a series of compounds in which two benzene rings (A- and B-ring) are linked by a central three-carbon bond, which can be further classified into flavonoids, flavonols, flavanones, and flavanonol, flavanoid-3-ols (also known as catechins), isoflavones, chalcones, and anthocyanidins. Flavonoids have different antioxidant activities, and their antioxidant activity is closely related to their structures. It is generally believed that the ortho-diphenolic hydroxyl group on the B ring plays a major role in the antioxidant activity of the flavonoids. Many flavonoids such as echinoside, quercetin, naringenin, taxol, tea polyphenols, soy isoflavones, trihydroxychalcone and cyanidin show significant antioxidant properties.

4.3 Phenolic acids

Phenolic acid refers to a class of compounds having several phenolic hydroxyl groups on the same phenyl ring. The phenolic acids found in natural plants can be divided into three categories: hydroxybenzoic acid and its derivatives (such as protocatechuic acid, gallic acid, syringic acid); ellagic acid and its derivatives ( such as 3-hydroxyphenylacetic acid); hydroxycinnamic acid and its derivatives (such as chlorogenic acid, ferulic acid, caffeic acid, rosmarinic acid, coumaric acid, sinapic acid). The relationship between the antioxidant capacity of phenolic acids and their chemical structure is the same as that of flavonoids, that is, the phenolic acids with adjacent phenolic hydroxyl groups have much stronger antioxidant capacity than those without phenolic hydroxyl groups. For example, chlorogenic acid, caffeic acid, and rosmarinic acid having a catechol structure have stronger antioxidant capacity than ferulic acid and sinapic acid having only one hydroxyl group.

5.Active peptides
Peptide-like substances with antioxidant properties are called bioactive peptides. Researchers have extracted various peptides with antioxidant activity from proteins of different plant origins. However, only glutathione is a natural antioxidant peptide that we can understand its molecular structure and conduct related mechanism researches. More natural antioxidant peptides such as soybean peptide, corn peptide, wheat peptide, rice bran peptide and peanut peptide are present in various natural proteases, are low molecular mixed peptides having certain antioxidant activity.

6.Polysaccharides
Polysaccharides are natural high molecular weight substances which are polymerized from more than 10 kinds of monosaccharides. A large number of studies have indicated that the polysaccharide compounds isolated and extracted from plants have the effects of inhibiting lipid peroxidation, scavenging free radicals, and inhibiting linoleic acid oxidation. More than 100 kinds of plant polysaccharides such as rosalaevigata polysaccharide, astragalus polysaccharide, Achyranthes polysaccharide, and garlic polysaccharide have anti-oxidation effects, and plant polysaccharides with antioxidant activity are also constantly being discovered.

7.Other
Many ingredients such as anthraquinones (Atractylon, Ginger monoterpene), anthraquinones (tanshinone, rhubarb anthraquinone) and melatonin are effective plant-derived antioxidants.