Chemical Constituents of Green Tea
Xanthine Derivatives, Vitamins, and Catechins
Tea has been considered since ancient times to have medicinal properties. Western chemists in the 19th century began to isolate and characterize the chemical constituents of many foods and beverages, including tea, and to prepare them synthetically. The tea beverage contains xanthine derivatives such as caffeine, theophylline, and theobromine, and the glutamide derivative theanine. These substances have well-known stimulant properties, and have also been reported to have beneficial effects on memory and on the immune system.1 Tea also contains many nutritional components, such as vitamin E, vitamin C, fluoride, and potassium.
The astringency of the beverage is due to phenolic constituents known as catechins, which make up a group of compounds that are closely related to tannins. Broadly speaking, a tannin is a gallic acid ester of a carbohydrate or phenolic compound, and includes compounds derived from wood that are unrelated to tea. Tannins are acidic due to the phenolic hydroxyl groups on the gallic acid moiety. They also act as antioxidants, and form complexes or chelates with metals. All of the tannins in tea are catechins, which as a group are hydroxylated flavanols and their gallic acid esters. These compounds make up roughly half of the hot water-soluble material in tea beverages. Some types of catechins are also contained in other plants, but the "tea catechins" are unique to the tea plant. During fermentation process, these catechins undergo phenolic oxidative coupling reactions that yield red-colored catechin dimers such as thearubigins (proanthocyanidins). These latter account for the darker color of the beverages produced from oolong and black tea, and also have physiological activity similar to that of the catechins including antibiotic and immunomodulatory effects. Green tea does not contain appreciable amounts of these catechin dimers. The chemical structures of the most important constituents and their building blocks are shown below.
Catechins are believed to have a range of beneficial health effects such as neuroprotective activity, and anti-inflammatory, antiulcer, antiviral, antibacterial, and antiparasitic effects. The most studied catechin in relation to health contributing potential is epigallocatechin 3-gallate (EGCG), which constitutes 50-75% of the total flavonoid content in green tea. EGCG is a potent antioxidant, and acts to inhibit a number of physiologically important enzymes. There is some evidence that it might have therapeutic applications in the treatment of some types of cancer, chronic fatigue syndrome, endometriosis, neurodegeneration, periapical lesions, regulating the HIV viral load, Sjögren's syndrome, and spinal muscular atrophy, to name a few.