The word phytochemical is derived from phyto or plant. Its study involves understanding the chemical composition of plants used in medicine. Phytochemicals produced in plants are secondary compounds responsible for metabolic activities and defense purposes. Phytochemicals are produced by specific biochemical pathways, which occur inside the plant cells. Phytochemicals can range from medicinally useful agents to deadly poisons. A number of phytochemicals isolated from plant material are used in the pharmaceutical drug industry today.

Alkaloids

Alkaloids are basically nitrogen bases. The amino acids act as building blocks for biosynthesis of alkaloids. A majority of alkaloids contain a pyridine, quinoline, and isoquinoline or tropane nucleus and are responsible for physiological effects in man or in animal. The side chains in alkaloids are derived from terpene or acetate. Alkaloids have basic properties and are alkaline in reaction, turning red litmus paper blue.

Alkaloids combine with acids to form crystalline salts without the production of water. A majority of alkaloids exist in solid form like atropine and they contain oxygen. Some alkaloids like lobeline or nicotine occur in liquid from and contain carbon, hydrogen and nitrogen. Alkaloids have one peculiarity regarding solubility in organic solvents. They are readily soluble in alcohol and sparingly soluble in water.

The salts of alkaloids are usually soluble in water. In nature, alkaloids exist in many plants: in larger proportion in the seeds and roots often in combination with vegetable acids. Alkaloid solutions are intensely bitter. The names of alkaloids ends in the suffix—ine. The salts of alkaloids are official. Codeine, atropine, morphine, ergotamine and ephedrine are common examples. Alkaloids are responsible for physiological effects in man or animals. The physiological effects are due to secondary metabolites arising from bio-chemical pathways operating in the plant cell. Alkaloids constitute the largest group of secondary chemical constituents.

Alkaloids are a significant source of pharmaceutical drugs. More than 12,000 alkaloids are known to exist in green flora and only a few have been exploited for medicinal purpose. With the help of scientific procedures like chromatography and mass spectroscopy it is now possible to determine the molecular formulas and chemical structures of active constituents of medicinal herbs.

Bitters

Bitter principles are basically glycosides and are found commonly in plants of Genitiaceae. They are chemically unrelated but possess the common property of an intensely bitter taste. Although this group of drugs is not used today, once upon a time they were given to promote appetite and aid digestion. The bitters act on gustatory nerves, which results in increased flow of saliva and gastric juices. From a chemistry point of view, the bitter principles contain lactone group. They may be diterpene lactones e.g., Andrographolide or Triterpenoids e.g., Amarogentin. The bitters have no action in general.

Some bitter principles are known to be astringent due to the presence of tannic acid. Gentiana lutea is the plant known to contain astringent bitter principles. They should not be prescribed with metals, as they are known to cause gastro-intestinal upset. Bitters have no detailed account in Herbal Materia Medica and are of not much historic importance. Some like Amarogentin have recently received some importance because of antiprotozoal activity. Andrographolide is being investigated for Anti-Aids activity. Bitter also possesses aromatic properties due to the presence of volatile oils, e.g.; Citrus aurantium (orange peel). They are sometimes used as flavoring agents.

Quassinoids

They are triterpenes with a pentacyclic ring system having lactone and methylene-oxygen ring bridge linking C8 and C13.

Limonoids

They are modified triterpenes with or derived from a precursor with a 4, 4,-8- trimethyl-17- furanylsteroid skeleton.

Flavones and flavonoids

Flavonoids are important group of polyphenols, widely distributed in plant flora. Around 4,000 flavonoids are known to exist and some of them are pigments in higher plants. Quercetin, kaempferol and quercitrin are common flavonoids present in nearly 70% of the plants. Soya flavones have recently gained importance due to a variety of pharmacological activities. Flavonoids are derived from parent compounds known as flavans.

Isoflavones (Phyto-estrogens)

Isoflavones are found in Glycine max (soybean). Clinical research has demonstrated soy isoflavones to be effective in menstrual diseases. They have antioxidant activity also. Isoflavones belong to a group of compounds known as phyto-estrogens.

Furanocoumarins

Furanocoumarins are photosensitizing agents used in the treatment of pigment disorders. Ayurveda, the ancient science of India, has described the use of bawachi (Psoralia corylifolia) for the treatment of leucoderma. Psoralens isolated from the medicinal herb, are reputed drugs in the field of dermatology.

Furanocoumarins are formed when furor ring is joined with coumarins. The plants of Rutaceae, Leguminosae and Apiaceae are rich sources of furanocoumarins. Depending upon the structure, the furanocoumarins are divided into linear and angular types. Furanocoumarin containing preparations are used externally as well as internally for treatment of leucoderma, psoriasis and skin carcinoma.

Furochromones

They are group of coumarins, derived from benzopyrone. They are related to furanocoumarins and are present in plants of family Apiaceae and Rutaceae.

Hydroxycoumarins

They represent another group of coumarins, which are widely distributed in Apiaceae and Gramineae.

Glycosides

They are water-soluble constituents, found in the cell sap. They are colourless, crystalline substances containing carbon, hydrogen and oxygen. Some glycosides are peculiar in having nitrogen and sulphur. Glycosides are neutral in reaction. Chemically, glycosides contain a carbohydrate (glucose) and a non-carbohydrate part (aglycone or genin). Alcohol, glycerol or phenol represents aglycones. A glycoside can be readily hydrolysed into its components with ferments or mineral acids.

Glycosides differ in their solubility in water. Some are soluble in ether and alcohol. Amygdalin found in almonds is a familiar example of a glycoside. Benzeldehyde is the decomposition product of amygdalin, responsible for odour and taste of almonds. Glycosides are optically active and are levorotatory.

Phenolic Compounds

They are widely distributed in plant flora. They constitute an important part of glycosides (phenolic glycosides), flavonoids, napthodianthrones and tannins. Acylphloroglucinols are group of phenolic compounds having significant antidepressant activity.

Phenylpropanoids

They contain a three-carbon side chain attached to phenol. Hyroxycoumarins, phenylpropenes and lignans are common examples of phenylpropanoids.

Resins

Resins are obtained by oxidization of volatile oils. Resins are brittle, non-volatile, solid substances. Sometimes resins are among the products of oxidization of terpenes. The chemical composition of resins is very complex and contains various compounds including acids.

Resins are soluble in alkalis, alcohol and insoluble in water. They are obtained from plant exudates and are produced in special ducts.

Oleoresins

They are natural products of resin mixed with volatile oils.

Gum-resins

They are plant exudates and are mixtures of gum and resin and often volatile oils. When gum- resins are dissolved in water, gum becomes soluble and resin is kept in suspension. Asfoetida is a familiar example.

Balsams

They are combinations of resins or oleoresins with aromatic acids like benzoic acid or cinnamic acid or both. They are viscous and obtained from the trunk of certain plants.

Saponins

They are glycosides found in a number of plants. Saponins are regarded as high molecular weight compounds in which a sugar molecule is combined with triterpene or steroid aglycone. Saponins have a characteristic feature of frothing. The term saponin is derived from Saponaria vaccaria; a plant, which abounds in saponins and once upon a time was used as a soap. Saponins are soluble in water and insoluble in ether. Saponins like glycosides or hydrolysis give aglycones.

Tannins

They are widely distributed in plant flora. They are phenolic compounds of high molecular weight. Tannins are soluble in water and alcohol and are found in root, bark, stem and outer layers of plant tissue. Tannins have a characteristic feature to tan, i.e. to convert the things into leather. The tannins are acidic in reaction and it is attributed to the presence of phenolic or carboxylic group. Tannins form complex with proteins, carbohydrates, gelatin and alkaloids.

Terpenes

They are flammable unsaturated hydrocarbons, existing in liquid form. They are found in essential oils, resins or oleoresins. They are used as intermediaries for the synthesis of sesquitrepenes and terpenoids. They are classified as mono, di or triterpenoids.

Sesquiterpenes

They constitute a significant group of phytochemicals. The sesquiterpenes are widely distributed in plant flora particularly in Compositae.

Sterols

They are derivatives of steroids. Some chemical constituents present in plant flora resemble steroids. Modern clinical studies have supported their role as anti-inflammatory and analgesic agents. Beta-sitosterol is the most commonly studied sterol compound isolated from a number of medicinal herbs and it has been seen as effective in reducing serum cholesterol levels.

Withanolides

They are a group of naturally occurring oxygenated ergostane type steroids having lactone in side chain and 2-en-1-one system in ring A. Withanolides which are considered to be responsible for various applications of the herb as adaptogen (anti-stress) and immunomodulator.

Lectins

They are structurally diverse, carbohydrate binding proteins that bind reversibly to specific mono or oligosaccharides. Abrin and ricin are familiar examples.