Chemical classes among natural products
In chemistry, substances are divided into groups according to their different characteristics, e.g., functional groups, possible ring systems, whether they are saturated or unsaturated. This gives a structure to the naming process, which allows the division into lists and tables for substances. Chemists can often easily understand the structure of a certain substance just by its name. It has been found useful within the chemistry of natural products to have a separate division for natural substances. One way has been to divide them into groups according to how they are produced in the plant, thus increasing the understanding of the plant’s enzyme systems and how substances are formed. However, by tradition some classes are rather based on their characteristics. Some important chemical classes within natural product chemistry are listed below:
Alkaloids are alkaline, nitrogen-containing hydrocarbon from plants. They often have a strong medicinal effect on animals and humans. There are many important drugs among alkaloids but also powerful toxins and narcotic substances. Examples of alkaloids are atropine, quinine, colchicine, morphine, nicotine and strychnine.
Terpenes are a kind of unsaturated hydrocarbon which is frequently found in essential oils and gives taste and smell to many spices and flowers. Examples of terpenes are cineol, geraniol, glycyrrhizin, limonene, menthol, pinen and thymol.
Glycosides are substances that are formed by sugar bound to another substance – aglycon. Many medically active substances are glycosides. Some interesting ones are flavonoids of various kinds (rutin), ginsenosides, cardio-active glycosides (e.g., digitonin) and laxative antrachinon glycosides (glucofrangulin and sennoside).
Phenylpropanes are substances with an aromatic ring and a three-carbon side chain. They often have a strong smell and taste. Together with monoterpenes they make up the substances in essential oils. Some well-known examples are anetol from aniseed, eugenol from cloves and cinnamic aldehyde from cinnamon.
Coumarins develop as a result of a closed ring of cinnamic acid (a phenylpropane) and therefore consist of at least two rings. Amongst coumarins and furanocoumarins should be mentioned coumarin, umbelliferon and psoralen.
Flavonoid is a generic term for a number of colour pigments that are found in plants. Depending on their structure they can be yellow, red or blue pigments. Some are sometimes called vitamin P, since they affect permeability in the capillaries. Among the common aglycons are apigenin, luteolin and quercetin.
Steroids are composed of two important groups of steroid hormones and cardio-active glycosides. Steroid hormones are, in turn, divided into two classes: 1. sexual hormones(e.g., progesterone, testosterone and prednisolone) and 2. adrenal gland cortex hormones (e.g., cortisone and prednisolone). Sexual hormones were the basis of the development of contraceptive products, i.e., they are found in contraceptive pills, “the Pill”.
Saponins are glycosides that reduce surface tension in water solutions and can destroy the cell membrane of red blood cells. Fish that find themselves in a solution of saponins die, which, for instance, is the reason for treating fishing lakes with rotenone. Some known saponins are glycyrrhizin, rotenone and senegin-II.
Tannic substances are known in everyday language as tannic acid or tannin. They are divided into two groups: hydrolysable and condensated tannic substances depending on the structure of their component parts.
Carbohydrates consist of carbon, hydrogen and oxygen and are formed in the plant’s photosynthesis from carbon dioxide and water. They are divided into types of sugar (e.g., glucose, fructose and sucrose) as well as big molecules such as starch and cellulose.
Amino acids, peptides, proteins. Amino acids contain two functional groups: a carboxyl group and an amino group. When two or more amino acids join, peptides are formed. Very big peptides are called proteins. Plants can produce their own amino acids but humans, and most animals, must get certain amino acids through the food chain in order to survive. Many snake toxins are peptides or proteins. Insulin is a hormone and a drug that consists of polypeptides.
Suggested reading:
Samuelsson, G., Drugs of Natural Origin. 5th ed., Swedish Pharmaceutical Press, Stockholm, 2004