Indole (chemical formula C8H7N) is a fragrant organic compound having a bicyclic structure that comprises a six-membered benzene ring combined with a five-membered nitrogen-containing pyrrole ring. This makes indole a heterocyclic organic compound. Indole is found extensively in the nature. In addition, a variety of bacteria also produce this organic compound. Since indole is an intercellular solitary molecule, this aromatic heterocyclic organic compound governs a variety of features of bacterial physiology. For instance, indole regulates plasmid stability, spore formation, biofilm formation, resistance to drugs and virulence. The amino acid tryptophan is derived from indole and it is the antecedent of the neurotransmitter called serotonin.

The name indole is basically a portmanteau of two works - indigo and oleum. The compound has been given this name because indole was isolated for the first time by treating indigo dye with oleum.

Indole chemistry is a fall-out of the study of the indigo dye. In fact, it is possible to convert indigo to oxindole and isatin. Later, in 1866, German chemist named Adolf van Baeyer reduced oxindole to form indole utilizing zinc dust. In 1869, Baeyer proposed that the compound be named indole.

Indies are vital antecedents of various other substances that are produced inside the human body and, hence, they have been studied extensively and utilized in lifestyle as well as therapeutic applications. As discussed earlier, indole was first discovered by a German chemist named Adolf van Baeyer in 1866. He was studying the properties of zinc dust and succeeded in reducing oxindole to form indole from the zinc dust. Following the discovery of this compound, indole turned out to be vital ingredients of the textile industry. Hence, further studies were undertaken to study the compound and eventually it was realized that this compound played a crucial role in the human body. Subsequently, study of indolic nucleus in materials such as auxin and tryptophan has led the scientists to understand the mechanisms of these substances in our body better.

Until the close of the 19th century, specific derivatives of indole included essential dyestuff. Scientists' interest in indole increased in the 1930s when people came to learn that the indole substituent (an atom or group other than hydrogen on a molecule) is naturally present in several vital alkaloids, such as auxins and tryptophan. Even today, indoles are an area that generates keen interest among scientists and further research on this subject is on.

Indole is an aromatic compound and is in a solid state in room temperature. This compound has various uses in the fragrance industry and its applications in the pharmaceutical industry are increasing by the day. Basically, indoles are produced in the human body in the form of a by-product during the degradation process of tryptophan - an amino acid. This compound is generally processed within the body and it accumulates in human feces. When high concentrations of indoles are present in feces it emits a potent and repulsive smell. However, it is surprising to note that when in extremely low concentrations indoles have a pleasant and flowery odour. Therefore, low concentrations of this compound are often used as ingredients in flower fragrances, such as orange blossom.

The chemical structure of indoles is complex and this is responsible for stabilizing them to protein structure. Any structure that encloses indole or the compound's derivative can help a protein or enzyme to form its right structure or correct the structure of the enzyme or protein. There are several processes of this action and one anticipated method is that this compound easily forms hydrogen bonds. It is worth mentioning here that hydrogen bonds are essential for formation of proteins. Proteins that enclose pyrrole rings that have an association with the indolic structure are able to resist heat as well as chemical treatment better compared to the proteins that do not have these rings. In present times, many harvested indoles have their origin in coal tar. This is because indoles are found in their most stable states in coal tar.

Precisely speaking, indole alkaloids are a cluster of particular chemicals that are present in several plants worldwide, plants such as periwinkle and snakeroot. One particular derivative of indole - indole-3-carbinol - is abundantly present in cruciferous vegetable such as cabbage, cauliflower and broccoli. This indole derivative or indole-3-carbinol possesses antioxidant, anti-carcinogenic and anti-artherogenic properties. These properties and their beneficial results have been established in several randomly controlled researches. Further studies are needed to ascertain the various applications of indole-3-carbinol and other similar derivatives of indole in treating a variety of lifestyle diseases.

As aforementioned, bacteria are capable of producing indole during the degradation process of the amino acid tryptophan. Moreover, this compound is naturally present in human feces, where its odour is intensely fecal. However, in extremely low concentrations, indole has a flowery scent and forms an ingredient in several perfumes and flower scents like orange blossoms.

Indole has a corresponding substituent and it is known as indolyl.

It is worth mentioning here that indole goes through electrophilic substitution. In other words, substituted indoles are basically structural elements of (for some compounds they are synthetic antecedents) the tryptamine alkaloids such as neurotransmitters melatonin and serotonin which have been derived from the amino acid tryptophan. There are other inolic compounds and these include plant hormone auxin (such as indolul-3-acetic acid or IAA), the anti-inflammatory drug called indomethacin, tryptophol, the betablocker pindolol in addition to dimethyltryptamine - a hallucinogen that occurs naturally.