Introduction:

The rational categorization of the Terpenes has been established based on the number of isoprene (or isopentane) units incorporated in the fundamental molecular skeleton:

Terpenes                  Isoprene units     Carbon atoms

1) Monoterpenes                2                       10

2) Sesquiterpenes               3                       15

3) Diterpenes                     4                        20

4) Sesterpenes                    5                        25

5) Triterpenes                     6                        30

6) Carotenoids                    8                        40

7) Rubber                          > 100                  > 500

Mono-, sesqui-, di- and Sesterpenes include the isoprene units associated in a head to tail fashion. The triterpenes and carotenoids (that is, tetraterpenes) have two C15 and C20 units correspondingly joined head to head. 

Monoterpenes:

These are the Terpenes which have been known for some centuries as the components of fragrant oils acquired from flowers, leaves and fruits. Monoterpenes, having sesquiterpenes, are the major constituents of essential oils. As some, like camphor, take place in a close to pure form, most take place as complex mixtures, often of isomers hard to separate. Such essential oils have many actions, like allelochemical functions among the plants and between plants and predators. The role in wound healing was as well noticed.

Acyclic Monoterpenes:

Among the natural molecules, the given are well known and encompass some of the structural isomers.

Fig: Acyclic Monoterpenes

The defensive role of simple Terpenes has been illustrated as for more complex compounds. Ocimene and linalool (having farnesene) were illustrated to be formed by de novo biosynthesis in plants damaged via insect herbivores. Such compounds to be expected mediate the interaction among herbivores and their natural enemies, fascinated by Terpenes. 

Monocyclic Monoterpenes:

They are mainly derived from cyclohexane having an isopropyl substituent. The most general are:

Fig: Monocyclic Monoterpenes

Limonene is the significant volatile emitted through the holm oak (that is, Quercus ilex), and acts as allelochemical in inhibiting seed germination of the other plant species.

Bicyclic Monoterpenes:

Fig: Bicyclic Monoterpenes

Pinene is, as limonene, an allelochemichal emitted via the roots of Quercus ilex. Camphor and pinene are as well allelochemicals emitted through Salvia leucophylla. Most of the monoterpenes have antitumor activity in the animal and cell models. They have as well antioxidant properties, -Terpenes and hydroxytyrosol being among the most efficient. 

Sesquiterpenes:

Sesquiterpenoids are stated as the group of 15 carbon compounds derived through the assembly of 3 isoprenoid units and they are mostly found in higher plants however as well in invertebrates. Sesquiterpenes, having monoterpenes, are a significant constituent of essential oils in plants. They are the most diverse group of Isoprenoids. In plants, they function as the pheromones and juvenile hormones. Sesquiterpene structures present some acyclic, mono-, bi-, tri- and tetracyclic systems. 

Acyclic sesquiterpenes:

The acyclic representative are as well termed as farnesans, word derived from the fundamental structure, farnesol. Farnesol and nerolidol are extremely common and are isolated from essential oils of different sources.

Fig: Acyclic sesquiterpenes

Farnesol is broadly distributed in numerous essential oils like neroli, citronella, cyclamen, lemon grass, rose, tuberose, musk and balsam. This is employed in perfumery to highlight the odors of perfumes. Furthermore, it is a natural pesticide for mites and is as well as pheromone for some insects and mammals, comprising elephants (that is, territorial marking, individual recognition, mate attraction). 

Farnesene, an analogue of the farnesol, is recognized to act as an alarm pheromone in the aphids. Released throughout predator attack, it causes aphids to stop feeding, disperse and provide birth to winged (instead of wingless) forms that leave their host plants. 

Fig: Farnesene

Nerolidol is present in the neroli, ginger, lavender, jasmine, tea tree and lemon grass. The aroma of nerolidol is woody and reminiscent of the fresh bark. This is employed as a flavoring agent and in perfumery. It was as well illustrated to be produced via the leaves of a large number of plant species in response to herbivory insects and then to be converted to a C11-homoterpene (4, 8-dimethyl-1, 3, 7-nonatriene) that fascinates the predatory insects.

Fig: C11-homoterpene

Cyclic sesquiterpenes:

Abscisic acid plays an important role in plants in the regulation of stomatal closure via regulating ion channel activities and water exchanges across the plasma membrane of guard cells.

Fig: Abscisic acid

The Cyclic ADP-ribose (cADPR) has been illustrated to mediate signaling of Abscisic acid in the drought-stress response leading to the activation of gene transcription and to stomatal closure. Abscisic acid has as well a variety of roles in the growth of plant, bud and seed dormancy, germination, cell division and movement. This induces storage protein synthesis in seeds and might be comprised in defense against the insect attack. 

Cadalene consists of the cadinane skeleton and is present in essential oils and in numerous plants. This is employed as a biomarker in the paleobotanic studies. In connection by the retene (1-methyl-7-isopropyl phenanthrene), it facilitates the estimation in sediments of the significance of Pinaceae in earliest forests.

Fig: Cyclic sesquiterpenes

Gossypol is the sesquiterpene dimer found in cotton which is made up from two cadinane units. All the cotton plant includes gossypol. Those Terpenes take place as a mixture of two enantiomers however each consists of various biological activities. For non-ruminant animals (such as, rodents, chickens and humans), (-)- gossypol is considerably more toxic than the (+) enantiomers. It consists of anti-cancer properties and inhibits male fertility in the humans. In contrary, cotton plants having high levels of (+)-gossypol are resistant to the insect damage. Such Terpenes should be eliminated from the plant parts and oil prior to use as animal foods.

Capsidiol is the sesquiterpenoid compound which accumulates in tobacco Nicotiana tabacum and chili pepper Capsicum annuum in response to the fungal infection. This is considered as a phytoalexin. 

Fig: Capsidiol

Diterpenes:

They encompass 20 carbon atoms and are derived from the geranylgeraniol pyrophosphate. They are of fungal or plant origin and are mainly found in resins, gummy exudates and in the resinous high-boiling fractions remaining after the distillation of essential oils. 

Diterpenoid groups which are physiologically active comprise:  vitamin A activity (that is, retinol), phytohormones which regulate the growth of plant and germination, example: gibberellins, fungal hormones which stimulate the switch from asexual to sexual reproduction, example: trisporic acid; disease resistance agents (that is, phytoalexins), example: casbene and podocarpic acid, the anticancer drug, taxol, from the bark of the yew tree, the cancer promoter, phorbol, and natural cannabinoids.

The diterpenes encompass exceptionally open chain, as found in the geranylgeraniol or phytol that makes a part of chlorophyll and the side chain of vitamin E and K, and crocetin that is a diacid diterpenoid and the lipid portion of the crocins, glycosylated derivatives present in the saffron.

Steviol is the aglycone of stevia's sweet glycosides, one of them being made up by substituting one hydrogen atom (that is, bottom) having glucose through an ester link, and the other hydrogen atom (top) with a disaccharide (like, glucose and rhamnose). The steviol glycosides are responsible for the sweet taste of the leaves of the stevia plant (Stevia rebaudiana, Asteraceae). Such compounds are 40 to 300 times sweeter than sucrose. They are made to be employed in sweet drinks. 

Retene is present in the tars acquired by distillation of resinous wood, it is a significant pollutant removed by the paper factories. This diterpene is present in geological sediment where it is made up by diagenesis from abietic acid, some intermediates having been well-known.

Dehydroleucodine was isolated from Artemisia douglasiana, a well-liked medicine in the Argentina and was illustrated to encompass some physiological and therapeutic properties: anti-proliferative activity in G2 phase, cytoprotective agent for the gastric ulcers and a general antioxidant. 

Cafestol and kahweol are mainly present in high concentrations (up to 18% diterpene esters) in the oil derived from coffee beans. The mere difference between cafestol and kahweol is an additional double bond present in the second cycle of kahweol. Such diterpenes are esterified having one fatty acid (C14 to C24), Palmitic and linoleic acids being the main esterified fatty acids. 

Phorbol is the diterpene isolated in the year 1934 from croton oil (that is, seeds of Croton tiglium). Different fatty acid esters of phorbol have significant biological properties, the most notable of which is the capacity to act as tumor promoters via the activation of protein kinase C as they mimic diacylglycerols. The most general phorbol ester is phorbol-12-myristate-13-acetate (PMA) that is employed as a research tool in the models of carcinogenesis.

Cannabinoids are the group of diterpenes present in Cannabis (that is, Cannabis sativa L). All such substances are structurally associated to tetrahydrocannabinol (THC) and are able to bind to particular cannabinoid receptors.

Sesterpenes:

They are mainly derived from geranylfarnesol pyrophosphate and encompass 25 carbon atoms. Sesterpenes were isolated from insect protective waxes and from fungal sources.

Variously unsaturated and branched Sesterpenes, termed as Haslenes, were found in the species of diatomaceous algae. They are broadly distributed and rich in marine sediments. Some of the haslenes were found to be made by an iatom Haslea ostrearia according to the culture temperature and were illustrated to contain cytostatic.

Triterpenoids:

They make a large group of natural substances that comprises steroids and as result sterols. Squalene is the instant biological precursor of all Triterpenoids. Squalene epoxide (2,3-oxidosqualene) is made by the enzyme squalene epoxidase that make use of NADPH and oxygen to oxidize the squalene. This metabolic step is primary in sterol biosynthesis leading to the manufacture of lanosterol or cycloartenol.

Squalane is a fully saturated derivative of squalene. Present in sebum, it is mostly employed as a component in numerous cosmetic products. This is obtained via hydrogenation of squalene extracted from the olive oil. The large groups of steroids, comprising sterols, are present in extremely small amounts in bacteria however at larger amounts in animals and plants and while the hopanoids are extremely rich in prokaryotes where they substitute cholesterol.

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