Terpenes or isoprenoids are organic compounds that consist of lipids, hydrocarbons, alcohols, acids, etc., that is, several types of substances, which however have in common that they all derive from isoprene. Therefore, they represent a vast family of compounds with variable properties and functions, depending on their structure and plant or animal origin.
Terpenes can be broken down into isoprene units, which consist of a five-carbon hydrocarbon called 2-methyl-1,3-butadiene: an alkene. These units are joined by biochemical processes to form terpenes, which in turn can undergo functional changes in their structures.
We have, for example, myrcene (upper image), which is extracted mainly from myrtle oil and other plants. Note that it is a hydrocarbon, but in any of its double bonds it can form functional groups or cause the structure to be enclosed. Thus, there are acyclic and cyclic terpenes, the latter being the most common.
Terpenes make up the famous essential oils of plants, characterized by their pleasant fragrances. They are also found in waxy, greasy, volatile substances, which mostly serve as precursors of vitamins or hormones.
Terpenes have the following characteristics:
- Terpenes are generally volatile aromatic compounds.
- They are unsaturated hydrocarbon molecules (they have double bonds between some of their carbon atoms) that are found in all living organisms, but they are particularly abundant in the essential oils of many vegetables.
- Those terpenes that have oxygen atoms are known as “terpenoids”. These compounds perform different functions in living organisms and are surprisingly diverse.
- Not only are they produced by plant species, but they are also abundant in many animals, where they perform just as important functions.
Isoprene unit and bonds
In the highest part of the upper image we have the structure of isoprene and its two double bonds. The positions of these double bonds mark two ends: a head and a tail, which are where the molecule can bond with other isoprene units during the formation of terpenes.
For example, under the isoprene unit we see two isoprene molecules that have bonded to each other. However, the molecule on the left (green box) linked using its tail, while the one on the right (blue box) linked using its head. If we count the number of total atoms it will equal 10.
Isoprene molecules can also be linked by another type of bond: glue-glue. This completely changes the structure obtained, where now the methyl groups point in opposite directions in the carbon skeleton.
In general, the tail-head type unions predominate over those of the tail-tail or head-head type. However, all three forms are possible, which is why the structures of polysoprenoids can be very diverse; and furthermore, the carbon skeleton is capable of closing to form rings or cycles.
Classification according to the number of carbon atoms
The general formula for isoprenoids or terpenes is (C 5 H 8 ) n , where n is equal to the number of isoprene units.
If n is equal to 1, we will have the isoprene, C 5 H 8 . When n is equal to 2, instead, we will obtain our first isoprenoid, (C 5 H 8 ) 2 , which is called monoterpene, having 2 isoprene units and 10 carbon atoms. Thus, we have the sesquiterpenes ( n = 3), the diterpenes ( n = 4), etc.
Function of terpenes
Terpenes intervene in endless physiological processes in animals and plants. For example, monkeys and sesquiterpenes make up the essential oils, saps, and tissues of some plants.
Di and triterpenes, being heavier, predominate in tree resins, which can be used to attract or repel insects from the surroundings; or in the oils or liquid fats of animal species. And on the other hand, tetraterpenes or other heavier specimens, make up pigments, fats, waxes, or generally solid substances.
Many substances with a large role in animal physiology, such as vitamins A, E and K, as well as ubiquinones, are synthesized from terpenes that serve as substrates for very effective enzymes. Hence, the functions of such terpenes are the subject of molecular biology or advanced biochemistry.
Examples of terpenes
Some examples of terpenes will be shown below, accompanied by a brief structural and functional description.
Above we have some geraniums and the structure of geraniol, a volatile alcohol that is responsible for the smell of roses. As can be seen, it is a monoterpene with an OH group, because if we count its carbon atoms it will equal 10, which divided by 5 we will have 2 isoprene units.
In limonene, whether its R or S enantiomer, we have a cyclic monoterpenic isoprenoid, responsible for the smell of oranges or lemons, respectively.
Note that it is a hydrocarbon, since it does not have functional groups or heteroatoms. The isopropyl group, or the presence of two geminal CH 3 groups , reveal that we are dealing with an isoprenoid or terpene.
Α-Selinene is one of the members of the Seline family. It consists of a cyclic sesquiterpene hydrocarbon. If we count its carbon atoms we will have that it is equal to 15, which divided by 5 gives us 3 isoprene units. This isoprenoid is partly responsible for the characteristic odor of celery.
Cembrene, a cyclic diterpenic hydrocarbon, is extracted from the oil of pines and some corals. Again, if we count its carbon atoms we will have 20 of them, which divided by 5 gives us 4 isoprene units linked together.
In natural rubber we have multiple isoprene units linked to form large polymer chains. It is therefore a polysoprenoid ( n is much greater than 8).
One of the most prominent examples of isoprenoids or terpenes is squalene, which, as can be seen above, is a lipid hydrocarbon of considerable length. It has a total of 30 carbon atoms, which divided by 5 gives us 6 isoprene units, which is why it is classified as a triterpene.
In squalene, the tail-tail bond between carbons 12 and 13 of the chain can be appreciated once again. It is not easy for many isoprenoids to discern what type of junctions it has, as their structures are sometimes confusing at first glance. If squalene were to fold into rings, any kind of cyclic structures could form.
Carotenoids are terpenes that function as natural pigments in many plant, fungal, and bacterial species. They are essential for the synthesis of vitamin A in animals and must be consumed with food, since the human body and that of other mammals is unable to synthesize them.
The oil extracted from the leaves of basil, bay leaf and bitter orange is rich in acyclic monoterpenoids known as “triene”, among which β-myrcene and the conformational isomers of β-ocimene stand out.