The mechanisms of action of essentials oils can in a first approximation be explained by twelve important groups of constituents. Oils whose main components belong to the same group will exhibit similar effects. But naturally each oil has its own specific qualities in addition to its general properties. The table below provides an overview of the most important qualities of monoterpenes, sesquiterpenes, and phenylpropanes.
All phenylpropanes contain an aromatic ring structure just like the phenols. For instance, cinnamic aldehyde (found in cinnamon bark oil and cassia oil) and eugenol (found in clove bud oil and clove leaf oil) are the two members of the group with stimulant and irritant but also strong antiseptic effects. Other important phenylpropanes, such as estragol (also known as methyl chavicol) in basil and tarragon oil, and anethol, which is found in anise oil, are much less aggressive and have a definite stabilizing influence on the autonomic nervous system. They also have strong antispasmodic effects, especially (as one might guess from the culinary usage of these oils) in the digestive tract.
Essential oils contain only monoterpenes (10 carbon atoms) and sesquiterpenes (15 carbon atoms) in notable quantities. Diterpenes (20 carbon atoms) are encountered only in small quantities and only in a few oils, such as sclareol in clary sage oil (Salvia sclarea).
The varying molecular size of monoterpenes and terpenoids and their higher homologues has practical implications for aromatherapy. Monoterpenes are so small that their effects are so greatly influenced by the nature of a functional group that they can be categorized accordingly. For example, terpene aldehydes have calming effects, while terpene alcohols are antiseptic and
This method of classification cannot readily be applied to the larger sesquiterpenes, because here the influence of the functional group is less dramatic. The effects of these molecules are determined as much by their unique molecular structure as by a functional group, and vary significantly from molecule to molecule.
The bulk (approximately 90 percent) of the components of essential oils are monoterpenes and sesquiterpenes. For a listing of the most important characteristics the functional groups lend to the terpene molecules, see the table on Influence of Functional Groups on the Effects of Essential Oil Components (top). Monoterpenes are the dominant components of citrus and needle oils, as well as oils derived from herbs and spices. Oxygen-containing monoterpenes, such as geraniol, linalol, nerol,and citral or citronellal, are among the most widely distributed natural terpene compounds. We often find that oils consist primarily of monoterpenes and about 10, 15, or 20 percent sesquiterpenes.
High monoterpene content can often be recognized by looking at an oil. Oils like eucalyptus are mostly clear, mobile (have a low viscosity), and volatile. These properties reflect the small molecular size of the terpene molecules. Oils with high sesquiterpene content, such as patchouli or sandalwood, have a much higher viscosity. Their color varies from yellow or dark yellow to brown. Because of their lasting aroma and their low volatility, they are often used as fixatives or base notes.
Phenol itself (see diagram above) does not exist in the plant world. In contrast to the most common phenols found in essential oils, such as thymol and carvacrol, it is a carcinogen. In addition to a hydroxyl group (OH), there is also a short carbon chain attached to the aromatic ring. These natural substances have nothing in common with phenol, which is poisonous and derived from mineral oil. Rather, they are a perfect example of the advantages natural substances hold over synthetic ones. The antiseptic effect of natural thymol is significantly greater than that of synthetic phenol, but thymol is practically nontoxic compared to its synthetic namesake.
Essential oils with high concentrations of the monoterpene-phenols thymol and carvacrol are thyme, ajowan, oregano, summer savory, and winter savory (mountain savory).
Monoterpene alcohols share some similarities with phenols. The hydroxyl group is, however, not attached to an aromatic ring system but to a “normal” terpene structure. They are mild and belong to the gentlest and most useful terpene molecules. Often, they are astoundingly effective against microorganisms, but nontoxic to humans. Essential oils containing primarily monoterpene alcohols are therefore especially suitable for everyday use in hygiene and skin care. Oils with high monoterpene alcohol content are usually distinguished by their pleasant scent and a tonifying effect, especially on the nerves.
Essential oils with a high content of the monoterpene alcohollinalol are coriander, rosewood, petitgrain, and the linalol chemotype of thyme. Neroli and clary sage also contain high levels of linalol.
Alcohols, including terpene alcohols, react easily with acids to form a new chemical compound: a so-called ester. In the plant world, esters are almost always strongly aromatic, often with a distinctive, fruity note.
Esters are therefore used in the flavor industry especially for foods with fruit flavors. In aromatherapy, oils with high ester content are used for their balancing and antispasmodic effects. This antispasmodic action is dependent on the chain length of the acid half of the molecule. Esters of formic acid (one carbon atom) are the main component in geranium oil, and their antispasmodic influence is only mild. Geranium (Pelargonium) is often used in massage oils because its gentle effect is desirable for this application.
Esters of acetic acid (two-carbon atoms), so-called acetates, can be found in everlasting, Inula graveolens, rosemary (verbenone type), and fir (Abies siberica), as well as bay laurel and cardamom, which is a time-tested remedy for digestive cramps. Esters of acids with five carbon atoms are the main components of Roman chamomile, one of aroma therapy’s strongest antispasmodics.
Esters of acids with seven carbon atoms have the strongest antispasmodic effect, and can be found in ylang ylang and in mandarin petitgrain.
Esters also act as fungicides, and are useful in preventing fungal and yeast infections. Geranium oil, which does not have a very strong antibacterial effect but is an effective fungicide, is especially significant in treating yeast infections.
The citrus like aroma of melissa is well known and easy to recognize. The aldehydes responsible for this effect are citral and citronellal. They are important starting materials for the chemical industry and are extracted in large quantities from the oils of the citronella grasses.
Essential oils with a high aldehyde content have sedative and anti-inflammatory effects. The anti-inflammatory effect is most pronounced if the oil is administered in low concentrations, diluted in either air or liquid. The sedative and antispasmodic effects of citral and citronellal are stronger in relatively lower concentrations (citral is also an effective antiviral agent). Higher concentrations or doses above a certain saturation level will lessen rather than strengthen the effects. In 1973 Wagner and Sprinkmever demonstrated this behavior of the sedative effects of various terpene compounds. Lemongrass, for example, if used undiluted, can even have a mild irritant quality.
The salient quality of essential oils containing ketones is the stimulating effect on cell and tissue regeneration. Applied externally, thuja oil, and especially everlasting (Helichrysum italicum), can give spectacular results for healing wounds or promoting the generation of new tissue. For this reason everlasting oil often is used in regenerative skin care.
An entirely different aspect of the monoterpene ketones is their mucolytic effect, in other words their ability to facilitate the loosening and elimination of mucus.
Toxicity of ketones
A variety of ketones can have toxic effects: they attack parts of the nervous system. For this reason, it is absolutely essential to use oils with a high ketone content in correct dosages (see Toxicity of Essential Oils> Essential oils containing ketones: Neurotoxic). Oils with a high thujone content, such as sage, mugwort, wormwood, thuja, and hyssop (with pinocamphone content), should be used with special caution at any time.
Because the ketone content of essential oils such as eucalyptus (E. globulus) or rosemary (verbenone type) is relatively unproblematic, these oils can be used for their mucolytic effects.
In general, ketones stimulate the formation of tissue, have mucolytic effects, dissolve fats, and are potentially neurotoxic.
A large number of essential oils contain cineole, a terpene oxide also known as eucalyptol. The characteristic structural element of this compound is an oxygen atom that is integrated into a terpene ring system. Especially high concentrations of this molecule are found in the various eucalyptus and melaleuca oils. This compound has strong antiviral and expectorant (meaning it stimulates the bronchi to cough up and eliminate mucus) effects.
Interestingly, the oil of creeping hyssop (Hyssopus officinalis var. decumbens) is the only commercially available oil with a content of linalol oxide. Because of its outstanding antiviral and expectorant effects, this oil is used in aroma medicine for the treatment of bronchial infections.
All citrus oils (with the exception of bergamot) contain up to 90 percent or more monoterpene hydrocarbons. The typical aroma of these oils stems not from the terpenes but from strongly aromatic trace components. Lemon oil, for example, is composed primarily of limonene. Characteristic for its aroma is the 3.5 percent content of citral. Grapefruit oil (Citrus paradisi) can contain up to 90 percent monoterpenes, but its characteristic aroma comes from traces of nootkatone (CIS-ketone) and pararnenthenthiol. Mandarin oil (Citrus reticulata) contains up to 90 percent monoterpenes, but its aroma comes largely from methvlanthranilate, which is also responsible for mandarin’s calming effects. Orange oil (Citrus sinensis) can contain up to 95 percent of the monoterpene limonene, but its characteristic aroma comes from sinensal (aldehyde), and a variety of other trace components.
Other important essential oils that are com- posed primarily of monoterpenes are pepper oil, various pine oils, nutmeg, juniper, and turpentine. Angelica oil also contains primarily monoterpenes. The warm, musky scent of angelica is brought about by macrocyclic lactones (cyclic esters) and ketones (especially pentadecanolide). These lac tones and ketones make up only about 0.2 to 0.4 percent of angelica oil.
The qualities of sesquiterpenes differ significantly from those of the monoterpenes. They are primarily anti-inflammatory, cooling, and antiallergenic. It is somewhat problematic to summarize the properties of these compounds in a stream-lined way for the purposes of aromatherapy, because they are chemically very different. It is still reasonable, however, to characterize the effect of sesquiterpenes as anti-inflammatory and anti-allergenic. Chamazulene, which is an active component of German chamomile and has a special calming effect, and carvophyllene, which is found in various concentrations in many different oils, belong in this group.
A wide spectrum of action exists among sesquiterpenes containing functional groups with oxygen. While it was possible to classify the monoterpenes according to functional groups, this is not possible with the sesquiterpenes, because their effects are too varied to enable simple categorization. The influence of the relatively larger molecular structure equals that of the functional group.
The interplay between chemical structure and effect is more varied with sesquiterpene alcohols than with monoterpene alcohols. The disscussion of sesquiterpene alcohols shall therefore be limited to a few well-researched examples. In general, we can characterize the effects of sesquiterpene alcohols as antiallergenic, liver stimulating, stimulating glandular secretions, and anti-inflammatory.
Bisabolol, a natural sesquiterpene alcohol, is the strongest anti-inflammatory substance in German chamomile (Matricaria recutita). Comprehensive pharmacological studies have shown that the effect of this alcohol is even stronger than that of chamazulene, which lends chamomile oil its distinctive blue color. The main components of the most valuable chemotype of German chamomile are the hydrocarbons farnesene and chamazulene, as well as up to 35 percent (-)?-bisabolol, the most effective variant of this molecule.
Other plants from the Compositae family (Asteraceae) have proven to be a reservoir of pharmacologically active components. Echinacea (Echinacea purpurea and Echinacea angustifolia) is normally used in the form of an extract or tincture, but the essential oil derived from its roots has immune stimulating qualities, which lately have been attributed to a number of sesquiterpene alcohols.
Sandalwood is composed almost exclusively of the alcohol santalol (fifteen carbon atoms). It is traditionally used for bladder or urinary tract infections and heartburn. Authors of different studies have disagreed on its antiseptic qualities: some recommend this oil for the treatment of various urinary problems, while others have observed no therapeutic effects. One can assume that the effects of sandalwood oil are influenced more by immune-rnodulant properties than by antibiotic ones. Other oils with a high content of alcohols with fifteen carbon atoms are niaouli (Melaleuca quinquenervia viridifiora), ginger, patchouli, vetiver, carrot seed, and spikenard.
In general, sesquiterpene alcohols tonify muscles and nerves, reduce congestion in the veins as well as in the lymphatic system, and have moderate antimicrobial effects. Some of the sesqui terpene alcohols in essential oils have unique qualities: viridiflorol, found in niaouli or sage, has an estrogenlike influence and works as a tonic on the veins; spathulenol, found in verbena (Lippia citriodora), acts as a fungicide; cedrol, found in atlas cedar, is a tonic for the veins; santalol, found in sandalwood, is a tonic for the heart; and carotol, from carrot seed oil (Daucus carota) , stimulates the regeneration of liver cells.
Sesquiterpene ketones and sesquiterpenes with other functional groups can have very interesting effects, but these effects vary greatly from oil to oil.