Journal:The cannabis terpenes

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Full article title The cannabis terpenes
Journal Molecules
Author(s) Sommano, Sarana R.; Chittasupho, Chuda; Ruksiriwanich, Warintorn; Jantrawut, Pensak
Author affiliation(s) Chiang Mai University
Primary contact Email: sarana dot s at cmu dot ac dot th
Editors Valgimigli, Luca
Year published 2020
Volume and issue 25(24)
Article # 5792
DOI 10.3390/molecules25245792
ISSN 1420-3049
Distribution license Creative Commons Attribution 4.0 International
Website https://www.mdpi.com/1420-3049/25/24/5792/htm
Download https://www.mdpi.com/1420-3049/25/24/5792/pdf (PDF)

Abstract

Terpenes are the primary constituents of essential oils and are responsible for the aroma characteristics of the Cannabis plant. Together with cannabinoids, terpenes illustrate a potential synergic and/or entourage effect, with their interactions having only been speculated on for the last few decades. Hundreds of terpenes have been identified that additionally add to the overall cannabis sensory experience, contributing largely to the consumer’s experiences, as well as the market price. These terpenes also enhance many therapeutic efforts, especially as aromatherapy. To shed light on the importance of terpenes in the cannabis industry, the purpose of this review is to morphologically describe sources of cannabis terpenes and to explain the biosynthesis and diversity of terpene profiles in different cannabis chemotypes.

Introduction

Cannabis sativa L. or Cannabis is a herbaceous annual that has a long history of use around the world as fiber, food, oil, and medicine. Depending on the purpose of utilization, cannabis can be called by different names, for example, “hemp” as a fiber and textile crop, and “recreational cannabis,” also known in the United States as "marijuana." Aside from its quality as an industrial textile, the psychoactive properties have placed a stigma on Cannabis plants as an illicit drug, with many informal names including pot, dope, grass, weed, Mary Jane, bud, hash, bhang, kef, ganja, locoweed, reefer, doob, spliff, toke, and roach. It has been forbidden to grow in many countries due to its psychoactive constituents.[1] From a medical perspective, many recent studies have advised that an increase in cannabis use is associated with psychiatric symptoms, including depression and anxiety.[2][3] However, many users still exclusively endorse its recreational purpose.[4][5] As a result, there has been a strong movement toward correcting negative attitudes about Cannabis, and attempts have been made in trying to remove this plant from narcotic lists. In Thailand, for instance, as of 2020, cannabis strains such as กัญชง (hemp) and กัญชา (marijuana) are legally grown for industrial fiber or medicinal purposes based on the controlled level of active metabolites, including cannabinoids such as Δ9–tetrahydrocannabinol (THC) and cannabidiol (CBD).[6] While primary focus has been paid to the bioactive functions of the cannabinoids, the hydrocarbon terpenes could also potentially offer interesting entourage effects that could ideally synergize or downstream their effects.[7] Eminently, with the rise in the legal cannabis industry, interest has grown around cannabis terpenes as they contribute many of the different aromatic characteristics that influence the diverse varieties of cannabis strains.[8]

Within the scope of this review, we provide the general background history of cannabis discovery and the importance of terpenes. The taxonomy and morphology of cannabis, particularly in regards to the localization of its terpenes, are discussed. More importantly, the chemistry, biosynthesis, and diversity of terpenes in different cannabis genotypes are of major interest in this review.

The cannabis discovery and its importance as a source of terpene

Cannabis has a long history dating back approximately to just after the Ice Age, as cord and textile scraps made of cannabis fiber have been found in historic caves in the Czech Republic.[9][10] In the nineteenth century, the plant was recorded as originating near the southern area of the Caspian Sea near Iran (Figure 1b).[11][12] It was later confirmed by the chemotaxonomy of the essential oil from cannabis of diverse origins, and most of the cannabis phenotypes collected around the globe had chemical ingredients similar to those of Central Asian origin.[13] In previous days, it was known as the original fiber plant in Asian culture. Seed and seed oil extracts were also used as food.[11][14] The first record of cannabis in the literature in China can be dated to approximately 5,000 years ago, as written by emperor Chen Nong, who was then known as the father of Chinese agriculture. The Chinese alphabet's “Ma” was created using the mimic of the cannabis drying process (Figure 1a). The letter was adjusted to describe the male plant, “his” separated from the female plant, “chu” for the quality of the fiber.[15][16] In 500 BC, the use of cannabis spread worldwide from Asia to Europe and to Africa through the silk road. In the nineteenth century, hemp was popular in the western world as a fiber crop that had superior qualities.[17][18] It is not too surprising therefore that cannabis is today known as an ideal plant in terms of sustainability. In a period of rapid industrial growth, hemp became the industrial crop as countries raced against one another toward modernity. In addition to the world textile industry, the plant began to be known for its medical uses. It was believed that during the nineteenth century, there were thousands of cannabis medicines available, produced by more than 280 manufacturers.[19] However, the growth and interest in this fiber crop crashed after the attempt to add it to the narcotic list with opium during the 1925 International Opium Convention in Geneva.[19]


Fig1 Sommano Molecules21 25-24.png

Fig. 1 The evolution of the Chinese characters of the word “Cannabis sativa” or “Ma” (a), and the place of origin of the cannabis plant in the southern Caspian sea near Iran (b)

The first cannabinoid isolates used for medicinal purposes were produced in Czechoslovakia, and CBD was fully characterized for the first time in 1963, followed by the psychoactive THC in the following year.[20][21][22] The discovery of cannabinoid receptors, CB1 and CB2, together with the full comprehension of the endocannabinoid system, helped us recognize the medicinal benefits of this plant.[23][24] In 1942, Simonsen and Todd[25] were the first researchers to separate terpene fractions from cannabinoids in the literature, and p-Cymene was reported as a main constituent from Egyptian hashish. Only in the past years have the terms "synergic" and "entourage effect" appeared in speculations by chemists all over the world in relation to cannabis compounds, included the terpenes.[26] The proposed synergies were first described as routes for the molecular regulation of endogenous cannabinoid activity.[27] Russo[28] later proposed the unique therapeutic effect of cannabis terpenes that possibly play a role on the entourage effects of the medicinal properties of the cannabinoids. This phytocannabinoid-terpenoid synergy could potentially enhance the treatments of pain, inflammation, depression, anxiety, addiction, epilepsy, cancer, fungal, and bacterial infections.[7][28][29][30][31]

Taxonomy and localization of cannabis terpenes

Cannabis belongs to the small family of flowering plants known as Cannabaceae, which includes hop and Celtis berry. It is a dicotylate angiosperm that gives incomplete (lacking of petals) and also imperfect flower types with the separated sexual organs. The flower bears only pistils, known as pistillate flowers (or female flowers), and those with only the stamens are called staminate (or male flowers). In nature, Cannabis produces either male or female flowers (dioecious); however, under short-day conditions, it may give both male and female flowers or monoecious. Plants are able to grow as high as three meters, or smaller depending on the varieties and the conditions of growth. The flowers often initiate as groups of flowers or as an axillary bud (Figure 2a and 2b). The stem consists of the outermost layer of the epidermis, which is thin and coarse, and the primary and secondary layers that provide the better fiber quality. The innermost, which is the lignified secondary blast fiber, give the best fiber quality.[32][33]


Fig2 Sommano Molecules21 25-24.png

Fig. 2 The Cannabis plant var. Kees’ Old School Haze (available at https://dutch-passion.com) (a) and their flower buds var. Gorilla Glue (available at http://www.seedstockers.com) (b)

Leaves are palmate with five to seven leaflets. The male flower (Figure 3a) has no petals, usually with five yellowish tepals, and five anthers yielding pollen. The female flower (Figure 3b) had a single-ovulate and is enriched with trichome structures, which are the localization points of cannabinoids and terpenes, as shown in Figure 3c. These terpenes are responsible for the defense and interaction with herbivores and pests.


Fig3 Sommano Molecules21 25-24.png

Fig. 3 Cannabis staminate flowers (a) and pistillate flowers (b), with the trichome structures as the localization of cannabinoids and terpenes of Kees’ Old School Haze (available at https://dutch-passion.com) (c).

Taxonomists classified Cannabis plants into three species in the early days: (i) C. sativa Linnaeus, (ii) C. indica Lamarck, and (iii) C. ruderalis.[34][35] Today, many researchers are convinced that Cannabis that grows commercially is C. sativa L., but the subvariety “sativa” should be known as hemp and the subvariety “indica” should be called recreational cannabis or marijuana. The differences in these sub-varieties are shown in Table 1.

Table 1. Differences between cannabis (marijuana) and hemp
Characteristics Cannabis (Marijuana) Hemp
Genus Cannabis sativa L. Cannabis sativa L.
Sub-variety indica sativa
Utilized organs leaves, flowers, stems and seeds containing trichomes stem
Level of psychoactive THC High (>1%/DW) Low
Medicinal CBD Can be high Can be high
Leaf Broad, darker leaf color Thinner and greenish
Content of terpene (Rosin) High (gluey) Low

The most usable part for hemp is the stem in particular, while parts usually with trichomes are the most usable for cannabis. The level of THC is graded as >2% of dry weight, and flowers give a much higher terpene content, which becomes sticky to the touch.[36] Some researchers do not agree with the separation of the two by the chemical compositions. Morphologically, the leaf of cannabis is broader and the color is darker compared to that of hemp. Many recent studies have attempted to separate the combinations of terpene composition in several species of Cannabaceae where it is apparent that hemp and the Indica cannabis are closely related. For example, hemp can also yield terpene profiles similar to those of marijuana.[37]

Three types of glandular trichomes are characterized based upon their surface morphology, namely bulbous, sessile, and stalked (Figure 4a).[38] Bulbous trichomes are the smallest, while sessile trichomes appear on the epidermis with a short stalk and a globose head comprised of a multicellular disc of secretory cells and a subcuticular metabolite storage cavity. Similarly, the stalked trichomes are slightly larger with a globose head elevated above the epidermal surface by a multicellular stalk.[39] In cannabis, the sessile and stalked trichomes differ not only in morphology, but they also have distinct fluorescent properties, number of cells in their secretory disc, and terpene profiles.[40] The stalked glandular trichomes of mature flowers have a globular head consisting of an enlarged disc greater than eight secretory cells known to be rich in cannabinoids and monoterpenes (Figure 4b). The sessile trichomes are mainly found on sugar leaves (Figure 4c). They have eight secretory cells that produce fewer cannabinoids and higher proportions of sesquiterpenes.


Fig4 Sommano Molecules21 25-24.png

Fig. 4 Different trichome structures for Cannabis plants (a); the stalked types that are available on the floral surface (b); and sugar leaf structures with the presences of trichomes of Cannabis sativa L. var. Kees’ Old School Haze (available at https://dutch-passion.com) (c).

To separate the trichomes, the flowers are usually pre-frozen or freeze-dried and then are gently rubbed on sieve mesh. The trichomes separated in this process are known as kief, which can be pressed to make hash. In Nepal, hash is hand-shaped into balls, also known as wax or “Charas.”[41] Hash oil, on the other hand, is the concentrated hash that has been dissolved in organic solvents such as alcohol, propane, or butane.[42][43] The extraction allows pigments such as chlorophylls and other contaminants to be extracted along with the terpenes, resulting in a dark green color extract. After extraction, the solvent is then removed by evaporation either by direct heat or under a vacuum, resulting in the oil product with high viscosity.

Terpene biosynthesis in cannabis

The energy required for plant growth and development derives from photosynthesis, respiration, and transpiration with O2, CO2, nutrients, and water. The energy is restored in the form of primary chemical ingredients that plants later exploit. These primary metabolites include carbohydrates, lipids, proteins, and nucleic acids. However, during cycles of growth and reproduction, plants might be challenged by stresses including hard environmental conditions, pests, and herbivores. Plants then produce different groups of compounds called secondary metabolites that are used as defenses to those challenges. For example, it can produce compounds that draw in pollinators, including birds to help them in the fertilization process or seed dispersion.[44] These compounds are produced in different forms and are exploited for their biological functionalities[45]; for example, alkaloids such as morphine and codeine in opium give psychoactive and pain relief activity to mammals. Phenolics and flavonoids found in the skins of fruits and berries possess antioxidant activity.[46] Sulfur containing compounds such as allicin in garlic can be used to reduce lipoglycerides in the blood and also have the ability to stimulate appetite.[47] Saponin glycoside in soap nuts can be used as a surfactant[48], and finally, the terpenoids, which are main ingredients found in plants containing essential oils[49], are used as food additives, and some depict psychoactive ability and aroma characteristics such as those found in cannabis.

Terpenes are hydrocarbons with small isoprene units linked to one another to form chains, while terpenoids are oxygen-containing terpenes. Four types of terpenes/terpenoids are usually found in the Cannabis plant[26]:

  • monoterpenes (10C) of two isoprene units
  • sesquiterpenes (15C) of three isoprene units
  • diterpenes (20C) of four isoprene units
  • triterpenes (30C) of six isoprene units

To date, more than 200 volatiles have been reported from the different cannabis genotypes, of which 58 monoterpenes and 38 sesquiterpenes have been characterized.[50][51][52][53] Figure 5a illustrates a chromatogram of the terpene extract from the floral tissue of cannabis. Among others, the major monoterpene components are limonene, β-myrcene, α-pinene, and linalool, with traces of α-terpinolene and tran-ocimene[54][55] (Figure 5b), while predominate sesquiterpenes are E-caryophyllene, caryophyllene oxide, E-β-farnesene, and β-caryophyllene.[56] The cannabinoids are biologically synthesized from diterpene structures to form phenol terpenoids, which account for almost a quarter of all metabolites.[26] Thus, the combination of the terpenes provides the unique aromas to different strains.

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Notes

This presentation is faithful to the original, with only a few minor changes to presentation. Some grammar and punctuation was cleaned up to improve readability. In some cases important information was missing from the references, and that information was added.