Acidic Cannabinoid Decarboxylation

Abstract

Introduction:

Cannabis is a valuable plant, cultivated by humans for millennia. However, it has only been in the past several decades that biologists have begun to clarify the interesting Cannabis biosynthesis details, especially the production of its fascinating natural products termed acidic cannabinoids.

Discussion:

Acidic cannabinoids can experience a common organic chemistry reaction known as decarboxylation, transforming them into structural analogues referred to as neutral cannabinoids with far different pharmacology. This review addresses acidic and neutral cannabinoid structural pairs, when and where acidic cannabinoid decarboxylation occurs, the kinetics and mechanism of the decarboxylation reaction as well as possible future directions for this topic.

Conclusions:

Acidic cannabinoid decarboxylation is a unique transformation that has been increasingly investigated over the past several decades. Understanding how acidic cannabinoid decarboxylation occurs naturally as well as how it can be promoted or prevented during harvesting or storage is important for the various stakeholders in Cannabis cultivation.

Introduction

Among the earliest domesticated plants was Cannabis, first cultivated in Central Asia thousands of years ago.¹ The taxonomy of Cannabis is rather complicated and has been well explained by McPartland's extensive treatise.² In summary, the genus of Cannabis was once thought to comprise up to three putative species: Cannabis sativa, Cannabis indica, and Cannabis ruderalis. However, this classification has become controversial and extensive interbreeding has likely made these distinctions rather hopeless.³ Consensus now appears to be that C. indica and C. ruderalis are simply varieties of C. sativa. In this review, the reference to Cannabis will apply only to the genus. Cannabis is a very special plant in many ways and this discussion will focus on just one intriguing chemistry property of it; namely, the reaction known as acidic cannabinoid decarboxylation. However, to fully appreciate how unique this transformation is in the plant world, a brief summary of Cannabis botany and biosynthesis now follows.

Cannabis Botany and Biosynthesis

A useful review of general Cannabis botany has been published by Farag and Kayser⁴ and the discussion here will address the subject at a high level, leaving more details for later. Cannabis is an annual plant with two important growth phases. The vegetative stage is marked by gradual plant development and followed by a flowering (or inflorescence) period with the accelerated biosynthesis of key natural product chemicals in the emerging flowers. Cannabis is also dioecious, simply meaning that male and female flowers reside on separate plants. Male staminate flowers develop anthers, which open to release wind-driven pollen for the awaiting female Cannabis pistillate flowers. Having served their purpose, the male Cannabis plants soon die so as not to compete with female plants for limited space or important resources. This destiny for the male plants ensures the development of fruit called achenes (often mistakenly described as seeds) in the female Cannabis plants. Interestingly, since unpollinated female Cannabis flowers (also known as “sinsemilla”) do not produce achenes, more of their biosynthetic energy can be focused on creating greater amounts of diverse natural products.⁵ For this reason, there has been heightened interest among Cannabis growers to more quickly identify male and female plants,⁶ thereby better controlling plant field location by gender and optimizing the degree of crop pollination. A very detailed discussion describing the architecture and florogenesis of the female Cannabis inflorescence has recently been published by Spitzer-Rimon et al.⁷

The biosynthesis and secondary metabolism of Cannabis have been progressively elucidated over the past several decades.^8–10 During the flowering period, the female Cannabis plant skillfully creates hundreds of diverse natural products. Significant amounts of these compounds are lower molecular weight, volatile terpenes (linear or cyclic unsaturated hydrocarbons),^11,12 providing a characteristic fragrance to the Cannabis flower. Terpenes are clearly important natural products but they are only marginally involved in this discussion. The other major category of Cannabis natural products pertinent to this review are termed cannabinoids or alternatively “phytocannabinoids” to emphasize their natural plant derivation^13–15 and distinguish them from the endogenous cannabinoids known as endocannabinoids. Structurally, a cannabinoid can be described as a composition hybrid of a cyclic terpene fused to a para-alkyl substituted aromatic resorcinol ring. There are now well more than a hundred such cannabinoids isolated from Cannabis, with new ones frequently being discovered and characterized.¹⁶ The absolute and relative amount of each cannabinoid in any plant depends on many factors, including the Cannabis variety, the plant organ being examined, and the timing in its growth cycle. Besides terpenes and cannabinoids, Cannabis also contains lesser amounts of alkaloids, amino acids, carbohydrates, esters, fatty acids, flavonoids, hydrocarbons, ketones, proteins, and steroids. Clearly, the Cannabis chemical diversity is almost unrivaled in the plant kingdom.

Acidic and Neutral Cannabinoid Pairs

Figure 1 displays six of the major Cannabis constituents grouped in pairs; namely, delta-9-tetrahydrocannabinolic acid (THCA), delta-9-tetrahydrocannabinol (THC), cannabidiolic acid (CBDA), cannabidiol (CBD), cannabigerolic acid (CBGA), and cannabigerol (CBG). For the sake of brevity, only these abbreviations will now be used to identify each cannabinoid. The pairing of the structures in Figure 1 is both deliberate and purposeful. This juxtaposition emphasizes the important structural chemistry relationship between an acidic cannabinoid (with a carboxylic acid group [CO₂H]) on the left and its conversion to a neutral cannabinoid (with removal of the carboxylic acid) on the right. However, going forward in this discussion, when just the general term “cannabinoid” is used, it will refer collectively to both acidic and neutral cannabinoids. Acidic cannabinoids have also been referred to as “pre-cannabinoids” in a lengthy magnum opus by Hanus et al.¹⁴ and they exist because the enzyme systems creating them require carboxylic acid functionalized substrates.^17,18 It is also important to note that these acidic cannabinoids are the final products in an enzymatic reaction cascade involving the conjunction of separate polyketide (resorcinol) and mevalonate (terpene) biosynthetically parallel pathways. Loss of a carboxylic acid as carbon dioxide is a very common organic chemistry reaction termed “decarboxylation.” Although acidic cannabinoid construction is enzymatic, the final acidic cannabinoid decarboxylation step (giving a neutral cannabinoid) has been widely accepted as non-enzymatic in nature. Only three pairs of important acidic and neutral cannabinoids are displayed in Figure 1 but quite a few others have been isolated from Cannabis as well, including compounds that contain a propyl instead of a pentyl resorcinol linear side chain.¹⁹ Importantly, these enzyme-created acidic cannabinoids of Figure 1 are very different and not to be confused with the acidic cannabinoids resulting from the human metabolism of neutral cannabinoids. Although the biosynthetically assembled acidic cannabinoids of this review are decorated with a carboxylic acid group on their aromatic resorcinol ring, many of the acidic cannabinoid human metabolites incorporate a carboxylic acid on their terpene ring instead.²⁰

FIG. 1.

Acidic and neutral cannabinoid pairs. CBD, cannabidiol; CBDA, cannabidiolic acid; CBG, cannabigerol; CBGA, cannabigerolic acid; THCA, delta-9-tetrahydrocannabinolic acid; THC, delta-9-tetrahydrocannabinol.

There are several interesting cannabinoids worthy of separate mention. First, cannabinol (CBN; Fig. 2) and its acid analogue cannabinolic acid (CBNA)²¹ have been recognized as Cannabis oxidation artefacts. CBN has the distinction of being the first cannabinoid whose structure was elucidated as well as synthesized by Roger Adams at the University of Illinois in 1940.²² Since CBN is the ultimate oxidative degradation product for a number of cannabinoids including THC, the ratio of CBN to THC has been proposed as an approximate age indicator for Cannabis samples stored at ambient temperature.²³

FIG. 2.

Structures of CBNA, CBN, and THCA-B. CBN, cannabinol; CBNA, cannabinolic acid.

The second compound to consider has been one of the more unusual tales among acidic cannabinoids. THCA (Fig. 1) was first discovered by Korte et al.²⁴ In 1969, Mechoulam isolated from hashish and fully characterized another minor isomer of THCA with its carboxylic acid in the alternate aromatic resorcinol ring position.²⁵ Mechoulam suggested an appropriate name for this second THCA isomer as THCA-B (Fig. 2) to differentiate it from the original acid. Curiously, the hashish sample that Mechoulam investigated contained only 0.5% of THCA-B and “very little or none” of THCA. THCA-B displayed some unique spectroscopic properties and could even be purified as a crystalline solid with an unusually high melting point (184–185°C) for a cannabinoid. However, on decarboxylation, both isomers THCA and THCA-B afforded the same product, THC. In contrast to most other cannabinoids, there has been very little published literature on THCA-B according to SciFinder.^® (All future references to the chemistry literature in this review, especially where metrics are cited, made use of the SciFinder chemistry database). Although some investigators have searched for but failed to isolate THCA-B from other Cannabis sources,²⁶ chemists from the University of Mississippi and elsewhere have, indeed, separately confirmed its existence in fresh Cannabis.^27,28 The identity of THCA-B was further corroborated later by a definitive Norwegian X-ray crystal study.²⁹ Interestingly, in many descriptions of the key THCA-producing Cannabis enzyme THCA synthase, THCA-B has often been omitted from biosynthesis Figures and discussion,³⁰ still leaving open the curious details of its biosynthetic origin. One final point of Cannabis vernacular nomenclature requires clarification. Especially when authors have simultaneously discussed both the A and B isomers of THCA, they have often abbreviated the original and predominant isomer THCA-A to clearly distinguish it from THCA-B. That convention will now be followed here as well.

Acidic Cannabinoid Decarboxylation: A Chemically Unique Transformation

Acidic cannabinoid decarboxylation is a remarkable and unique chemistry event from several perspectives and these are now considered. The organic chemistry literature contains more than 53,000 journal articles regarding the general decarboxylation reaction, both enzymatic and synthetic. Although acidic cannabinoid decarboxylation exclusively involves the replacement of an aromatic carboxylic acid group with a hydrogen atom (often termed “protodecarboxylation”), this specific conversion amounts to only a small fraction of the larger body of decarboxylation chemistry literature. Unlike acidic cannabinoid decarboxylation, most other general decarboxylations have been catalyzed (to lower their activation energy) and have often been coupled to the construction not of carbon–hydrogen bonds but carbon–carbon, carbon–halogen, carbon–nitrogen, carbon–phosphorus, and carbon–sulfur bonds. Although acidic cannabinoid decarboxylation has been largely considered a thermal process, the greater collection of general decarboxylation reactions have also employed more unusual energy forms, including electrochemistry³¹ and even lasers.³² Also, depending on the reaction environment and catalysis of the carboxylic acid substrate, general decarboxylation can occur by means of several possible mechanisms. Clearly, acidic cannabinoid decarboxylation constitutes a very unique subset of general decarboxylation chemistry.

Second, in the context of the many thousands of known plant and marine natural products, the acidic cannabinoid decarboxylation narrative is also very rare. It is hard to imagine another close parallel where a family of natural product carboxylic acid precursors have experienced decarboxylation to afford a second collection of corresponding neutral natural product analogues. The chemistry literature has documented more than 550,000 published articles on “natural products” to date and much <1% of these involve non-enzymatic decarboxylation like those of acidic cannabinoids. However, many of these references have not described the decarboxylation of a natural product itself but the use of decarboxylation as a synthetic tool to construct a natural product. The closest analogy that comes to mind in comparing acidic cannabinoid decarboxylation with other natural products chemistry has perhaps been the intriguing conversion of ibotenic acid to muscimol in the toxic mushroom Amanita muscaria. This decarboxylation has also been described as predominantly a thermal process, observed mostly during storage of the harvested mushroom.³³ Interestingly, by using the solvent dimethyl sulfoxide we exploited this facile decarboxylation chemistry to convert ibotenic acid into [³H] muscimol with tritiated water at just ambient temperature overnight (Fig. 3). In our radiosynthesis, a tritium atom was regiochemically substituted for the excised carboxyl group.³⁴ However, even this ibotenic acid transformation is still an imperfect analogy for acidic cannabinoid decarboxylation in that it is a single natural product example and not a family of them. Also, the ibotenic acid precursor in our synthesis experienced an alkyl decarboxylation, not a much more difficult aromatic decarboxylation as in the case of acidic cannabinoids.

FIG. 3.

Conversion of ibotenic acid to [³H] muscimol using ³H₂O, DMSO, rt, 16 h. DMSO, dimethyl sulfoxide.

Finally, regarding the neutral cannabinoid product and its medicinal chemistry, acidic cannabinoid decarboxylation has also been unusual in its outcome. To date, the pharmacology of acidic and neutral cannabinoids has been found to be very different as exemplified by the pair THCA-A and THC.^35,36 Only rarely in medicinal chemistry does the pharmacology of a substance become significantly altered by removal of just a single functional component like a carboxylic acid group.

Acidic Cannabinoid Decarboxylation: A Uniquely Conflicted Market Segment Event

The preceding section described how very special acidic cannabinoid decarboxylation is from several chemistry perspectives. However, acidic cannabinoid decarboxylation is also an unusual event from a market segmentation standpoint, considering the various stakeholders in the expanding Cannabis cultivation industry. The chemistry literature has recorded more than 150 patents (awarded and applications) going back to 1981, describing some aspect of acidic cannabinoid decarboxylation. In fact, the pace of intellectual property in this specialized area is accelerating with nearly 60% of all the patents in this collection filed in just the past 3 years alone. Consequently, there is clearly a large Cannabis market segment with a vested interest in promoting acidic cannabinoid decarboxylation. However, in sharp contrast, there is also an emerging portion of the Cannabis research community actively exploring the pharmacology and medicinal use of acidic cannabinoids. These latter researchers certainly have a completely different application perspective and for this separate Cannabis opportunity, acidic cannabinoid decarboxylation is an anathema. Obviously, from diametrically opposite market and product viewpoints, the acidic cannabinoid decarboxylation reaction is a conflicted chemistry transformation that can be either a blessing or a curse.

When and Where Does Acidic Cannabinoid Decarboxylation Occur?

It is important next to consider the several circumstances where acidic cannabinoids might undergo decarboxylation, discussing them in the logical time progression of Cannabis plant growth, harvesting/processing, storage, and human consumption. Living Cannabis itself is the obvious starting point for potential acidic cannabinoid decarboxylation, and it is essential to first understand exactly where acidic cannabinoids are biosynthesized in planta. Although it has been recently demonstrated that Cannabis roots are devoid of cannabinoids,³⁷ they have been detected to some extent in Cannabis achenes,³⁸ vegetative leaves,³⁹ and pollen.⁴⁰ However, the major site of acidic cannabinoid biosynthesis and accumulation has been found to occur in the tiny structures called trichomes,⁴¹ which abundantly decorate the female Cannabis flower.

The earliest documented investigation of Cannabis trichomes has been attributed to Italian researchers Briosi and Tognini working in Milan in 1894.⁴² After their detailed and well-illustrated botanical study, other occasional publications on Cannabis trichomes appeared over the years as reviewed by Dayanandan and Kaufman.⁴³ However, the most extensive and sustained research on Cannabis trichomes and their correlation with cannabinoid content began with Paul G. Mahlberg at Indiana University, identifying three distinct types of Cannabis glandular trichomes as bulbous, capitate-sessile, and capitate-stalked trichomes.^44–62 Because Cannabis trichomes are so small and fragile, several innovative technologies have recently been developed to study them either individually or collectively for cannabinoid content. These methods have included trichome storage cavity microsuction,⁶³ two-photon microscopy with intrinsic autofluorescence,⁶⁴ specialized nuclear magnetic resonance (NMR) techniques,^65,66 megaelectronvolt (MeV) secondary ion mass spectrometry (MS),⁶⁷ and Raman spectroscopy.^68–70

A comprehensive and practical review has recently appeared outlining the significant pitfalls that should be avoided when analyzing cannabinoid trichome contents, especially during gas chromatography (GC) where a heated injection port and column can easily cause artefactual acidic cannabinoid decarboxylation.⁷¹ This thermal reaction should be considered when interpreting neutral cannabinoid content results by GC analysis in the older literature. This is particularly true if some form of carboxyl group protective derivatization (such as trimethylsilyl [TMS] group installation) was not utilized for any acidic cannabinoids present. For this reason, high-performance liquid chromatography (HPLC) coupled with MS are generally preferred analytical methods for acidic cannabinoids. In summary, these many investigations indicated that capitate-stalked trichomes are the most biosynthetically active in Cannabis, producing acidic cannabinoids with their concentration increasing during the female plant flowering period. Two studies are particularly illustrative: First, as determined by LCMS, the relative trichome concentrations of THC and CBD were found to be ∼1–2% of that for their precursors THCA-A and CBDA.⁶⁶ Also, a second study using quantitative NMR spectroscopy of trichome content (with low-temperature fluid handling) measured the peak concentrations of THC and CBD (during week 7 of the Cannabis flowering period) as about 0.5 mg/g of trichome weight.⁶⁵ Therefore, measurable acidic cannabinoid decarboxylation does occur to a limited extent even in the living Cannabis plant during its flowering stage.

The next opportunity for more significant acidic cannabinoid decarboxylation would be during Cannabis harvesting and processing, either in the form of whole flower removal or by botanical liquid extraction. As already mentioned, there have been a growing number of patents and journal articles whose technology was purposely designed to promote decarboxylation during Cannabis harvesting. Not surprisingly, when microwave heating (ethanol, 150°C, 10 min) has been employed, essentially complete acidic cannabinoid decarboxylation was observed.⁷² Several publications have also addressed the role of organic solvents in extracting cannabinoids from Cannabis and found that cannabinoid stability was very solvent dependent.^73,74 Besides HPLC, alternative analytical methods have been explored to track the progress of acidic cannabinoid decarboxylation. One of the more promising analytical approaches for monitoring decarboxylation has been by infrared (IR) spectroscopy. In a very recent paper, changes in the IR spectrum band intensity of heated Cannabis flowers have been correlated with the decreasing amounts of THCA-A/CBDA and the increasing concentration of their products, THC/CBD.⁷⁵ In particular, the authors noted that the growing intensity of certain IR bands near 1510, 1430, 1260, 1180, 1040, and 835 cm⁻¹ signaled the beginning of decarboxylation for THCA-A and CBDA. Some of these decarboxylation-related IR bands could also be highly informative, such as the unique strong IR band at 1424 cm⁻¹, which was specific for THC but not CBD.

When Cannabis has been harvested without a deliberate thermal decarboxylation step (to preserve its original acidic cannabinoids), storage of the plant contents would be the next occasion for potential acidic cannabinoid decarboxylation. With the nutritional and therapeutic use of Cannabis increasing, there has been a greater urgency to understand all chemical changes in the cannabinoid profile during storage, including acidic cannabinoid decarboxylation.^76–80 One intriguing finding in these studies has been that acidic cannabinoids still embedded in the Cannabis inflorescence matrix appear to be more stable and less likely to undergo decarboxylation than isolated acidic cannabinoids.^35,77 This has been a common observation dating back to some of the earliest Cannabis stability studies.²⁶ Two especially novel investigations of extremely old Cannabis have been published. A sample of more than 80-year-old dried Cannabis flower tops was analyzed by GC-MS with TMS derivatization.⁸¹ The study revealed that the major cannabinoid detected was CBN with lesser amounts of CBNA. However, the most surprising result in this sample was the discovery of both THCA-A and THC, the longevity of which the author attributed to storage in darkness. A second and even more fascinating “archeological” examination of ancient Cannabis from a 2700-year-old grave site in Central Asia by Russo et al. identified (HPLC and MS analysis) several neutral cannabinoids.⁸² Unsurprisingly, CBN dominated the sample profile and strongly suggested the initial presence of THC. Interestingly, some CBD was also found but as expected, no volatile terpenes were present. A remarkable photomicrograph in Figure 2B⁸² clearly displayed archaic amber capitate-sessile trichomes on a recovered Cannabis leaf fragment. Evidence gathered at the excavation site indicated that this early Cannabis was likely used “for pharmaceutical, psychoactive or divinatory purposes.”

Having considered the actual Cannabis plant as well as its harvesting and storage conditions, one final aspect of acidic cannabinoid decarboxylation to address is human consumption and metabolism. To probe the conversion of THCA-A to THC during smoking, several simulated smoking experiments have been reported.^83,84 Both studies came to practically the same conclusions. They discovered that the decarboxylation of THCA-A to THC was essentially complete during the smoking process. However, the recovery of THC in each study was only about 30%. The authors reasoned that the missing THC mass balance could likely be attributed to its combustion during the simulation.

Currently, consensus among Cannabis researchers has been that at least for THCA-A, little if any decarboxylation of it occurs in vivo.^14,36 For instance, in an investigation of rat metabolism, no physiological conversion of THCA-A to THC was observed.⁸⁵ Since analytical methods have been perfected to measure acidic cannabinoids such as THCA-A in human blood and urine,^86,87 the detection of significant THCA-A in human fluids would also appear to rule out any appreciable decarboxylation of it in humans. However, more direct human metabolic studies of acidic cannabinoids are now beginning to appear using different forms of administered medical Cannabis. Care often needs to be taken in the design and interpretation of these acidic cannabinoid investigations. Depending on the route of acidic cannabinoid administration, artefactual decarboxylation can potentially occur as in the case of THCA-A and CDBA given by vapor inhalation.⁸⁸

Kinetics and Mechanism Studies of Acidic Cannabinoid Decarboxylation

Several laboratories have carefully interrogated the chemistry of acidic cannabinoid decarboxylation, arriving at kinetics and mechanism conclusions. In one of the initial studies, Mechoulam converted CBGA to CBG in refluxing methanol with sodium hydroxide,⁸⁹ although this alkaline method would not necessarily be indicative of the decarboxylation environment for most acidic cannabinoids. Interestingly, Mechoulam later found that in 2 h CBDA could be partially decarboxylated in refluxing benzene (80°C) but completely converted in 2 h to CBD in refluxing xylene (139°C).⁹⁰ These reactions were also conducted under the inert atmosphere nitrogen, indicating an increased awareness that oxygen could cause unwanted side products during acidic cannabinoid decarboxylation. These results were also early and compelling evidence for the largely thermal nature of acidic cannabinoid decarboxylation. Japanese workers investigated the decarboxylation of THCA-A, noting how relatively easy it was to accomplish, completed in refluxing benzene in only 7 h.⁹¹ Leisztner was among the first to systematically study acidic cannabinoid decarboxylation, finding that the reaction exhibited first-order kinetics.⁹² In particular, he examined the decarboxylation of THCA-A and CBDA on various surfaces in open reactors as analyzed by HPLC.

Other recent mechanism studies have also reported comparable results and corroborated that acidic cannabinoid decarboxylations demonstrated first-order kinetics.^93–97 Perrotin-Brunel, citing an earlier related precedent of salicylic acid decarboxylations,^98,99 employed molecular modeling calculations to support a keto-enol mechanism as a likely lower-energy THCA-A decarboxylation pathway.⁹⁷ Khan at the University of Mississippi determined that the THCA-A decarboxylation rate constant was higher than that of both CBDA and CBGA.⁹⁶ Also, he observed that the THCA-A decarboxylation (110°C) was a relatively clean and quantitative transformation, resulting in few if any side products. In contrast, Khan reported that the CBDA and CBGA decarboxylations (110°C) were more complex and accompanied by several unknown side reactions, resulting in product loss of 18% for CBDA and 53% for CBGA. Moreno also confirmed that the decarboxylation of THCA-A to THC was faster than that of both CBDA and CBGA.⁹⁴ Importantly, she also conducted these decarboxylations in the inert atmosphere nitrogen and concluded that the presence of oxygen during decarboxylation led to oxidized side products and diminished amounts of neutral cannabinoids. Employing a simplified decarboxylation kinetic model (with independent acidic cannabinoid calculations), Moreno found that optimization of the THCA-A to THC conversion was ideally a combination of shorter reaction time and higher temperature. Conversely, the CBDA to CBD decarboxylation was best performed for a longer reaction time and lower temperature. Although most decarboxylation studies have reported results at higher temperatures, a very recent publication (ahead of print) examined the rate of acidic cannabinoid decarboxylation at near ambient temperature for long periods of time.⁹³ Studies such as this will be valuable to establish Cannabis product shelf life.

Interestingly, very recent results of Canadian researchers also suggested that a specific decarboxylation mechanism may be possible for acidic cannabinoids in a more lipophilic environment.⁷⁶ The CBDA decarboxylation affording CBD was investigated in hempseed oil heated to 85°C. It was found in this study that antioxidants such as α-tocopherol (vitamin E) were able to significantly decrease the decarboxylation rate of CBDA. With added α-tocopherol, the half-life of CBDA in hempseed oil at 85°C was increased from about 4 to 17 days. The authors did not propose an exact mechanism for this impressive result but suggested further study. However, α-tocopherol is a well-known free radical scavenger¹⁰⁰ and free radical decarboxylations are common in organic chemistry.¹⁰¹ The topic of a free radical mechanism for acidic cannabinoid decarboxylation has received relatively little attention in the vast Cannabis literature. Nevertheless, under the right circumstances, acidic cannabinoids may well be able to decarboxylate via a free radical pathway.

Sunlight Ultraviolet Radiation and Its Relationship to Cannabinoid Chemistry

At this point in the discussion, we have only considered thermal energy sources as the primary cause of acidic cannabinoid decarboxylation and this includes sunlight.

However, the ultraviolet (UV) contribution of sunlight cannot be ignored. The correlation of sunlight UV radiation on the accumulation of plant medicinal compounds has been extensively reviewed,¹⁰² and the specific study of UV radiation and its effect on Cannabis dates back nearly 40 years. Several of these early papers in the Cannabis literature have described a curious relationship between the sunlight UV component and plant cannabinoid content. Perhaps the first suggestion that sunlight UV energy might promote cannabinoid formation in some way was the work of Pate¹⁰³ and later Lydon.¹⁰⁴ After an extensive literature review, Pate concluded that Cannabis grown at higher altitudes produced relatively larger amounts of THC (but less CBD) than those grown at lower altitudes. He also proposed that increased exposure to UV-B radiation (280–315 nm) could function as an evolutionary selection pressure for Cannabis at higher altitudes, with the plant beneficially employing THC as a UV-B protection agent. Although not drawing the same evolutionary conclusions as Pate, Lydon also believed that elevated Cannabis plant altitude with increased UV-B exposure in some way stimulated cannabinoid synthesis. However, Lydon supported this concept with his own experimental work, demonstrating a linear relationship of increasing Cannabis THC content (GC analysis) with a larger UV-B plant dose.

More recently, there have been at least two unique studies^105,106 that have measured (at the same altitude) the correlation of individual Cannabis plant flower height and cannabinoid content. In the first paper, Bernstein discovered that the accumulation of most cannabinoids (GC analysis) in Cannabis flowers (medical cultivar NB100) at the top of a plant (with more direct UV exposure) was significantly greater than that of lower flowers (with more UV protection) on the same plant. In particular, the amount of THC was 12% in upper plant flowers and only 6% in lower flowers of the Cannabis studied. Bernstein also noted that “the chemical variation and its regulation in cannabis is a sorely neglected topic” and recommended further study to explain this fascinating cannabinoid yield variation.¹⁰⁵ Although not explicitly stated by these authors, it appears that their Cannabis sample preparation and (underivatized) GC analyses were performed in such a way as to convert any THCA-A present to THC.

The earlier discussion raises another interesting question about sunlight UV. That acidic cannabinoid decarboxylation happens to at least a small extent in the Cannabis plant itself has already been reviewed here. Based on the publications cited so far, the minimal thermal threshold for a reasonable rate of acidic cannabinoid decarboxylation would be seemingly difficult to achieve in a Cannabis plant during normal summer temperatures even factoring in additional solar radiation heating of leaves.¹⁰⁷ It is intriguing to consider whether sunlight UV energy might also facilitate the small amount of acidic cannabinoid decarboxylation observed in the plant. Surprisingly, there have been relatively few studies concerning cannabinoid photochemistry in general^90,108–111 and by a literature review, it appears that no one has yet reported the photolytic decarboxylation of an acidic cannabinoid at the laboratory bench. However, the decarboxylation of many other non-cannabinoid acidic organic compounds by sunlight or simulated sunlight has been well precedented in the organic chemistry literature.^112–117 In fact, for several of these non-cannabinoid examples, sunlight-catalyzed decarboxylation was a main photolytic pathway. Although the possibility of acidic cannabinoid photochemical decarboxylation is clearly very speculative, the photobiology of Cannabis and its correlation with cannabinoid yield appears to have now captured the imagination of many Cannabis growers.^118–121

Future Directions

Having reviewed the important and intriguing topic of acidic cannabinoid decarboxylation as part of the larger Cannabis chemistry story, it is certainly interesting to ask: What is left to explore? Consider just a few of the many potential future topics and related questions about both acidic cannabinoids and their decarboxylation.

In a recent review on the biosynthesis of minor cannabinoids, Kayser commented that the Cannabis trichome is not just some “passive container” but is more of a “chemical reactor,” producing acidic cannabinoids and other complex metabolites as well.¹²² Clearly, more research needs to be accomplished to fully understand all the interesting biophysical details of the trichome microenvironment. THCA-B is a unique minor acidic cannabinoid¹²³ and it would be fascinating to understand the factors that account for its formation in some varieties of Cannabis.

Unlike the rather straightforward X-ray crystal determination of THCA-B more than 45 years ago,²⁹ the X-ray crystal structure of THCA-A has only been performed in just the past several months employing a novel extraction–crystallization system using liquid carbon dioxide at 5°C.¹²⁴ However, it may be possible using the very new technique of microcrystal electron diffraction (MicroED) to obtain the crystal structure of THCA-A and related lower melting cannabinoids in a more routine way. MicroED has been successfully employed recently to structure elucidate a variety of small molecules utilizing just powder samples of them.^125,126

Although there is growing interest in the medicinal use of acidic cannabinoids such as THCA-A, their decarboxylation remains a troublesome challenge to solve.³⁵ A number of investigators have reported that acidic cannabinoids sequestered in Cannabis inflorescences are less inclined to decarboxylate.⁷⁷ It would be valuable to learn what causes this increased acidic cannabinoid stability in Cannabis and whether this same stabilizing matrix effect chemistry can be exploited outside of Cannabis to prevent decarboxylation. It is well known that esters of carboxylic acids protect them against decarboxylation. From time to time, terpene esters of acidic cannabinoids have been isolated¹²⁷ and it would be interesting to determine how prevalent such esters are in various varieties of Cannabis.

Several authors have independently reported the curious fact that THCA-A decarboxylates faster than CBDA. The structures of these two acidic cannabinoids are clearly very similar, except for the more constrained ring system in THCA-A. To the best of my knowledge, no explanation has been advanced in the peer-reviewed literature for this intriguing decarboxylation rate disparity and it would be very useful to learn the explanation. In reviewing the when and where of acidic cannabinoid decarboxylation, the area having the most limited data would appear to be that of human metabolism. A human metabolism study using different administration methods of acidic cannabinoid containing medical Cannabis is another valuable future direction for further decarboxylation investigation.

Conclusion

Certainly, the domestication of plants was a watershed accomplishment in human history and the development of civilization. Cannabis was an early example of this agricultural breakthrough, providing valuable materials to mankind for millennia. The expanding knowledge of Cannabis biology in the past several decades has revealed the unique importance of acidic cannabinoid decarboxylation to neutral cannabinoids. Much has been learned about this simple transformation, yet unanswered questions regarding it provide new and exciting topics for further exploration.

Abbreviations Used

CBD

cannabidiol

CBDA

cannabidiolic acid

CBG

cannabigerol

CBGA

cannabigerolic acid

CBN

cannabinol

CBNA

cannabinolic acid

GC

gas chromatography

HPLC

high-performance liquid chromatography

IR

infrared

MicroED

microcrystal electron diffraction

MS

mass spectrometry

NMR

nuclear magnetic resonance

THC

delta-9-tetrahydrocannabinol

THCA

delta-9-tetrahydrocannabinolic acid

TMS

trimethylsilyl

UV

ultraviolet

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received.

Cite this article as: Filer CN (2022) Acidic cannabinoid decarboxylation, Cannabis and Cannabinoid Research 7:3, 262–273, DOI: 10.1089/can.2021.0072.