We once again go beyond cannabi¬noids and look at the importance and chemistry of flavonoids in cannabis.
Cannabis is known for being a complex plant with thousands of chemical compounds. The most economically important compound for many years was the cannabinoids, particularly tetrahydrocannabinol (THC). In the past decade we have begun to understand the importance of other groups of natural cannabis compounds such as terpenes and now flavonoids. Flavonoids are naturally occurring secondary metabolic products that can have important functions within the cannabis plant and benefit consumers by their health and healing properties. In this column, we once again go beyond cannabinoids and look at the importance and chemistry of flavonoids in cannabis.
There are thousands of compounds in botanicals. Cannabis is rich in many vitamins, nutrients, and beneficial compounds. Historically, cannabis has been harvested for cannabinoids, but over the past decade there has been increased research and interest in the other active and beneficial compounds such as fatty acids, terpenes, antioxidants, and flavonoids. Many of these beneficial compounds are metabolite compounds produced as an end product of metabolism. Metabolites are small molecules that have many functions including defense, pigments, pheromones, odorants, and catalysts for many biological processes.
Primary metabolites are involved in growth, development, and reproduction. If a primary metabolite is missing or disrupted, the organism can degrade or die. Examples of primary metabolites are amino acids, antioxidants, and vitamins. Secondary metabolites are not directly involved in critical processes, but have secondary functions. An absence of a secondary metabolite does not necessarily mean the organism’s death. Examples of these secondary metabolites include compounds such as terpenes and flavonoids.
Flavonoids are secondary plant, algae, or fungus metabolites composed of polyphenolic compounds. Phenyl groups are cyclic molecules (C6H5) closely related to benzene (Figure 1). Flavonoids generally have a 15-carbon structure with two phenyl rings and one heterocyclic pyran ring (Figure 2).
In 1664, Robert Boyle first described flavonoid experiments with plants and colors in his book Experiments and Considerations Touching Colours (1,2). The first flavonoid was identified by a Hungarian biochemist and Nobel Prize winner, Albert Szent-GyÃ¶rgyi in 1930. The name for flavonoids has it root in the Latin flavus meaning yellow and describes the function as pigments in plants. Flavonoid pigments are found to produce several ranges of colors including yellows, blues, and reds with all the mix of colors in between. Color is perceived by optical system as wavelengths of light absorbed and reflected by a substance. Certain chemical compounds such as flavonoids and elements can reflect or transmit energy in visible light bands, which then are seen as particular colors (see Table I). Visible light is in the range of ~400–700 nm. It is within this range of light that humans perceive color. Outside that range of light we find other types of light and energy such as X-ray, ultraviolet (UV), and infrared (Figure 3). (See upper right for Table I, click to enlarge.)
Chemicals that allow for the absorption and reflection of light are said to have chromophores. In these molecules the energy difference between molecular orbitals is in the energy range of visible light. Visible light hits the electrons in the orbital and excites them moving the electron from a ground state to an excited state. This excitation produces a change in the shape of a molecule called a conformational change.
Chromophores are specific structural forms or interactions that allow for visible light to be transmitted. One of the most common chromophores is a conjugated Ï-bond. Pi-bonds are double bonds within the p-orbital of a molecule. Conjugated Ï-bonds occur when three or more p-orbitals within a molecule form alternating double and single bonds within a molecule as seen in the alternating double and single bonds within the flavonoid phenyl rings (Figure 1).
Additional structures within a molecule can also be important in the perception of color by altering or increasing the transmission of visible light wavelengths. An auxochrome is a functional group in the molecule; attached to the chromophore. These functional groups (such as alcohols, carboxylic acids, and nitrogen containing compounds) change the ability of the chromophore to absorb light. This change can alter the wavelength or intensity of the absorption and therefore change the visible color observed. It is the ability to absorb and reflect light that allows flavonoids to function to capture wavelengths of light, protect plants from UV radiation, attract pollinators, and defend against predators (3).
These phytochemical properties make flavonoids powerful phytonutrients and antioxidants, which make them in demand for human nutrition and health. Antioxidants have been a buzz-word in human health for several decades. These compounds inhibit oxidation, which is the loss of electrons that creates free radicals with unpaired valence electrons. These radical ions, atoms, or molecules are highly reactive and are important in many biological processes, but due to their highly reactive nature they can also cause unplanned changes or damage to other molecules, cells, and tissues causing disruptions to health and disease. Often these radicals can cause chain reactions that damage cells. Antioxidants can stop these chain reactions. Antioxidants can be compounds that are intentionally produced to preserve products and foods from oxidation such as preservatives and antirust compounds or antioxidants can be naturally occurring compounds that inhibit oxidative free radicals from undergoing chain reactions and causing cell damage and disease. Studies have shown that flavonoids can produce positive results in combatting heart disease, diabetes, and some cancers (4–6).
Flavonoids can be categorized into three general classes by backbone structure: bioflavonoids, isoflavonoids, and neoflavanoids. Bioflavonoids (or flavonoids) are designated as compounds where the phenyl ring is attached to the C2 carbon of the benzopyran ring. Isoflavonoids have their attachment to the C3 carbon and neoflavonids have the attachment of the phenyl ring at the C4 carbon of the benzopyran ring (Figure 4).
There are dozens of subgroups for all of the structural flavonoid groups. Table II shows important subgroups and functional groups of flavonoids. These groups produce the antioxidant effects and color pigmentations in many plants. Reds, blues, and purples are produced by anthocyanins. Yellows are produced by many flavonoids including anthoxanthins, chalcones, and aurones. Proanthocyanids produce black, brown, tan, and red pigments. (See upper right for Table II, click to enlarge.)
Cannabis is rich in many beneficial compounds including flavonoids. It is estimated that up to 10% of the cannabis plant compounds are flavonoids with up to almost 3% by weight in dried leaves and buds. The list of flavonoids in cannabis depends on which strains are being examined and the color of the plants. There are a dozen basic cannabis flavonoids found to some degree in the majority of the strains. Cannabis has also been found to have its own flavonoids exclusive to cannabis species (cannaflavins A, B, and C). Cannaflavins were first discovered by a researcher in London in 1986. Marilyn Barrett at the University of London identified cannaflavin A and cannaflavin B (Figure 5).
The majority of cannabis flavonoids are either flavones or flavonols, which are anthoxanthins. These compounds are primary responsible for yellow colors (Table III). (See upper right for Table III, click to enlarge.)
Cannabis is known for variations in colors and patterns. As plants grow they are primarily green and yellow. As the plant matures, many color variations can occur in the leaves and buds. The primary color of cannabis is green from the presence of chlorophyll. During the changes in temperature, however, such as during changes of season the production of chlorophyll stops and other colors can present as other compounds masked by chlorophyll become apparent.
Some of the more spectacular colors of cannabis are blues, purples, reds, and pinks due to flavonoids known as anthocyanins. These compounds are often dependent upon pH. These compounds are antioxidants and give the red and blue hues of grapes, berries, and cannabis especially strains such as Purple Haze, Blueberry, and Blue Dream. Red and pink colors tend to be rare, but include strains like Pink Panther. Yellow and oranges can be seen in strains with high carotenoids (terpenoids) or flavonoids such as Lemon Kush and Orange Bud.
Flavonoids in cannabis are antioxidants, colorants, and plant protectants. These compounds are not psychoactive but can play a role in suggested entourage effects where the flavonoids present in cannabis act in a synergistic action with other compounds such as the cannabinoids and terpenes to produce antioxidant, antidepressant, anti-inflammatory, and disease fighting properties. Just as in the case of the importance of terpenes in cannabis, the importance of flavonoids will need a lot more investigation and testing to truly elucidate the beneficial effects of these compounds for health.
Patricia Atkins is a Senior Applications Scientist with SPEX CertiPrep and a member of both the AOAC and ASTM committees for cannabis.
P. Atkins, Cannabis Science and Technology3(3), 24–27 (2020).