Flavonols are compounds of flavonoid nature. They are a subgroup of flavonoids that possess very interesting properties and benefits for the human body. These types of natural chemicals are of plant origin, widely present in foods such as fruits, vegetables, legumes, and spices.
There are different types according to their chemical structure. Some are of immeasurable value due to their antioxidant effects, while others, on the other hand, manage to act against inflammation and even have properties for memory.
Contents
- 1 What are Flavonols
- 2 Properties
- 3 Types of Flavonols
- 3.1 3-Hydroxyflavone
- 3.2 Azaleatin
- 3.3 Fisetin
- 3.4 Galangin
- 3.5 Myricetin
- 3.6 Quercetin
- 3.7 Isoquercetin
- 3.8 Gossypetin
- 3.9 Kaempferol
- 3.10 Rutin
- 4 Foods and Plants with Flavonols
- 5 References
What are Flavonols
They are plant pigments included within the flavonoid family. We can find different types of natural chemical compounds, and depending on their chemical structure, their backbone or spinal column will be designated as one type or another.
Flavonols have a backbone of 3-hydroxyflavone (3-hydroxy-2-phenyl chromen-4-one), with hundreds of them existing depending on the position of the phenolic groups –OH.
Although their name (flavonol) is very similar to that of other flavonoids, they are not the same as flavanols. They also have differences in the composition of their rings and phenolic group compared to flavones, thus providing other properties and benefits for humans when ingested through foods.
Properties
They are known for being present in many foods and for functioning as pigments in plants. Grape flavonols are responsible for giving a yellow or white color to the grape fruit, as well as imparting a yellow color to white wine. In grapes, they can be found in the skin along with anthocyanins, a class of flavonoid present in the skin of black grapes.
They also participate in the hue of red wine, with their presence being lower in this beverage than the other bioflavonoids that express their intensity with characteristic darker tones typical of Rioja.
In plants, flavonols function as protective agents against UV rays, being considered to have potential applications as antioxidants in humans. Although this property is still not fully understood, there are many indications that it could be the case because other plant metabolites considered flavonoids mostly act as powerful antioxidants and protectors of the cardiovascular system, as well as preventive substances for inflammatory and neurodegenerative diseases.
Like other polyphenolic compounds, flavonol gives color to plant flowers.
- Antibacterial.
- Antiviral.
- Antioxidant.
- Inhibitors of tumor growth (breast cancer in vitro).
Types of Flavonols
- 3-Hydroxyflavone.
- Azaleatin (2-(3,4-Dihydroxyphenyl)-3,7-dihydroxy-5-methoxy-4H-1-benzopyran-4-one).
- Fisetin (3, 7, 3′, 4′-tetrahydroxyflavone).
- Galangin (Norizalpinin or 3,5,7-Trihydroxyflavone or 3,5,7-triOH-Flavone).
- Myricetin (3,5,7-Trihydroxy-2-(3,4,5-trihydroxyphenyl)-4-chromenone or Cannabiscetin or Myriceto).
- Quercetin (2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one).
- Isoquercetin (2-(3,4-Dihydroxyphenyl)-5,7-dihydroxy-3-[(2S,3R,4S,5S,6R)-3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2-yl]oxychromen-4-one).
- Gossypetin (gossypetin 3,5,7,8,3 ‘, 4’-dihydroxyflavone).
- Kaempferol (Naringenin-chalcone synthase).
- Rutin (quercetin-3-rutinoside or rutoside or soforin).
3-Hydroxyflavone
As we have already mentioned, 3-Hydroxyflavone is the basis of all flavonols. It is the backbone of all of them, so it is present in every type of flavonol.
It has the ability to perform intramolecular proton transfer in the excited state (ESIPT). This characteristic of 3-Hydroxyflavone allows it to be used as a fluorescent probe for the study of cell membranes. Also as a base for intermembrane proteins.
Azaleatin
Azaleatin is a methylated flavonol. Also within the group of flavonoids. As a flavonol, it is present in most plants called «Plumbago,» with more species being discovered every day that possess this type of flavonoid. It can also be found in fruits such as pecans.
Azaleatin is also considered a tetrahydroflavone and a monomethoxyflavone, meaning quercetin with the hydroxy group at position 5 replaced by a methoxy group.
Fisetin
It acts as a pigment in plants, giving yellow color to leaves and flowers. Fisetin is a flavonoid and polyphenol, being present in many fruits, vegetables, and greens.
The discoverer of the flavonol fisetin was Josef Herzig in the year 1891.
The difference of fisetin from other antioxidant flavonols and flavonoids is that fisetin is toxic. It also has mutagenic properties, causing mutations on DNA. However, besides these side effects, it also has the ability to reduce the toxicity of aflatoxins, toxins present in corn crops, peanuts, and other nuts infected by fungi.
Since it has mutagenic action, it can be used in medicine as an activator of sirtuins, enzymes that affect cellular metabolism by regulating the expression of certain genes (epigenetics) in animals and plants. This property makes it able to function as an anti-aging and calorie-restricting product.
Like other flavonols, fisetin also improves cognition, in this case, long-term memory.
Galangin
Galangin has antibacterial and antiviral properties. It is a flavonol that acts by inhibiting the growth of cells that produce breast cancer. Some studies relate galangin to topical hydroquinone to reduce vitiligo symptoms, a disease that affects the skin and hair (causing white patches due to lack of melanin).
The use of galangin for vitiligo was indicated in a study conducted in May 2014 by Hugo SX and his team, where after administering galangal rhizome orally for 30 days, there was an increase in the presence of melanin in hair follicles.
Myricetin
Myricetin is a flavonol with a molecular formula of C15H10O8. It is a type of flavonoid or polyphenolic compound attributed with antioxidant properties. It acts by blocking damage to biological tissues that can be caused by free radicals.
Foods containing myricetin include berries, nuts, red wine, and most fruits.
There are many similarities with other types of flavonols, having a very similar structure to other well-known ones such as quercetin, fisetin, and the flavone luteolin. Its functions are very similar and it can be found in its natural form as myricitrin, a glycoside.
The properties of myricetin are antioxidant and anti-cholesterolemic, the latter property linked to the reduction of LDL cholesterol like another flavonol, gossypetin. There are also some in vitro studies indicating that it could have benefits in preventing prostate and pancreatic cancer.
Quercetin
Quercetin is a flavonol present in fruits and vegetables in the form of O-glycoside. This type of flavonoid is very common, being one of the most ingested by humans through diet.
Its presence in foods can be found along with rutin and naringenin, also present in citrus fruits such as oranges, lime, and tomatoes. Generally, the more red the food is, the richer it is in quercetin.
Studies show that the benefits of quercetin when administered with resveratrol, a very powerful antioxidant, can reduce the formation of adipose tissue, with further research needed to delve into its therapeutic applications. Nowadays, there are many supplements enriched with the flavonol quercetin, but it is not proven to have beneficial effects on human health. Its nutritional value and potential medicinal properties are unknown.
Isoquercetin
Isoquercetin is the isomer of quercetin (3-O-glucoside of quercetin), and some pharmaceutical companies like Merck have developed an anti-wrinkle cream (RonaCare) indicated to prevent premature skin aging and expression signs and wrinkle formation on the face.
According to the company developing this product, it has antioxidant and sun radiation-blocking properties. It’s worth noting that Ronacare, in addition to isoquercetin, includes other cosmetic molecules.
Gossypetin
Known as Gossypetin, gossypetin is one of the most studied flavonols. Its scientific name is 3,5,7,8,3′,4′-hexahydroxyflavone and has been isolated from the plant Hibiscus sabdariffa.
Gossypetin has antibacterial properties and inhibits the growth of certain enzymes related to neurosynaptic function. It is an antagonist of TrkB (Tropomyosin receptor kinase B), also acting as a radioprotector.
In its natural form, gossypetin is present in plants such as Godmania aesculifolia and in St. John’s Wort (Tridax procumbens) or hypericum.
It is considered a selective agonist of the tropomyosin receptor kinase B (TrkB) and since it can penetrate the blood-brain barrier, it has applications for the treatment of Alzheimer’s disease and in psychiatric disorders such as depression, amyotrophic lateral sclerosis, and cognitive deficits. Its properties act on the central nervous system.
Gossypetin may promote memory enhancement and improve emotional learning.
It also has antioxidant properties like most polyphenols.
Kaempferol
It can be obtained from many foods such as hazelnuts, green tea, broccoli, and Brussels sprouts. Kaempferol is a water-soluble flavonol, soluble in hot ethanol and ethyl ether.
From kaempferol, glycosides have been extracted that are having medicinal applications such as astragaline and kaempferitrin. This flavonol is highly regarded for its antidepressant properties.
Its anticancer action can also be detected when applied in a mixture with other flavonoids (quercetin + myricetin and kaempferol). Access full information on the properties of kaempferol.
Rutin
It is a type of flavonoid glycoside. Rutin or quercetin-3-rutinoside is naturally present in plants. Sometimes rutin is referred to as vitamin P, although it is not exactly a vitamin.
Its chemical structure places it in the flavonol quercetin and the disaccharide rutinose, possessing properties to inhibit platelet aggregation and to slow vascular permeability. It makes blood flow smoother, less thick, which brings enormous health benefits.
It also exhibits anti-inflammatory activity. Rutin is a glycoside with significant biological activity, also highlighting its effects on the transformation of glucose into sorbitol and the inhibition of aldose reductase activity.
In terms of the circulatory system, it can increase the thickness and resistance of capillaries, being administered to hemophilia patients.
In another context related to combating cholesterol, the administration of rutin along with ferulic acid decreases the risk of heart diseases. It lowers the level of LDL cholesterol toxicity.
Foods and Plants with Flavonols
- Propolis.
- Alpinia officinarum.
- Helichrysum aureonitens.
- Alpinia galanga rhizome (Alpinia officinarum Hance).
- Mango (Mangifera indica).
- Noble Rhubarb.
- Sikkim Rhubarb.
- Bauhinia longifolia.
- Rheum nobile.
- Rhododendron (Rhododendron mucronatum).
- Eucryphia.
- Cape Plumbago, blue, Jasmine (Plumbago capensis, Ceratostigma willmottianum, etc.).
- Pecan (Carya illinoinensis, pecan).
- Strawberries.
- Apples.
- Persimmons.
- Onions.
- Cucumbers.
- Tea.
- Onion.
- St. John’s Wort (Tridax procumbens).
The flavonol called galangin is found in propolis from bee honey, in the rhizome of Alpinia galanga, in Alpinia officinarum, and in the plant Helichrysum aureonitens.
One of the richest foods in quercetin is onions, but it is also present in broccoli and thyme.
References
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- Smith, Gerald J., & Markham, Kenneth R. (1998). Tautomerism of flavonol glucosides: relevance to plant UV protection and flower colour. Journal of Photochemistry and Photobiology A: Chemistry, 118(2), 99-105.
- Si D, Wang Y, Zhou YH, et al. (March 2009). “Mechanism of CYP2C9 inhibition by flavones and flavonols”. Drug Metab. Dispos. 37 (3): 629-34.
- Ciolino, H. P.; Yeh, G. C. (1999). «The flavonoid galangin is an inhibitor of CYP1A1 activity and an agonist/antagonist of the aryl hydrocarbon receptor». British Journal of Cancer 79 (9/10): 1340–1346.
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- Herzig, J. (1891). «Studien über Quercetin und seine Derivate, VII. Abhandlung» [Studies on Quercetin and its Derivatives, Treatise VII]. Monatshefte für Chemie (in German) 12 (1): 177-90.
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- Hugo SX, Liu XM, Ge CH, Gao L, Peng XM, Zhao PP, et al. “The effects of galangin on a mouse model of vitiligo induced by hydroquinone”. Phytother Res 2014; 28: 1533-1538.
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