Flavonols

{{short description|Class of plant and fungus secondary metabolites}}

{{distinguish|Flavanols}}

File:Flavonol num.svg

Flavonols are a class of flavonoids that have the 3-hydroxyflavone backbone (IUPAC name: 3-hydroxy-2-phenylchromen-4-one). Their diversity stems from the different positions of the phenolic –OH groups. They are distinct from flavanols (with "a") such as catechin, another class of flavonoids, and an unrelated group of metabolically important molecules, the flavins (with "i"), derived from the yellow B vitamin riboflavin.

Flavonols are present in a wide variety of fruits and vegetables. In Western populations, estimated daily intake is in the range of 20–50 mg per day for flavonols. Individual intake varies depending on the type of diet consumed.{{cite journal |vauthors=Cermak R, Wolffram S |title=The potential of flavonoids to influence drug metabolism and pharmacokinetics by local gastrointestinal mechanisms |journal=Curr. Drug Metab. |volume=7 |issue=7 |pages=729–44 |date=October 2006 |pmid=17073577 |doi=10.2174/138920006778520570 |url=http://www.bentham-direct.org/pages/content.php?CDM/2006/00000007/00000007/0004F.SGM |archive-url=https://archive.today/20120720144359/http://www.bentham-direct.org/pages/content.php?CDM/2006/00000007/00000007/0004F.SGM |url-status=usurped |archive-date=2012-07-20 |url-access=subscription }}

The phenomenon of dual fluorescence (due to excited state intramolecular proton transfer or ESIPT) is induced by tautomerism of flavonols (and glucosides) and could contribute to plant UV protection and flower colour.{{cite journal | last1 = Smith | first1 = Gerald J. | last2 = Markham | first2 = Kenneth R. | year = 1998 | title = Tautomerism of flavonol glucosides: relevance to plant UV protection and flower colour | journal = Journal of Photochemistry and Photobiology A: Chemistry | volume = 118 | issue = 2| pages = 99–105 | doi=10.1016/s1010-6030(98)00354-2}}

Besides being a subclass of flavonoids, flavonols are suggested by a study of cranberry juice to play a role along with proanthocyanidins, in the juice's ability to block bacterial adhesion, demonstrated by the compressing the fimbria of E. coli bacteria in the urinary tract so as to greatly reduce the ability of those bacteria to stay put and initiate an infection.{{cite web|url=http://medicalxpress.com/news/2016-07-juicy-news-cranberries.html|title=Juicy news about cranberries|website=medicalxpress.com|access-date=13 April 2018}} Flavonol aglycones in plants are potent antioxidants that serve to protect the plant from reactive oxygen species (ROS).{{Cite journal|title=Enhancement of oxidative and drought tolerance in Arabidopsis by overaccumulation of antioxidant flavonoids.|last=Nakabayashi R, Yonekura-Sakakibara K, Urano K, Suzuki M, Yamada Y, Nishizawa T, Matsuda F, Kojima M, Sakakibara H, Shinozaki K, Michael AJ, Tohge T, Yamazaki M, Saito K. | pmid=24274116 | doi=10.1111/tpj.12388|volume=77|issue=3|pmc=4282528|journal=Plant J|year=2014 |pages=367–79}}