Flavan-3-ols (sometimes referred to as flavanols) are a subgroup of . They are derivatives of that possess a 2-phenyl-3,4-dihydro-2 H-chromen-3-ol skeleton. Flavan-3-ols are structurally diverse and include a range of compounds, such as catechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, , , . They play a part in plant defense and are present in the majority of plants.
Chemical structure
The single-molecule (monomer) catechin, or isomer epicatechin (see diagram), adds four hydroxyls to flavan-3-ol, making building blocks for concatenated polymers (
) and higher order polymers (
).
Flavan-3-ols possess two chiral carbons, meaning four occur for each of them. They are distinguished from the yellow, ketone-containing flavonoids such as quercitin and rutin, which are called flavonols. Early use of the term bioflavonoid was imprecisely applied to include the flavanols, which are distinguished by the absence of ketones. Catechin monomers, dimers, and trimers (oligomers) are colorless. Higher order polymers, anthocyanidins, exhibit deepening reds and become .
Catechin and epicatechin are , with (–)-epicatechin and (+)-catechin being the most common optical found in nature. Catechin was first isolated from the plant extract catechu, from which it derives its name. Heating catechin past its point of decomposition releases pyrocatechol (also called catechol), which explains the common origin of the names of these compounds.
Epigallocatechin and gallocatechin contain an additional phenolic hydroxyl group when compared to epicatechin and catechin, respectively, similar to the difference in pyrogallol compared to pyrocatechol.
Catechin gallates are gallic acid of the catechins; an example is epigallocatechin gallate, which is commonly the most abundant catechin in tea. and are oligomeric flavan-3-ols.
In contrast to many other , flavan-3-ols do not generally exist as in plants.
Biosynthesis of (–)-epicatechin
The flavonoids are products from a
cinnamoyl-CoA starter unit, with chain extension using three molecules of
malonyl-CoA. Reactions are catalyzed by a type III PKS enzyme. These enzymes use
coenzyme A esters, and have a single active site to perform the necessary series of reactions: chain extension, condensation, and cyclization. Chain extension of 4-hydroxycinnamoyl-CoA with three molecules of malonyl-CoA gives initially a polyketide (Figure 1), which can be folded. These allow Claisen-like reactions to occur, generating
Aromaticity.
Fluorescence-lifetime imaging microscopy can be used to detect flavanols in plant cells.
- Figure 1: Schematic overview of the flavan-3-ol (–)-epicatechin biosynthesis from tyrosine (Tyr) or phenylalanine (Phe) in plants. Enzymes are indicated in blue, abbreviated as follows:
- :
Aglycones
| + Flavan-3-ols
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Dietary sources
Flavan-3-ols are abundant in
derived from the tea plant
Camellia sinensis, as well as in some
Cocoa solids (made from the seeds of
Theobroma cacao), although the content is affected considerably by processing, especially in
chocolate.
Flavan-3-ols are also present in the human diet in fruits, in particular
Pome,
berry,
, and
wine.
Their content in food is variable and affected by various factors, such as
cultivar,
Food processing, and preparation.
Bioavailability and metabolism
The
bioavailability of flavan-3-ols depends on the
Human nutrition, type of compound and their
Stereochemistry configuration.
While monomeric flavan-3-ols are readily taken up, oligomeric forms are not absorbed.
Most data for human metabolism of flavan-3-ols are available for monomeric compounds, especially
Catechin. These compounds are taken up and metabolized upon uptake in the
jejunum,
mainly by
O-methylation and glucuronidation,
and then further
Drug metabolism by the
liver. The colonic
Human microbiota has also an important role in the metabolism of flavan-3-ols and they are catabolized to smaller compounds such as 5-(3′/4′-dihydroxyphenyl)-γ-valerolactones and
hippuric acid.
Only flavan-3-ols with an intact (epi)catechin moiety can be metabolized into 5-(3′/4′-dihydroxyphenyl)-γ-valerolactones (image in Gallery).
Possible adverse effects
As catechins in
green tea extract can be
Hepatotoxicity,
Health Canada and EFSA have advised for caution,
recommending intake should not exceed 800 mg per day.
Research
Research has shown that flavan-3-ols may affect
Vascular system,
blood pressure, and
blood lipids, with only minor effects demonstrated, as of 2019.
In 2015, the European Commission approved a
health claim for
containing 200 mg of flavanols, stating that such intake "may contribute to maintenance of vascular elasticity and normal blood flow".
As of 2022, food-based evidence indicates that intake of 400–600 mg per day of flavan-3-ols could have a small positive effect on cardiovascular
.
==Gallery==
External links
