Anthocyanins: definition, structure and pH

CONTENTS

What are anthocyanins?

Anthocyanins are a subgroup of flavonoids, therefore they are polyphenols, which give plants their distinctive colors.
They are water soluble pigments and are present in the vacuolar sap of the epidermal tissues of flowers and fruit.
They are responsible for the colors of the most of the petals, fruits and vegetables, and of some varieties of cereals such as black rice. In fact, they impart red, pink and purple to blue colors to berries, red apples, red grapes, cherries, and of many other fruits, red lettuce, red cabbage, onions or eggplant, but also red wine.
Together with carotenoids, they are responsible for autumn leaf color.
Finally, anthocyanins contribute to attract animals when a fruit is ready to eat or a flower is ready for pollination.

They are bioactive compounds found in plant foods that have a double interest for man:

  • the first one, a technological interest, due to their effects on the organoleptic characteristics of food products;
  • the other due to their healthy properties, being implicated in the protection against cardiovascular risk.
    In fact:

in vitro, they have an antioxidant activity, due to their ability to delocalize electrons and form resonance structures, and a protective role against oxidation of low density lipoproteins (LDL);

like other polyphenols, such as catechins, proanthocyanidins and other uncolored flavonoids, they can regulate different signaling pathways involved in cell growth, differentiation and survival.

Chemical structure of anthocyanins

The basic chemical structure is flavylium cation (2-phenylbenzopyrilium), which links hydroxyl (-OH) and/or methoxyl (-OCH3) groups, and one or more sugars.
The sugar-free molecule is called anthocyanidins.

Basic Skeleton of Anthocyanins
Fig. 1 – 2-Phenylbenzopyrilium

Depending on the number and position of hydroxyl and methoxyl groups, various anthocyanidins have been described, and of these, six are commonly found in vegetables and fruits:

  • pelargonidin
  • cyaniding
  • delphinidin
  • petunidin
  • peonidin
  • malvidin
Antocyanins
Fig. 2 – Antocyanins

Anthocyanins, as most of the other flavonoids, are present in plants and plant foods in the form of glycosides, that is, linked to one or more sugar units.
The most common sugars present in these natural pigments are:

The sugars are linked mainly to the C3 position as 3-monoglycosides, to the C3 and C5 positions as diglycosides (with the possible forms: 3-diglycosides, 3,5-diglycosides, and 3-diglycoside-5-monoglycosides). Glycosylations have been also found at C7, C3′ and C5′ positions.
The structure of these molecules is further complicated by the bond to the sugar unit of different acyl substituents such as:

  • aliphatic acids, such as acetic, malic, succinic and malonic acid;
  • cinnamic acids (aromatic substituents), such as sinapic, ferulic and p-coumaric acid;
  • finally, there are pigments with both aromatic and aliphatic substituents.

Furthermore, some anthocyanins have several acylated sugars in the molecule; these anthocyanins are sometimes called polyglycosides.

Depending on the type of hydroxylation, methoxylation and glycosylation patterns, and the different substituents linked to the sugar units, more than 500 different anthocyanins have been identified that are based on 31 anthocyanidins. Among these 31 monomers:

  • 30% are from cyanidin;
  • 22% are from delphinidin;
  • 18% are from pelargonidin.

Methylated derivatives of cyanidin, delphinidin and pelargonidin, namely peonidin, malvidin, and petunidin, all together represent 20% of the anthocyanins.
Therefore, up to 90% of the most frequently encountered anthocyanins are related to delphinidin, pelargonidin, cyanidin, and their methylated derivatives.

Role of pH

The color of these molecules is influenced by the pH of the vacuole where they are stored, ranging in color from:

  • red, under very acidic conditions;
  • to purple-blue, in intermediate pH conditions;
  • until yellow-green, in alkaline conditions.

In addition to the pH, the color of these flavonoids can be affected by the degree of hydroxylation or methylation pattern of the A and B rings, and by glycosylation pattern.
Finally, the color of certain plant pigments result from complexes between anthocyanins, flavones and metal ions.
It should be noted that anthocyanins are often used as pH indicators thanks to the differences in chemical structure that occur in response to changes in pH.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Escribano-Bailòn M.T., Santos-Buelga C., Rivas-Gonzalo J.C. Anthocyanins in cereals. J Chromatogr A 2004:1054;129-141. doi:10.1016/j.chroma.2004.08.152

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8. doi:10.3945/​ajcn.111.028340

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231


Proanthocyanidins: definition, structure and absorption

CONTENTS

What are proanthocyanidins?

Proanthocyanidins or condensed tannins, also called pycnogenols and leukocyanidins, are polyphenolic compounds (in particular they are a flavonoid subgroup) widely distributed in the plant kingdom, second only to lignin as the most abundant phenol in nature.
They are present in high concentrations in various parts of the plants such as flowers, fruits, berries, seeds (e.g. in grape seeds), and bark (e.g. pine bark).

Together with anthocyanins and their oxidation products, and catechins, they are the most abundant flavonoids in human diet and it has been suggested that they constitute a significant fraction of the polyphenols ingested in the Western diet.
Therefore, condensed tannins should be taken into consideration when the epidemiological association between the intake of polyphenols, especially flavonoids, and chronic diseases are examined.

Chemical structure of proanthocyanidins

Condensed tannins have a complex chemical structure being oligomers (dimers to pentamers) or polymers (six or more units, up to 60) of catechins or flavanols, which are joined by carbon-carbon bonds.

Proanthocyanidins
Fig. 1 – Procyanidin Skeleton

They may consist exclusively of:

  • (epi)catechin, and they are named procyanidins;
  • (epi)afzelechin, and they are named propelargonidins;
  • (epi)gallocatechin, and they are named prodelphinidins.

Propelargonidins and prodelphinidins are less common in nature and in foods than procyanidins.

Depending on the bonds between monomers, proanthocyanidins have a:

  • B-type structure, if the polymerization occurs via carbon-carbon bond between the position 8 of the terminal unit and the 4 of the extender (or C4-C6);
  • A-type structure, less frequent, if monomers are doubly linked via an ether bond C2-O-C7 or C2-O-C5 plus a B-type bond.

Procyanidins

The most common dimers are B-type procyanidins, B1 to B8, formed by catechin or epicatechin; in B1, B2, B3 and B-4 dimers, the two flavanol units are joined by a C4-C8 bond; in B5, B6, B7 and B8 dimers the two units are joined by C4-C6 bond.

Proanthocyanidins
Fig. 2 – Procyanidins B1-B4

Procyanidin C1 is a B-type trimer.

Procyanidin A-2 is an example of A-type procyanidin.

Intestinal absorption of proanthocyanidins

Condensed tannins are poorly absorbed from the intestine; together with anthocyanins and gallic acid ester derivatives of tea catechins, they are the least well-absorbed polyphenols.
It seems that low molecular weight oligomers (2-3 monomers) may be absorbed as such while polymers are not.
In the systemic circulation, dimers reach concentrations of two orders of magnitude lower than those of catechins.
It seems that condensed tannins with a degree of polymerization greater than three transit into the stomach and small intestine without significant modifications, and then, into the large intestine, they are catabolized by colonic microflora, with production of phenylpropionic, phenilvaleric and phenylacetic acids. These degradation products have been suggested to be the major metabolites of proanthocyanidins in healthy humans.

Procyanidins and catechins

It had been proposed that the catabolism of procyanidins in the gastrointestinal tract lead to the release of monomeric catechins, thus indirectly contributing to their systemic pool in humans. In recent years, it has been shown that this does not happen because procyanidins do not significantly contribute to:

  • the concentration of catechin metabolites in the systemic circulation;
  • the total catechin metabolites excreted in the urine;
  • finally, they do not significantly affect plasma metabolite profile derived from catechol-O-methyltransferase activity.

Therefore, analyzing the potential health benefits associated with the intake of foods containing these phytochemicals, catechins and procyanidins should be considered distinct classes of related compounds.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Gu L., Kelm M.A., Hammerstone J.F., Beecher G., Holden J., Haytowitz D., Gebhardt S., and Prior R.L. Concentrations of proanthocyanidins in common foods and estimations of normal consumption. J Nutr 2004;134(3):613-617. doi:10.1093/jn/134.3.613

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Nandakumar V., Singh T., and Katiyar S.K. Multi-targeted prevention and therapy of cancer by proanthocyanidins. Cancer Lett 2008;269(2):378-387. doi:10.1016/j.canlet.2008.03.049

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8. doi:10.3945/ajcn.111.028340

Santos-Buelga C. and Scalbert A. Proanthocyanidins and tannin-like compounds: nature, occurrence, dietary intake and effects on nutrition and health. J Sci Food Agr 2000;80(7):1094-1117. doi:10.1002/(SICI)1097-0010(20000515)80:7<1094::AID-JSFA569>3.0.CO;2-1

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Wang Y.,Chung S., Song W.O., and Chun O.K. Estimation of daily proanthocyanidin intake and major food sources in the U.S. diet. J Nutr 2011;141(3):447-452. doi:10.3945/jn.110.133900


Catechins: definition, structure, green tea, black tea, cocoa

CONTENTS

What are catechins?

Catechins or flavanols, with flavonols such as quercetin, and flavones such as luteolin, are a subgroup of flavonoids among the most widespread in nature.
Flavanols and proanthocyanidins, together with anthocyanins and their oxidation products, are the most abundant flavonoids in human diet.

Chemical structure of catechins

Chemically they differ from many other flavonoids as:

  • they lack the double bond between positions 2 and 3 of the C ring;
  • they not have a keto group at position 4;
  • they have a hydroxyl group in position 3, and for this reason they are also called flavan-3-ols.
Catechins
Fig. 1 – Basic Flavanol Skeleton

Another distinctive feature of flavan-3-ols is their ability to form oligomers (two to ten units) or polymers (eleven or more units, up to 60 units) called proanthocyanidins or condensed tannins.

Catechins in foods

Flavanols commonly found in plant-derived food products are catechin, epicatechin, gallocatechin, epigallocatechin, and their gallic acid ester derivatives: catechin gallate, gallocatechin gallate, epicatechin gallate, and epigallocatechin gallate or EGCG.

Catechins
Fig. 2 – Flavanols

Flavanols present with higher frequency are catechin and epicatechin, which are also among the most common known flavonoids, and almost as popular as the related flavonol quercetin.
Cocoa and green tea are by far the richest sources in flavanols. In these foods the main flavonoids are catechin and epicatechin (cocoa is also a good source of epigallocatechin), but also their gallic acid ester derivatives, the gallocatechins.
However, they are also present in many fruits, especially in the skins of apples, blueberries (Vaccinium myrtillus) and grapes, in vegetables, red wine and beer, and peanuts.
As in many cases flavanols are present in the seeds or peels of fruits and vegetables, their intake may be limited by the fact that these parts are discarded during processing or while eaten.
Furthermore, in contrast to other flavonoids, catechins are not glycosylated in foods.
Proanthocyanidins, that is polymeric flavan-3-ols, are also commonly found in plant-derived food products. Their presence has been reported in the skin of peanuts and almonds, as in the berries.

Green and black tea

Green tea is an excellent source of flavonoids. The main flavonoids present in the leaves of the tea (as in cocoa beans) are catechin and epicatechin, monomeric flavanols, together with their gallate derivatives such as EGCG.
Epigallocatechin gallate is the most abundant catechin in green tea and it seems to have an important role in determining green tea benefits, as the reduction of:

  • vascular inflammation;
  • blood pressure;
  • concentration of oxidized LDL.

Black tea (fermented tea) contains fewer monomeric flavanols, as they are oxidized during fermentation of the leaves to more complex polyphenols such as theaflavins (theaflavin digallate, theaflavin-3-gallate, and theaflavin-3′-gallate, all dimers) and thearubigins (polymers).
Theaflavins and thearubigins are present only in the tea; their concentrations in brewed tea are between 50- and 100-folds lesser than in tea leaves.

It should be noted that tea epicatechins are remarkably stable to heat in acidic environment: at pH 5, only about 15% is degraded after seven hours in boiling water (therefore, adding lemon juice to brewed tea does not cause any reduction in their content).

Cocoa and cocoa products

Cocoa has the highest content of polyphenols and flavanols per serving, a concentration greater than those found in green tea and red wine. Most of the flavonoids present in cocoa beans and derived products, such as black chocolate, are catechin and epicatechin, monomeric flavanols, but also epigallocatechin, and their derivatives such as the gallocatechins; among polymers, proanthocyanidins are also important.

Fruits, vegetables, and legumes

Catechin and epicatechin are the main flavanols in fruits. They are found in many fruits in different concentrations, respectively, between 5-3 and 0.5-6 mg/100 g fresh weight.
On the contrary, gallocatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate are present in various fruits such as red grapes, berries, apples, peaches and plums, but in very low concentrations, less than 1mg/100 g fresh weight.
Except for lentils and broad beans, few legumes and vegetables contain catechins, and in very low concentrations, less than 1.5 mg/100 g fresh weight.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231