Tannins: chemical structure

HTs are molecules with a polyol (generally D-glucose) as a central core. The hydroxyl groups of these carbohydrates are partially or totally esterified with phenolic groups like gallic acid (-->gallotannins) or ellagic acid (--> ellagitannins). HT are usually present in low amounts in plants.

Some authors define two additional classes of hydrolyzable tannins: taragallotannins(gallic acid and quinic acid as the core) and caffetannins (caffeic acid and quinic acid)

Gallotannins:

• The phenolic groups that esterify with the core are sometimes constituted by dimers or higher oligomers of gallic acid (each single monomer is called galloyl)
• Each HT molecule is usually composed of a core of D-glucose and 6 to 9 galloyl groups
• In nature, there is abundance of mono and di-galloyl esters of glucose (MW about 900). They are not considered to be tannins. At least 3 hydroxyl groups of the glucose must be esterified to exhibit a sufficiently strong binding capacity to be classified as a tannin.
• The most famous source of gallotannins is tannic acid obtained from the twig galls of Rhus semialata. It has a penta galloyl-D-glucose core and five more units of galloyl linked to one of the galloyl of the core.

Ellagitannins:

• The phenolic groups consist of hexahydroxydiphenic acid, which spontaneously dehydrates to the lactone form, ellagic acid.
• Molecular weight range: 2000-5000.

HT properties:

• hydrolyzed by mild acids or mild bases to yield carbohydrate and phenolic acids
• Under the same conditions, proanthocyanidins (condensed tannins) do not hydrolyze.
• HTs are also hydrolyzed by hot water or enzymes (i.e. tannase).

Proanthocyanidins (condensed tannins)

PAs are more widely distributed than HTs. They are oligomers or polymers of flavonoid units (i.e. flavan-3-ol) linked by carbon-carbon bonds not susceptible to cleavage by hydrolysis.

• PAs are more often called condensed tannins due to their condensed chemical structure. However, HTs also undergo condensation reaction. The term, condensed tannins, is therefore potentially confusing.
• The term, proanthocyanidins, is derived from the acid catalyzed oxidation reaction that produces red anthocyanidins upon heating PAs in acidic alcohol solutions.
  • The most common anthocyanidins produced are cyanidin (flavan-3-ol, from procyanidin) and delphinidin (from prodelphinidin)
• PAs may contain from 2 to 50 or greater flavonoid units; PA polymers have complex structures because the flavonoid units can differ for some substituents and because of the variable sites for interflavan bonds.
• Anthocyanidin pigments are responsible for the wide array of pink, scarlet, red, mauve, violet, and blue colors in flowers, leaves, fruits, fruit juices, and wines. They are also responsible for the astringent taste of fruit and wines.
• PA carbon-carbon bonds are not cleaved by hydrolysis.
• Depending on their chemical structure and degree of polymerization, PAs may or may not be soluble in aqueous organic solvents.