27 May 2019

Tannins - protein binding polyphenolics

Natural toxin

Tannins are phenolic polymers that help plants deter herbivores through toxicity and impalatability. Tannins are found in a large variety of plants and fruits, and causes the astringent feeling of red wine and unripe apples.

The term tannin originates from the process ‘tanning’, where ‘tannin’ first was used to describe the compounds that could convert raw animal hides into leather. Through binding with collagen proteins of hides, the resistance to heat, microbes and water was increased.

Tannins are oligomeric compounds containing several phenolic groups. The molecular weight ranges from 500 to >20,000 Da, most being between 600 and 3000 Da. They are generally highly soluble in water. They can be separated into two main categories: (1) hydrolyzable tannins, containing a sugar unit in their core and easily hydrolyzable by weak acid or base, hot water or enzymes, and (2) condensed tannins, build exclusively from repeated flavonoid units by carbon-carbon bonds, also termed proanthrocyanins as they can be depolymerized into anthrocyanidins by acid catalysed oxidation in alcohol solutions.

Fig 1. A: Tannic acid, β (hydrolyzable tannin). B: Schematic drawing of condensed tannin. Click image for interactive 3D model of tannic acid.

The feeding deterring effect of the tannins stems from their ability to form complexes with numerous molecules (e.g. carbohydrates and enzymes involved in protein and carbohydrate digestion), especially proteins. This leads to both anti-nutritional effects as they bind to nutrients and gastrointestinal enzymes, and to astringency when binding to salivary glycoproteins. The interaction with proteins is often through hydrophobic and hydrogen bonding, with the phenolic groups being excellent hydrogen donors to carboxylic groups. The interaction leads to precipitation of the complexes, when tannins are in excess and can form a hydrophobic outer layer.

Hydrolyzable tannins are toxic to ruminants and microorganisms. The toxicity to ruminants occurs through the degradation by microbial metabolism and gastric digestion of the large molecules into smaller molecules, that can be absorbed through the gastroenteristial tract and cause hemorrhagic gastroenteritis, liver necrosis and kidney damage. This is known to occur e.g. when sheep and cattle feed on oaks.

In the plants, the tannins are located mainly in the vacuoles or surface wax, where they do not interfere with the plant’s own molecules for metabolism.    

SMILES (tannic acid): Oc1cc(cc(O)c1O)C(=O)Oc1cc(cc(O)c1O)C(=O)OC[C@H]1O[C@H](OC(=O)c2cc(O)c(O)c(OC(=O)c3cc(O)c(O)c(O)c3)c2)[C@H](OC(=O)c2cc(O)c(O)c(OC(=O)c3cc(O)c(O)c(O)c3)c2)[C@@H](OC(=O)c2cc(O)c(O)c(OC(=O)c3cc(O)c(O)c(O)c3)c2)[C@@H]1OC(=O)c1cc(O)c(O)c(OC(=O)c2cc(O)c(O)c(O)c2)c1

Further reading

  • Cornell University Department of Animal Science - Plants poisonous to livestock (accessed 20.05.2019, last updated 28.02.2019): Tannins: fascinating but sometimes dangerous molecules. https://poisonousplants.ansci.cornell.edu/toxicagents/tannin.html
  • Taiz, L., Zeiger, E., Møller, I.M., Murphy, A. (2015): Plant Physiology and Development. Oxford University Press. 6. ed. Appendix 4: Secondary metabolites. http://6e.plantphys.net/PlantPhys6e-appendix04.pdf
  • Reed, J.D. (1995): Nutrional toxicology of tannins and related polyphenols in forage legumes. Journal of Animal Science 73: 1516-1528.
  • Smeriglio, A., Barreca, D., Bellocco, E., Trombetta, D. (2017): Proanthrocyanidins and hydrolysable tannins: occurence, dietary intake and pharmacological effects. British Journal of Pharmacology 174: 1244-1262.