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Wednesday, January 30, 2013

Flavan-3-ols Are Also Called Catechins

The flavan-3-ols are also called catechins and they are found primarily in the grape seeds.1 They include (+)-catechin, (-)-epicatechin, (-)-epigallocatechin and (-)epicatechin gallate. Experiments have shown that there is an increase in catechin and epicatechin until veraison when those flavan-3-ols begin to decline. Because catechin and epicatechin have a bitter taste, the gradual ripening of seeds may be due to the decrease in the levels of catechin and epicatechin.
The structure of (+)-catechin differs from (-)-epicatechin only in their spatial arrangement of the atoms around the 2nd and 3rd carbon atoms in the pyran ring. I hope to illustrate that point with the following structural depictions.
The structures of (+)-catechin, (-)-epicatechin, and (-)-epigallocatechin shown below will consist of the following:
  • first as a bond-line structural representation
  • second as a 3D representation showing the relationship between the substituents on the second and third carbons
  • lastly as a space filling model
The structure of (-)-epicatechin is depicted below. The structure in the middle is a 3d representation and when compared with (+)-catechin above, shows the difference in the arrangement of the atoms connected to carbons 2 and 3.
(-)-Epigallocatechin shown below differs from (-)-epicatechin in having one more hydroxyl group (circled in red) attached to the phenyl ring.
In the case of (-)-epicatechin gallate, the stereochemistry around carbons 2 and 3 of the pyran ring is the same as in (-)-epicatechin and as in (-)-epigallocatechin, however, there is a gallic acid moiety that is esterified to carbon 3, circled in red as shown on the left.2
As mentioned above, catechins are important because they are contained in the seeds of the grape and decline after veraison. If bitter characters are detected in a wine, they may have come from these catechins. In addition, catechins are the monomeric units that can polymerize and lead to tannin formation, the subject of a future blog. Stay tuned!

References:
1. Douglas Adams, VI257, Lesson 8, pg. 50.
2. Jackson, Ronald S., Wine Science: Principles and Applications, Third Edition, Elsevier, Academic Press, 2008, pg. 283.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.

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