Biology Reference
In-Depth Information
the availability of hydrolyzable tannins required as references or
substrates because none of these delicate chemicals was, and still is,
commercially available (Gross, 2007).
3.2 Principal Structures and Definitions
Before discussing the biochemical events described in the subsequent
sections, it appears appropriate to insert a few comments on the
structural principles and conventional terminology in this field.
According to the classical definition formulated by Freudenberg (1920),
plant tannins are typically divided into condensed tannins and
hydrolyzable tannins. The first class, nowadays often referred to as
proanthocyanidins due to the liberation of colored anthocyanidins upon
treatment with alcoholic mineral acid (Scalbert, 1991), is of flavonoid
origin and is no subject of this article.
The second class, hydrolyzable tannins, is best described as
derivatives of gallic acid (3,4,5-trihydroxybenzoic acid,
1
; see Fig. 3.1),
this principal phenolic unit being esterified with the hydroxyl groups of a
central polyol moiety (usually β-
D
-glucopyranose). The simplest among
these esters, β-glucogallin (1-
O
-galloyl-β-
D
-glucopyranose,
2
), is known
as a natural product for more than a century. The fully acylated analogue,
1,2,3,4,6-penta-
O
-galloyl-β-
D
-glucopyranose (
3
), is regarded as the
immediate precursor of the two subclasses of hydrolyzable tannins,
i.e.
,
gallotannins and ellagitannins.
The latter,
i.e.
, ellagitannins, are the result of oxidative processes that
act on pentagalloylglucopyranose (PGG,
3
) to form linkages between
spatially adjacent galloyl residues, affording compounds like
tellimagrandin II (
4
), a simple monomeric ellagitannin. After eventual
hydrolytic release from the tannin core, the resulting 3,4,5,3',4',5'-
hexahydroxydiphenic (HHDP) acid (
5
) rearranges spontaneously to the
stable, rather insoluble dilactone, ellagic acid (
6
). It is interesting to note
that this artifact became name-giving for the whole group of natural
products. The second subclass, gallotannins, is characterized by
additional galloyl residues that are linked to the basic
pentagalloylglucose (
3
) core to afford
meta
-digalloyl residues (
7
) that
