The gallotannins 2,3,4,6-tetra-O-galloyl-D-glucopyranose (TGG) (2) and
1,2,3,4,6-penta-O-galloyl-b-D-glucopyranose (b-PGG) (3), found in many plant
families, are the major intermediates for the biosynthesis of hydrolysable tannins
(Gross 1993) (Hagenah and Gross 1993). Most of the galloyl units at the anomeric
centre of their D-glucosyl unit have the b configuration at the anomeric centre. But
in some cases like 1,4-di-O-galloyl-a-D-glucopyranose (4), the anomeric centre of
the D-glucopyranose has the a configuration (Khanbabaee and van Ree 2001).
Examples of some gallotannins are: tannic acid (Chinese gallotannin) (5), Turkish
gallotannin (6), acertannin (7), hamamelitannin (8) etc. (Fig. 21.1).
Tannic acid is a gallotannin with chemical formula given as C 76 H 52 O 46 . It
contains decagalloyl glucose (5) moiety and it is a commercially available tannin.
Tannic acid is found in the nutgalls formed by insects on twigs of certain oak trees
(Quercus infectoria and other Quercus species). While Chinese gallotannin contains
decagalloylglucose residue, Turkish gallotannins tends to have penta-, hexa-, or
heptagalloylglucose moiety.
21.3.2 Ellagitannins
Ellagitannins contains at least two galloyl units coupled to each other by C–C
bonds, but they do not form any catechin units. Ellagitannins are formed by the
first-stage biogenetic oxidative coupling of at least two galloyl units, yielding an
axially chiral hexahydroxydiphenoyl moiety or HHDP unit (9) (Khanbabaee and
Van Ree 2001). Remarkably, all ellagitannins, with HHDP units bound to the
polyol unit (basically, D-glucopyranose) in 2,3- or 4,6- or 1,6-positions, always
exert the (S)-configuration, while 2,4- or 3,6-coupled HHDP units show the (R)configuration generally (Feldman and Ensel 1994). Ellagitannins differ from gallotannins, in their galloyl groups linkage. Ellagitannins link their galloyl units
through C–C bonds, whereas gallotannins follows depside bond linkage (Radebe
et al. 2013). Some of the ellagitannins are chebulinic acid (10), corilagin (11),
geraniin (12), tellimagrandin II (13), casuarinin (14), potentillin (15) etc.
(Fig. 21.2).
21.3.3 Condensed Tannins
These tannins are derivatives of flavonoid (16), catechins (17), flavan-3–4-diol.
They do not contain sugar residues. Monomeric catechins and leukoanthocyanidins
(flavan-3–4-diol) (18) don’t have any tanning properties, but when they oligomerize
under acidic condition or enzymatic action, they can produce significant tanning
action (Khanbabaee and van Ree 2001) (Ferreira and Slade 2002). Condensed
tannins are the oligomeric and polymeric proanthocyanidins formed via linking
catechins one another in C-4 to C-6 or C-8 positions.
724
S. Mal and D. Pal
1,2,3,4,6-penta-O-galloyl-b-D-glucopyranose (b-PGG) (3), found in many plant
families, are the major intermediates for the biosynthesis of hydrolysable tannins
(Gross 1993) (Hagenah and Gross 1993). Most of the galloyl units at the anomeric
centre of their D-glucosyl unit have the b configuration at the anomeric centre. But
in some cases like 1,4-di-O-galloyl-a-D-glucopyranose (4), the anomeric centre of
the D-glucopyranose has the a configuration (Khanbabaee and van Ree 2001).
Examples of some gallotannins are: tannic acid (Chinese gallotannin) (5), Turkish
gallotannin (6), acertannin (7), hamamelitannin (8) etc. (Fig. 21.1).
Tannic acid is a gallotannin with chemical formula given as C 76 H 52 O 46 . It
contains decagalloyl glucose (5) moiety and it is a commercially available tannin.
Tannic acid is found in the nutgalls formed by insects on twigs of certain oak trees
(Quercus infectoria and other Quercus species). While Chinese gallotannin contains
decagalloylglucose residue, Turkish gallotannins tends to have penta-, hexa-, or
heptagalloylglucose moiety.
21.3.2 Ellagitannins
Ellagitannins contains at least two galloyl units coupled to each other by C–C
bonds, but they do not form any catechin units. Ellagitannins are formed by the
first-stage biogenetic oxidative coupling of at least two galloyl units, yielding an
axially chiral hexahydroxydiphenoyl moiety or HHDP unit (9) (Khanbabaee and
Van Ree 2001). Remarkably, all ellagitannins, with HHDP units bound to the
polyol unit (basically, D-glucopyranose) in 2,3- or 4,6- or 1,6-positions, always
exert the (S)-configuration, while 2,4- or 3,6-coupled HHDP units show the (R)configuration generally (Feldman and Ensel 1994). Ellagitannins differ from gallotannins, in their galloyl groups linkage. Ellagitannins link their galloyl units
through C–C bonds, whereas gallotannins follows depside bond linkage (Radebe
et al. 2013). Some of the ellagitannins are chebulinic acid (10), corilagin (11),
geraniin (12), tellimagrandin II (13), casuarinin (14), potentillin (15) etc.
(Fig. 21.2).
21.3.3 Condensed Tannins
These tannins are derivatives of flavonoid (16), catechins (17), flavan-3–4-diol.
They do not contain sugar residues. Monomeric catechins and leukoanthocyanidins
(flavan-3–4-diol) (18) don’t have any tanning properties, but when they oligomerize
under acidic condition or enzymatic action, they can produce significant tanning
action (Khanbabaee and van Ree 2001) (Ferreira and Slade 2002). Condensed
tannins are the oligomeric and polymeric proanthocyanidins formed via linking
catechins one another in C-4 to C-6 or C-8 positions.
724
S. Mal and D. Pal