Biology Reference
In-Depth Information
(HHDP) ester groups, which are produced by oxidative C-C coupling
between two galloyl groups (Haslam and Cai, 1994), are further
metabolized to form complex molecules via dehydrogenation or
intermolecular oxidative coupling with another galloyl group (Okuda,
2005). The products thus generated by these oxidative processes are
accumulated in high concentrations in some plants, such as
Euphorbia
,
Punica
and
Geranium
species (Okuda
et al.
, 1980, 2000]. The
ellagitannins found in these plant species feature
dehydrohexahydroxydiphenoyl (DHHDP) ester groups that exhibit very
interesting reactivity, hence contributing further to the structural
diversity of the ellagitannin natural products family.
4.2 Water Solubility and Hydrophobic Association of Ellagitannins
4.2.1 Structures of ellagitannins and their water solubility
The relation between the structures of hydrolyzable tannins and their
water solubility has been thoroughly examined (Tanaka
et al.
, 1997a).
The partition coefficients of a series of galloylated glucoses or
gallotannins between
n
-octanol and water are shown in Fig. 4.1
(
i.e.
, log P
vs
. number of galloyl groups). The water solubility of galloyl
glucoses is inversely correlated with the number of galloyl groups.
Pentagalloylglucose (PGG,
8
) was the most hydrophobic gallotannin
examined in this experiment. The increase of the number of galloyl
esters increases the global hydrophobic area of the molecule. However,
when two galloyl groups are connected via a biaryl bond, such as in
eugeniin (
9
, 1,2,3-tri-
O
-galloyl-4,6-(
S
)-HHDP-β-
D
-glucose, Nonaka
et
al.
, 1980), the water solubility is much higher than that of PGG
(
8
, see Fig. 4.2). It was then observed that the number of biaryl bonds in
each ellagitannin molecule is positively correlated with the water
solubility. Furthermore, sanguiin H-6 (
21
), an example of a dimeric
ellagitannin formed by intermolecular C-O coupling (Tanaka
et al.
,
1985), was also found much more hydrophilic than the monomeric
tannin. Overall, these results thus seem to indicate that the oxidative
metabolism of ellagitannins increases their water solubility.
