Biology Reference
In-Depth Information
Liquidambin (
25
) would thus constitute a molecular quoin for the
passage from glucopyranosic to
C
-glycosidic ellagitannins. As alluded to
above, a fast, probably enzymatically-controlled, galloylation at the O-5
center of the opened form of pedunculagin (
23
) would be the driving
force towards this passage by maintaining open the glucose core while
reinforcing the electrophilic character and accessibility of its C-1 center
under the form of an sp
2
-aldehydic type carbon.
Interestingly, Tanaka and colleagues managed to chemically convert
pedunculagin (
23
) into casuariin (
24
, 6%) and 5-desgalloylstachyurin
(
26
, 34%) by simply heating a solution of
23
in a pH 7.5 phosphate
buffer (Tanaka
et al.
, 1993). This non-enzymatic transformation may
also take place in some plants, but others, like
Liquidambar formosana
,
seem to have, for some reasons, evolved to better handle access to
C
-
glycosidic ellagitannins. Such plants might be equipped with a “trained”
5-
O
-galloyltransferase in order to biochemically forge the open-chain
aldehydic liquidambin (
25
), whose enhanced electrophilicity at C-1
could then facilitate the chemical establishment of the
C
-glycosidic bond
via nucleophilic addition. Of course, these thoughts are only speculative,
but they could constitute the basis of working hypotheses for future
studies aimed at tracking down the enzymes that might control the
genesis of
C
-glycosidic ellagitannins.
In any event, once monomeric
C
-glycosidic ellagitannins such as the
two epimeric pairs vescalagin/castalagin (
1
/
2
) and stachyurin/casuarinin
(
3
/
4
) are available, they serve as precursors for numerous other
C
-
glycosidic species. For example, ellagitannins such as the roburins A-E
and grandinin (
5
-
10
, see Fig. 9.2), the castaneanins A-D oligomeric
series (Tanaka
et al.
, 1996) exemplified in Fig. 9.3 by the pentamer
castaneanin D (
11
) and the flavano-ellagitannins
12
-
17
(Fig. 9.4) all
derive from
1
and/or
2
or
3
and/or
4
. Furthermore, all of these
compounds derive from a single chemical reaction type, that is the
substitution of the OH group at C-1 of their precursor by another
(nucleophilic) entity, which can be their precursor itself as in the case of
an oligomerization [
e.g.
, roburins A/D (
5
/
6
) and castaneanins A-D] or a
sugar unit like
D
-lyxose or
D
-xylose as in the case of the formation of
grandinin (
7
) and roburin E (
8
) or a flavanol moiety like catechin or
procyanidin B-3 as in the case of the formation of complex tannins such
