Biology Reference
In-Depth Information
old concept of tannins. They were mainly used for leathering and
staining, although some plants containing them have been used as
traditional medicines. Phlorotannins are highly unstable oligomers of
phloroglucinol (
i.e.
, 1,3,5-trihydroxybenzene) produced by algae that
have never been isolated without being first converted into their methyl
or acetyl derivatives, and as such they constitute a third but rather
peculiar group of tannins.
As for ellagitannins, although some members of this class of
hydrolyzable tannins were obtained early on, it is the isolation and
structural determination of over 500 pure compounds since 1975 from
various plants, many of which used in traditional medicines, that brought
remarkable changes in the definition and concept of “tannins” (Haslam,
1989, Okuda, 1995, 1999a, 2005, Okuda
et al.
, 1990, 1991, 1992a,
1993a, 1995, 2000, Quideau and Feldman, 1996).
1.1.1 About the chemical stability of ellagitannins
Geraniin, from
Geranium thunbergii
, which is one of the most popular
medicinal plants used in Japan, was isolated as crystals, thus allowing its
precise chemical analyses (Okuda
et al.
, 1982a) and X-ray
crystallography (Luger
et al.
, 1998). The purified crystalline geraniin
was surprisingly found to give almost no astringent taste, while its
capacity to bind to hemoglobin and basic compounds, as evaluated by the
relative astringency (RA) and relative affinity to methylene blue (RMB)
index values (see Section 1.6.2), are comparable to those of other main
tannins (Okuda
et al.
, 1985). The biological and pharmacological
activities, successively found for geraniin and other purified
ellagitannins, were remarkably different from those of the “tannins”
vaguely imagined in the past. The biogenetic sequences of these newly
found tannins allowed propositions about the chemical and biological
correlations among hydrolyzable tannins produced in nature. The old
concept of “tannins”, which merely meant mixtures of hardly identifiable
and unstable phenolics, has now been replaced by a new concept through
which tannins, particularly ellagitannins, can be considered in a way
similar to that of other types of natural organic products, such as
terpenoids and alkaloids.
Unlike the “tannins” of the old concept, these
