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What's
in a name?
Flavonoids are an
important class
of phenolic compounds; many of them are dietary antioxidants and
constituents of medicinal herbs. This month's pages will examine the
antioxidant flavonoids in detail; but first let's look at an overview of
phenolic phytochemicals to see how flavonoids fit in.
One thing that all of the different
phenolic compounds have in common is that their molecular structure includes what chemists call a phenol
(feen'-uhl), a ring of six carbon atoms with at least one -OH group
attached. Sometimes you will see "phenyl" (fen'-uhl) as part of a chemical
name; this also means the compound contains a phenolic ring. (However, not
all molecules with a six-carbon ring are classified as phenolic compounds;
for instance, phenylalanine is an amino acid).
Polyphenols
Some authors refer to this large class
of phytochemicals as 'polyphenols' (pah-lee-fee'-nallz), which is short
for "polyhydroxy
phenols." Polyhydroxy (pah-lee-hy-drahk'-see) means the characteristic six-carbon
ring has more than one ("poly-") of those -OH groups (also known as
"hydroxyl groups") attached to it. The vast majority of phenolic compounds
found in medicinal herbs and foods can accurately be called polyphenols.
Classifying
phenolic compounds
Below are the classes of phenolic
compounds that are important to herbalists. You can see that 'Flavonoids' is
one of twelve major classes according to this scheme. It also happens to be the
largest single class as far as total numbers of known compounds. About 2/3
of the polyphenols we obtain in our diets are flavonoids. Ten different
types of flavonoids are listed:
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There
are thousands of different phenolics
Together, these twelve groups represent
more than 8,000 different phytochemicals; about half of these are
flavonoids. Although the individual
compounds have widely varying physiological effects, they are related in
various ways by similarities in their chemical structures and biosynthetic
origins (see intermediate and advanced pages for structures and
relationships).
Phenolics can
occur in the form of glycosides
Many of these molecules, including many
of the
flavonoids, can occur
in plants in the form of glycosides (gly'-kuh-sidez), which means that they
have one or more simple sugars, such as glucose or galactose, attached to
them. "Glyco" comes from the Greek word for "sweet." The non-sugar part of
the molecule is called an aglycone (uh-gly'-kone), "a-" meaning
"without." The name of a molecule
sometimes indicates if it is a glycoside; for example, an anthocyanidin
+ a sugar = an anthocyanin. One type of
anthocyanidin is "Delphinidin,"
which is the aglycone of "Delphinin," the
glycoside. (Other kinds of molecules besides phenolics can occur as
glycosides too).
Variations on a theme
Not only are there thousands of phenolic compounds
known, but some of the smaller molecules can polymerize (pah-limuh'-rize), or
join together to form larger and more complex molecules. Different kinds of
molecules might be found in one polymer. For example, lignins are large
polymers of hydroxycinnamic acid "subunits"; proanthocyanidins are polymers of
flavanol subunits. Phenolic compounds can also be bound to certain
proteins, polysaccharides, or lipids. And if this isn't complex enough (!),
some molecules can have multiple isomers (ise'-uh-merz), forms where their
atoms are arranged slightly differently.
Synergistic
effects and undiscovered compounds
Even though chemists have elucidated the structures
of thousands of polyphenols, there are still many compounds in botanical
medicines that have not yet been fully
characterized. Many of these unknown molecules could have important
medicinal effects. It has been observed, for example, that certain
whole polyphenolic extracts have a greater antioxidant effect than any of their
known individual components. This could be due to both synergistic effects
and to the actions of unidentified compounds. Moreover, the phenolic composition of an
extract can change over time: aged red wine, for instance, has greater
antioxidant power than new red wine. Phytochemists believe that this may be
because of "complexation," the formation of new complex molecules as the
wine ages over time.
One can certainly make a case for the use of whole
foods, juices, herbs and full-spectrum extracts when considering the
probable synergistic effects among the many known and yet-to-be discovered
polyphenols. It is also useful to remember that fruits and vegetables (and
whole grains and legumes as well) are important dietary sources of
polyphenols. Within these sources, a rich interplay among the phenolic
molecules contributes to their established health-promoting effects.
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