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Flavonoid Antioxidants
For further information, see
Phenolics Advanced
page |
Flavonoids, the most abundant
polyphenols in the diet, can be classified
into ten groups based on differences in their chemical structures. The
intermediate page will use chemical structures this month, because it's hard to understand
the differences between flavonoids without seeing a picture. I've tried to explain the
"hieroglyphs" in the text accompanying them for herbalists who haven't
studied organic chemistry. See the glossary for
definitions as needed.
On the left below is the basic "flavan nucleus," the foundation structure upon
which flavonoids are constructed. If you look at all the flavonoid
structures, you will see that they have this pattern, or a variation of it,
in common. |
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In this map of a chemical structure, the point of each angle represents a
carbon atom. The lines between the points show chemical bonds between
adjacent atoms. The 'A' ring and the 'B' ring are made of six carbon atoms
each which are bonded together to form a special structure known as an aromatic ring.
The numbers next to each point are called "positions" on this
structure. At each position is a carbon atom where specific small groups of
atoms called
functional groups may attach. The A ring and the B ring are
attached to each other by a "three-carbon bridge" (shaded area). This
bent bridge, along with
an oxygen atom, makes up the 'C' ring. |
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The
different classes of flavonoid structures are distinguished by fairly minor variations on this
pattern. Below are the basic structures of eight of the different classes. Within
each of these classes, there are many further variations on the theme. Some
examples are given after the basic skeletons of each class. |
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Anthocyanidins and Anthocyanins: These molecules
are very similar to the flavan nucleus above. One difference is that
the oxygen atom has a positive charge on it; there are also two
double bonds in the C ring. There are many different kinds
of anthocyanidins and anthocyanins,
varying in the number and position of -OH groups, sugar groups, and other
functional groups attached. Some are quite
complicated, with parts of other flavonoid molecules attached to them. This
class of flavonoids contains the pigments that give certain fruits, vegetables
and herbs their dark red, blue, and purple colors. Many of them are
antioxidants. |
 Anthocyanidin
(aglycone). This kind of molecule lacks any
attached sugars. Notice the -OH group at position 3. Various anthocyanidins
have -OH groups at other positions on both the 'A' and 'B' rings. |
 Anthocyanin
(glycoside). In this skeleton, the -OH group at
position 3 has been replaced by one (or more) sugar molecules. In some
anthocyanins, there are additional sugar groups attached at other positions as well. |
 This is a
particular type of anthocyanidin aglycone known as Cyanidin. It's
composed of the basic anthocyanidin skeleton with four more -OH groups
attached at positions 5, 7, 4' and 5'. Cyanidin is found in Grapes,
Bilberry, Blueberries, Black Cherry, Cocoa powder, and many other medicinal
herbs and foods. |
 This is the
glycoside of cyanidin, known as Cyanin. It differs from
Cyanidin by having glucose
molecules replace the -OH groups at positions 3 and 5. It is found in Elderberry and many
of the same plants that contain cyanidin. |
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Proanthocyanidins: This group of important antioxidants contains
polymers made from multiple anthocyanidin-like molecules, known as
flavanols (see below). They are called proanthocyanidins because, if broken
apart with acid treatment, proanthocyanidins yield anthocyanidins such as
Cyanidin. Proanthocyanidin polymers consisting of
two to ten or more subunits have been identified.
Oligomeric proanthocyanidins (OPCs) are the water-soluble, short-chain
polymers. Proanthocyanidins are sometimes
referred to as "condensed tannins" and are responsible for
astringency in many foods and medicinal herbs. Red wine contains many complex
proanthocyanidins (extracted from grape skins and seeds); so do blueberries,
blackberries, strawberries, elderberries, and other red/blue/purple colored
plant parts. |
 This
is a proanthocyanidin composed of three linked subunits. The shaded area
represents one subunit, which is a flavanol known as Catechin (see
below). This proanthocyanidin is called Procyanidin C2.
(Why are some bonds represented as Wedges?). |
 Here is
Procyanidin B5, consisting of two subunits linked 'sideways' compared to
the architecture of Procyanidin C2.Some of the more complex
proanthocyanidins contain subunits linked in both ways. Some phytochemists believe that the larger, yet-to-be-identified molecules in
complex substances such as aged red wine could have fifty or more of these linked
subunits. |
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Flavanols: Let's look at a particular type of flavanol known as
a flavan-3-ol, which has an -OH group attached to the 3 position of
the basic flavan skeleton. The "-ol" ending comes from the word "alcohol"
which generally means "an organic molecule with an -OH group attached
(i.e., ethanol)." Flavan-3-ols are
the subunits of proanthocyanidins. Their structures are very similar to
those of anthocyanidins, except that there is no positive charge on the
oxygen atom and no double bonds in the C ring. |
 Here is the basic flavan-3-ol
skeleton. It's just a flavan nucleus with an -OH group attached to position
3 of the C ring. |
 This is
Catechin, a common flavan-3-ol that occurs in many plants. It's found in
Green tea, Cocoa powder, Red wine, Hawthorn, Bilberry, Motherwort, and other
herbs. It is also a common subunit of proanthocyanidin polymers such as
Procyanidin C2 above. Epicatechin is another common example; it
differs from Catechin only in the spatial orientation of its -OH
group. |
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Flavonols: Notice this word is
spelled with an "o" instead of with an "a" as in "flavanols".
This means that the molecule has a double-bonded oxygen atom attached to
position 4. They're still "-ols" because they retain the -OH group at
position 3 like the flavanols; but they
also have a double-bonded oxygen atom, which makes them like another class
of flavonoids known as "flavones" (see below). |
 This is the basic flavonol skeleton, with the -OH at position 3 and the =O at
position 4. It also differs from flavanols by
having a double bond between carbons 2 and 3 on the C ring. |
 Here's
the common flavonol, Quercetin. It's the most abundant flavonol
in the diet and is found in hundreds of herbs and foods. Onions are
especially rich in Quercetin. It has proven
antioxidant effects. |
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Flavones: Flavones are like flavonols, without the "-ol." In
other words, there is no longer an -OH group at position 3 on the central
ring. |
 Here's
the basic flavone skeleton, with the =O at position 4 and the double bond
between carbons 2 and 3. |
 This is
Apigenin, a flavone with -OH groups added to positions 5, 7, and 4'.
It's another very common flavonoid, appearing in many medicinal plants
and foods such as celery. Another flavone is luteolin, found in sweet red
peppers. |
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Flavanone: Take away the double bond between carbons 2 and 3 of
the flavone structure, and you have a flavanone. Notice the "o" has changed
back to an "a," which indicates that the flavanones
have a single bond between carbons 2 and 3, like the basic flavan
nucleus at the top of this page. |
 The
basic flavanone skeleton retains the =O, which makes it an "-one."
Many flavanones occur as glycosides; for example, hesperitin (aglycone) and
hesperidin (glycoside) occur in citrus along with naringenin (--->) |
 Naringenin, an antioxidant flavanone from citrus species, has -OH groups
attached at positions 5, 7, and 4'. Studies have indicated that it has
anti-inflammatory, anti-cancer, and liver protective effects. |
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Isoflavones: Isoflavones (also known as
isoflavonoids) are very similar to flavones, except the B ring is attached
to position 3 of the C ring, rather than to position 2 as in the flavones:
 To the left
is the basic flavone skeleton, with the B ring attached to position 2
of the central ring. On the right is the isoflavone skeleton,
which is exactly the same as the flavone skeleton but with the B ring
attached to position 3. "Iso" is short for "isomer." |
 Here is the
isoflavone Genistein, found in Red clover, Alfalfa, Peas, Soy & other legumes. It
consists of the basic isoflavone skeleton with -OH groups
attached at positions 5, 7, and 4'. Genistein is protective against
breast, prostate, and colon cancers and can help with hot flashes and
osteoporosis prevention. |
 This isoflavone,
Daidzein, is very similar to Genistein, only lacking the -OH
group at position 5. It's found in the same kinds of plants as Genistein
and acts in much the same way. Both of these isoflavones are anti-inflammatory
and show cardioprotective and mild antioxidant activities. |
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For in-depth information on flavonoids in berries, see this
interesting
doctoral dissertation from Finland. |
