338
RADICAL REACTIONS
Box 9.2 (continued)
OH
HO
OH
resveratrol
(stilbene)
O
OH
HO
OH
OH
O
OH
O
OH
OH
OH
epigallocatechin gallate
Quercetin, in particular, is almost always present in substantial amounts in plant tissues, and is a powerful
antioxidant, chelating metals, scavenging radicals and preventing oxidation of low-density lipoprotein. Flavonoids
in red wine (quercetin, kaempferol, and anthocyanidins) and in tea (catechins and catechin gallate esters) are also
demonstrated to be effective antioxidants. A particularly efficient agent in green tea is epigallocatechin gallate.
Resveratrol is another type of polyphenol, a stilbene derivative, that has assumed greater relevance in recent
years as a constituent of grapes and wine, as well as other food products, with antioxidant, anti-inflammatory,
anti-platelet, and cancer preventative properties. Coupled with the cardiovascular benefits of moderate amounts of
alcohol, and the beneficial antioxidant effects of flavonoids, red wine has now emerged as an unlikely but most
acceptable medicinal agent.
Vitamin C (ascorbic acid) is also a well-known antioxidant. It can readily lose a hydrogen atom from one of
its enolic hydroxyls, leading to a resonance-stabilized radical. Vitamin C is acidic (hence ascorbic acid) because
loss of a proton from the same hydroxyl leads to a resonance-stabilized anion (see Box 12.8). However, it appears
that vitamin C does not act as an antioxidant in quite the same way as the other compounds mentioned above.
O
HO
OH
O
O
O
O
HO
OH
O
OH
O
O
HO
OH
O
OH
HO
– H
vitamin C
(L-ascorbic acid)
HO
O
O
O
α-tocopherol
H
O
HO
OH
O
O
O
vitamin C (oxidized form)
tocopheryloxyl radical
resonance-stabilized radical
regeneration of vitamin E
from phenoxyl radical
The main function of vitamin C as a radical producer is to provide a regenerating system for tocopherol (see
above). Thus, tocopheryloxyl radicals are able to remove hydrogen atoms from vitamin C to regenerate functioning
molecules of tocopherol. A tocopheryloxyl radical may well be the agent that removes the first hydrogen atom
from vitamin C. A second such radical can then abstract a further hydrogen atom and produce the oxidized
tricarbonyl form of vitamin C.
RADICAL REACTIONS
Box 9.2 (continued)
OH
HO
OH
resveratrol
(stilbene)
O
OH
HO
OH
OH
O
OH
O
OH
OH
OH
epigallocatechin gallate
Quercetin, in particular, is almost always present in substantial amounts in plant tissues, and is a powerful
antioxidant, chelating metals, scavenging radicals and preventing oxidation of low-density lipoprotein. Flavonoids
in red wine (quercetin, kaempferol, and anthocyanidins) and in tea (catechins and catechin gallate esters) are also
demonstrated to be effective antioxidants. A particularly efficient agent in green tea is epigallocatechin gallate.
Resveratrol is another type of polyphenol, a stilbene derivative, that has assumed greater relevance in recent
years as a constituent of grapes and wine, as well as other food products, with antioxidant, anti-inflammatory,
anti-platelet, and cancer preventative properties. Coupled with the cardiovascular benefits of moderate amounts of
alcohol, and the beneficial antioxidant effects of flavonoids, red wine has now emerged as an unlikely but most
acceptable medicinal agent.
Vitamin C (ascorbic acid) is also a well-known antioxidant. It can readily lose a hydrogen atom from one of
its enolic hydroxyls, leading to a resonance-stabilized radical. Vitamin C is acidic (hence ascorbic acid) because
loss of a proton from the same hydroxyl leads to a resonance-stabilized anion (see Box 12.8). However, it appears
that vitamin C does not act as an antioxidant in quite the same way as the other compounds mentioned above.
O
HO
OH
O
O
O
O
HO
OH
O
OH
O
O
HO
OH
O
OH
HO
– H
vitamin C
(L-ascorbic acid)
HO
O
O
O
α-tocopherol
H
O
HO
OH
O
O
O
vitamin C (oxidized form)
tocopheryloxyl radical
resonance-stabilized radical
regeneration of vitamin E
from phenoxyl radical
The main function of vitamin C as a radical producer is to provide a regenerating system for tocopherol (see
above). Thus, tocopheryloxyl radicals are able to remove hydrogen atoms from vitamin C to regenerate functioning
molecules of tocopherol. A tocopheryloxyl radical may well be the agent that removes the first hydrogen atom
from vitamin C. A second such radical can then abstract a further hydrogen atom and produce the oxidized
tricarbonyl form of vitamin C.