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as an antioxidant and efficiently quenches
singlet oxygen. The chemical structure of
these molecules as well as the range of
their k
Q
values (1-2 × 10
8
mol.l
−1
.s
−1
) sug-
gest that the mechanism of singlet oxygen
suppression of this class of molecules is
similar to that observed for flavonoids, i.e.
due to a reversible electron transfer reac-
tion. Both a-tocopherol and b-carotene are
hydrophobic molecules with a high ten-
dency to localize in membranes. b-Carotene
is, however, a much more efficient
1
O
2
suppressor, suggesting that it should be
more efficient in protecting membranes
from damage initiated by
1
O
2
. In fact,
Stratton and Liebler have shown that, in
concentration conditions similar to those
found physiologically, b-carotene was
highly effective in protecting against for-
mation of oxidation products of membranes
and a-tocopherol was ineffective (Stratton
and Liebler, 1997).
Several other groups of molecules are
also known to suppress
1
O
2
with high effi-
ciency so that they could be considered as
antioxidant owing to their
1
O
2
suppression
abilities. Examples can include ascorbic
acid, histidine and catechins (Table 6.2).
Betanidines, found in high concentration in
beetroot, also seem to hold promising prop-
erties to protect against damage resulting
from
1
O
2
(Bonacin
et al
., 2009).
Carotenoids, which suppress
1
O
2
through triplet-triplet energy transfer,
present k
Q
values that are around two
orders of magnitude larger than those
observed for the other groups of molecules
cited in Table 6.2. However, it does not
mean that one should disregard the
1
O
2
suppressor abilities of flavonoid deriva-
tives and catechins, because these mol-
ecules may be present in different
concentrations and they certainly have dif-
ferent cellular and extracellular localiza-
tion domains. In fact, aqueous extracts of
plants, namely
Andrographis paniculata
and
Swertia chirata
, significantly protect
against oxidative damage induced by vari-
ous oxidants including
1
O
2
(Tripathi
et al
.,
2007). The difference in polarities among
these groups of molecules suggests possi-
ble synergistic roles of carotenoids and
flavonoids. Carotenoids are extremely
lipophilic (logP >15) and should either
work inside the structure of a membrane
protein or inside the membrane itself,
whereas most of the flavonoids are
hydrophilic and should work in aqueous
interfaces or in aqueous solutions.
6.8
Conclusions
Singlet oxygen plays important roles in
photo-induced damage in animals and
plants, causing damage to human skin and
decreasing crop yields.
1
O
2
is particularly
generated by the absorption of UVA-VIS
photons by naturally occurring photosen-
sitizers, whose triplets react with molecu-
lar oxygen. The reactivity of
1
O
2
with
electron-rich double bonds allows it to
react with several biomolecules, changing
their chemical structure and altering their
functions. In terms of the effects in mem-
branes, formation of lipid hydroperoxide is
the first step in lipid peroxidation that can
progress to chain break and loss of mem-
brane integrity. Nature has developed a
series of chemicals that protect biomol-
ecules from the damage caused by
1
O
2
. The
main quenchers found in nature suppress
1
O
2
by physical mechanisms. The fact that
all these molecules are well known anti-
oxidant agents and that they have high effi-
ciency in quenching
1
O
2
indicates that an
important part of their antioxidant activity
is due to the suppression of
1
O
2
. Carotenoids
are the most efficient
1
O
2
suppressors and
the mechanism of suppression is by
triplet-triplet energy transfer. Their main
site of localization is hydrophobic envi-
ronments, contrary to flavonoids and
catechins, which are also efficient
1
O
2
sup-
pressors, whose main quenching mecha-
nism is due to reversible electron transfer
reactions. The different environments and
suppressor mechanisms of these molecules
suggest a possible synergistic action of
carotenoids, flavonoids and catechins in
suppressing
1
O
2
and helping cells to keep
homeostasis under conditions of redox
misbalance.
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