Biology Reference
In-Depth Information
photosensitizer pigments. Plants, on the
other hand, make use of their secondary
metabolites in an appropriate and necessary
way to suppress
1
O
2
and other oxidizer
agents that could lead to destruction of bio-
logical systems.
Plants metabolize countless antioxi-
dants, such as flavonoids, anthocyanins,
ascorbic acid, carotenoids and vitamin A.
The antioxidants can be found in every vital
structure of plants, and they are part of their
defence mechanisms. Many of these sub-
stances such as lycopene and those made by
the shikimic acid pathway, are not biosyn-
thesized in animals, but have important roles
in the life of humans. The challenge now
would be to understand how these chemical
structures that have been known to exert dif-
ferent roles in human health could be play-
ing their role by deactivating singlet oxygen.
It is important to mention that singlet
oxygen is also used to synthesize natural
products. One example of the action of
1
O
2
in synthesis of natural products is the syn-
thesis of ascaridol (Fig. 6.13a), which is a
component of chenopodium oil (Schenck,
1954). Schenck and Ziegler, who carried out
this synthesis in 1941, expressed the view
that ascaridol may be generated in the plant
by a photosensitized reaction involving
chlorophyll, rather than by an enzymatic
process. Another example is in germacrano-
lides: costumonolide undergoes photooxy-
genation to obtain peroxycostumonolide
using methylene blue as photosensitizer
(Fig. 6.13b) (El-Feraly, 1977).
6.6
Carotenoids
Carotenoids are molecules constituted of
polyene conjugated systems and are shown
to be involved in the prevention of several
human diseases, such as cancer, coronary
heart disease, age-related eye retina macular
degeneration and eye lens cataracts (Foote
and Denny, 1968; Di Mascio
et al
., 1989;
Conn
et al
., 1991; Trekli
et al
., 2003). The
mechanism of disease prevention seems to
be primarily related to the prevention of
peroxidation processes. The ability of sev-
eral carotenoids to quench singlet oxygen
has been studied, and their general antioxi-
dant activities have also been extensively
studied (Gust
et al
., 1993; Edge
et al
., 1997;
Frank, 2003). As mentioned above, the
quenching of
1
O
2
by carotenoids is mainly
due to an efficient energy transfer through
transition
states
process
(triplet-triplet
h
v
, O
2
Chlorophyll
(a)
Ethanol
Ascaridol
OOH
R
R
h
v
, O
2
(b)
Methylene blue
Costunolide
Peroxycostunolide
Fig. 6.13.
Two examples of natural oxidative processes via singlet oxygen in plants.
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