Environmental Engineering Reference
In-Depth Information
•
-
and ascorbate (AscH
-
) can reduce the Fe
3
+
-chelate to form Fe
2
+
-
chelate (Eq.
6.1
), which subsequently reacts with H
2
O
2
to form HO
where O
2
•
in the Fenton
•
reaction (Eq.
6.2
). HO
then induces the oxidation of biomolecules (Eq.
6.3
) in
biological systems.
The presence of metals that can act as catalysts in biological systems is
caused by the fact that many biomolecules bind transition metals, especially pro-
tein moieties containing oxygen, nitrogen or sulfur atoms. The transition metals
are coordinated with biomolecules through the
d
-orbitals that can also permit the
simultaneous binding of a biomolecule and dioxygen, thus providing a bridge
between O
2
and the biomolecule (Miller et al.
1990
; Buettner and Jurkiewicz
1996
; Khan and Martell
1967
; Valentine
1973
). The major free radicals with
the highest reduction potential are HO
•
•
•
•
, RO
, LOO
, GS
, urate, and even the
•
tocopheroxyl radical (TO
). Ascorbate itself acts as an antioxidant by replacing a
potentially very damaging radical (X
•
), through the following reaction (Buettner
and Jurkiewicz
1996
; Buettner
1993
,
1988
):
(6.4)
ASCH
•
+
X
•
→
ASC
•−
+
XH
•
-
is the ascorbate radical, which has low reduction potential and does
not give additional reaction with O
2
to form further oxidizing species. The kinet-
ics of these electron/hydrogen atom transfer reactions are very fast, as is observed
for the equilibrium mixture of AscH
2
/AscH
−
/Asc
2
−
at pH 7.4 which has been
mentioned in earlier section (Buettner and Jurkiewicz
1996
; Buettner
1988
; Ross
et al.
1994
). Therefore, ascorbate is an excellent antioxidant from both a thermo-
dynamic and kinetic point of view, but Asc
2
−
or Asc
where Asc
•
-
can produce low levels of
•
-
by superoxide dismutase can prevent further free
radical oxidation in biological systems (Buettner and Jurkiewicz
1996
; Williams
and Yandell
1982
; Scarpa et al.
1983
; Winterbourn
1993
).
One of the most important impacts of harmful solar UV radiation in biologi-
cal systems is the skin cancer, which is generally induced by the photoinduced
production of free radicals. Formation of
1
O
2
within cell membranes is caused by
the photodynamic action of the photosensitizers photofrin and merocyanine 540
(Buettner and Jurkiewicz
1996
). The
1
O
2
reacts with the membrane lipids to form
lipid hydroperoxides (LOOH). Skin is a significant iron excretion site (Green et
al.
1968
), and Fe(II) can react with LOOH to form highly oxidizing lipid alkoxyl
radicals (LO
superoxide. The removal of O
2
•
) by a Fenton-like reaction (Eq.
6.5
):
Fe
2
+
+ LOOH
→
Fe
3
+
+ LO
•
+
OH
−
(6.5)
Applications of iron chelators to the skin can give photoprotection in the case
of chronic exposure to UV radiation, by reducing the formation of free radicals
(Bissett et al. 1991).
Second, another most important impact of HO
•
is the declining of plants by
•
the effect of HO
, which is formed in dew waters of plants by several sources
such as NO
2
-
, NO
3
-
and H
2
O
2
+
Fe
+
oxalate system present in the plants
