Chemistry Reference
In-Depth Information
2.5.2
Copper
There are a number of transition-metal ions in low oxidation states such as Cu(I)
which readily react with H
2
O
2
. In the case of Cu(I), the resulting intermediate
has also a very strong oxidative power, e.g., is capable of abstracting an H atom
from MeOH. At high MeOH concentration the chain length of this reaction de-
pends on the MeOH concentration, and it has thus been concluded that an oxi-
dant must be present which has not the properties of free
•
OH (Johnson et al.
1985, 1988). Evidence has been given that the intermediate, (H
2
O)
m
Cu
+
O
2
H
−
can decompose in acid solution into Cu
2+
and
•
OH, but at sufficiently high H-
donor concentration also react with the latter (Masarwa et al. 1988).
In the reduction of Cu
2+
to Cu
+
, Cl
−
may play an important role, because they
considerably stabilize the Cu
+
species (Gilbert et al. 1997). GSH is the major cel-
lular free thiol (close to 10
−2
mol dm
−3
). It readily reduces Cu(II) to Cu(I)
−
GSH
complexes. These react with hydroperoxides (Gilbert and Silvester 1997; Gilbert
et al. 1999). At low concentrations where these complexes are monomeric, the re-
action with H
2
O
2
gives rise to
•
OH [reaction (92)], but higher concentrations the
monomeric complexes aggregate [reaction (93)] and this aggregate undergoes
a two-electron oxidation [reaction (94)]. In contrast,
tert
-butylhydroperoxide
undergoes one-electron oxidation also with the oligomeric aggregates. This has
been rationalized by pointing out that at pH 7 the one- and two-electron poten-
tials are 0.46 and 1.32 V for H
2
O
2
and 1.9 and 1.7 for
tert
-butylhydroperoxide,
respectively. Thus a two-electron step is thermodynamically preferred for H
2
O
2
,
whereas the reverse is true for
tert
-butylhydroperoxide.
−
The above reactions may play a role in copper-mediated DNA damage, and the
effect of the
trans
-resveratrol, a naturally occurring phenolic antioxidant, has
been studied in this context (Burkitt and Duncan 2000).
Albumine
Cu
2+
complexes are similarly reduced by thiols and then react
with H
2
O
2
(Ozawa et al. 1993). The
•
OH radical thus formed has been detected
by spin trapping.
DNA strand breakage is caused by EDTA-Cu
2+
in the presence of a thiol
(Mukherjee and Chatterjee 1995). In this study it has, however, also been re-
ported that Cu
2+
(in the presence of H
2
O
2
and Cl
−
) nick DNA in the absence
of a deliberately added reductant. The latter observation has been corroborated
by Yamamoto and Kawanishi (1989), and it has been suggested that H
2
O
2
can
serve as a reductant under these conditions. The pattern of DNA fragments in
−
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