Chemistry Reference
In-Depth Information
Table 2 . 3.
Rate constants of Fenton-type reactions. For further details see Goldstein et
al. (1993) and Koppenol (1994)
k
/dm
3
mol
−
1
s
−
1
Couple
E°/V
pH
Fe
II/III
0.77
1
65
Fe(EDTA)
II/III
10
3
0.12
7
7
×
Fe(pyrophosphate)
II/III
10
5
7. 2
1
×
Fe(cyt.c)
II/III
0.27
7
2.5
Cu
I/II
10
3
0.16
1
4
×
Cu(OP)
I/II
10
3
0.17
7
2
×
Cr
II/III
10
4
−0.41
0
7
×
Ti
III/IV
0.1
0
540
V
II/III
−0.26
1
8.6
VO
2+
/VO
3+
1.0
0
∼
1
Ru(NH
3
)
II/III
0.067
7
∼
1.5
Co([14]aneN
4
)
II/III
0.44
1
10
3
4
×
et al. 1994). Under such conditions, dGuo is oxidized to 8-oxo-dGuo (Svoboda and
Harms-Ringdahl 2002). Complexation of Fe
2+
to the phosphate groups of DNA is
even more effective as the autoxidation of Fe
2+
is concerned (for the role of transi-
tion metal ions in the catalysis of the autoxidation of biomolecules, see Miller et al.
1990). EDTA and DPTA are often used to complex iron and prevent its interference
in autoxidation reactions, and there is an ongoing discussion as to the molecular
basis of the reasons why these ligands may act under certain conditions as an an-
tioxidant or a pro-oxidant (Engelmann et al. 2003).
Due to the ready reaction of complexed Fe(II) with O
2
and the generation of
•
OH in the course of this reaction, the sequence of the addition of the reagents
will be of a major importance especially when H
2
O
2
is used to induce a Fenton
reaction. In fact, when H
2
O
2
is added last and somewhat late, there may be no
Fe(II) left to induce the Fenton reaction proper. In many studies this potential
artifact has not been realized (e.g., Asaumi et al. 1996 as an example), and con-
clusion are potentially fraught with considerable errors.
When such reactions are allowed to proceed in the presence of 2
-deoxynucleo-
sides, different product ratios than generated by
ionizing radiation are observed
(Murata-Kamiya et al. 1998). This is likely to be due to the presence of both Fe(II)
and Fe(III) which may modify the
•
OH-induced reactions (Theruvathu et al. 2001b).
Such phenomena may be quite a general feature whenever Fe(II) [for rate constants
see Khaikin et al. (1996)] and/or another reductant (e.g., ascorbate) is involved.
′
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