Chemistry Reference
In-Depth Information
Both HO
2
•
and O
2
•
−
react with ascorbate [reactions (91)−(94); Nishikimi 1975;
Cabelli and Bielski 1983].
A
•
−
+ H
2
O
2
+ H
+
AH
2
+ HO
2
•
→
(91)
AH
2
+ O
2
•
−
A
•
−
+ H
2
O
2
→
(92)
AH
−
+ HO
2
•
A
•
−
+ H
2
O
2
→
(93)
AH
−
+ O
2
•
−
→
product(s)
(94)
10
4
dm
3
mol
−
1
s
−
1
. Kinetically, reactions (92) and (93) cannot be separated (
k
93
+ 0.36
k
92
= 1.2
×
10
4
dm
3
mol
−
1
s
−
1
, and that for
k
94
is 5
The value for
k
91
is 1.4
×
×
10
7
dm
3
mol
−
1
s
−
1
).
The ascorbate radicals also react very rapidly with O
2
•
−
/HO
2
•
[reactions (95)
and (96),
k
95
= 5
10
9
dm
3
mol
−
1
s
−
1
,
k
96
= 2.6
10
8
dm
3
mol
−
1
s
−
1
].
×
×
A
•
−
+ HO
2
•
→
products
(95)
A
•
−
+ O
2
•
−
→
products
(96)
The ascorbate radical is one of the radicals that do not react readily with O
2
, but
it reacts with O
2
•
−
. The product of this reaction is not yet known. There are other
radicals that have similar properties such as phenoxyl-type radicals. A promi-
nent member of this group is the vitamin E radical. In the phenoxyl radical series,
addition as well as ET have been discussed (Jonsson et al. 1993; d'Alessandro et
al. 2000). The reaction of the tyrosyl radical with O
2
•
−
is an example showing that
addition is the main route despite of its relatively high redox potential [reactions
(97)−(99); only one pathway is shown; Jin et al. 1993].
The
N
-centered tryptophan (Fang et al. 1998) and bisbenzimidazole radicals
(Adhikary et al. 2000) also do not react with O
2
, but readily with O
2
•
−
, the former
by addition. In the nucleobase series, a similar situation prevails for G
•
(
k
(G
•
+
O
2
•
−
) = 4.4
10
9
dm
3
mol
−
1
s
−
1
; von Sonntag 1994). Most of the reaction seems to
occur by ET reforming G plus O
2
, but in competition about 15% to lead to prod-
ucts such as Iz and 8-oxo-G (Misiaszek et al. 2004).
The O
2
•
−
/HO
2
•
radicals can be trapped by spin traps, but the rate constants
are low and their lifetime is often only short (Lauricella et al. 2004), i.e., they
give rise to the
•
OH-adduct radical upon hydrolysis (Chap. 3.4). To increase the
×
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