Chemistry Reference
In-Depth Information
These days, quite common procedures use for the Udenfriend reaction H
2
O
2
as an
•
OH source and catalytic amounts of Fe(III) which are reduced by the ascorbate,
for example, 5
10
−3
mol dm
−3
substrate, 5
10
−6
mol dm
−3
Fe(III), 15
10
−6
mol
×
×
×
dm
−3
EDTA, 50
10
−6
mol dm
−3
ascorbate (Halli-
well and Kaur 1997). Here, one has to take into account that both Fe(II)EDTA and
ascorbate (cf. Huie et al. 1987; Neta et al. 1989) reduce intermediate peroxyl radi-
cals [reactions (79)−(82); cf. Yurkova et al. 1999; for the one-electron reduction of
hydroperoxides see Phulkar et al. 1990].
10
−6
mol dm
−3
H
2
O
2
and 50
×
×
•
OH + RH
→
R
•
+ H
2
O
(79)
(80)
R
•
+ O
2
→
RO
2
•
RO
2
•
+ reductant
→
RO
2
H
(81)
RO
2
H + Fe
2+
RO
•
+ OH
−
+ Fe
3+
→
(82)
Because of this apparent importance of ascorbate not only in artificial systems
(for the
•
OH-production by multivitamin tablets, see Maskos and Koppenol 1991)
but also in the living cell where a concentration of ca. 10
−2
mol dm
−3
is typically
maintained, it seems adequate to digress for a short moment on its chemistry.
Ascorbic acid [
(244nm) = 10,800 dm
3
mol
−1
cm
−1
] is an acid (p
K
a
= 4.1)
[equilibrium (83)]. Thus at around pH 7 the monoanion [
ε
(265nm) = 14,500 dm
3
ε
mol
−1
cm
−1
] predominates.
The ascorbate ion is a reasonably strong reductant [E
7
(ASC
•
−
, H
+
/ASCH
−
) =
0.282 V; equilibrium (84)]. In the presence of its oxidized form, dehydroascorbic
acid [DHA,
(300nm) = 720 dm
3
mol
−
1
cm
−
1
], an equilibrium concentration of
ε
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