Chemistry Reference
In-Depth Information
CO
•−
is capable of selectively oxidizing guanines in the self-complementary
oligonucleotide duplex d(AACgCgAATTCgCgTT), dissolved in air-
equilibrated aqueous buffer solution (Table 5.2). Oxidative guanine base
damage to dNA was demonstrated [60]. The oxidation of two common fluo-
rescent probes, dichlorodihydrofluorescein (dCFH
2
) and dihydrorhodamine
(dHR), and their oxidized forms (dHR and Rh) by
CO
•−
have also been
studied (Table 5.2) [61]. Oxidations of dCFH
2
and dHR were suggested to
occur via a two-step process (e.g., Eqs. 5.19, 5.20):
DCFH CO
+
•−
→
DCFH HCO
•
+
−
(5.19)
2
3
3
2
DCFH
•
→
DCF DCFH
+
.
(5.20)
2
The reactions of
CO
•−
with synthetic nitroxide antioxidants have been
studied [65]. The reactivity of
CO
•−
radicals was independent of the ring size
and side chain of cyclic nitroxides. The second-order rate constants were deter-
mined in the range of (2−6) × 10
8
/M/s at pH > 9.0. In consideration of the
basic side chain of nitroxide, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-
(aminomethyl)-proxyl, the second-order rate constant was independent of the
pH in the studied range from 9.1 to 11.7. The nitroxides were shown to be the
most effective metal-independent scavengers of
CO
•−
radicals [65].
5.1.2 Peroxymonocarbonate
Peroxymonocarbonate (
HCO
−
) has been a known oxidant for many years [21].
The equilibrium reaction of bicarbonate with hydrogen peroxide rapidly forms
HCO
−
ions at near neutral pH (
t
1/2
≈ 300 seconds) (Eq. 5.21) [22]:
−
−
HCO H O
+
HCO H O
+
k
=
0 33
.
.
(5.21)
3
2
2
4
2
21
A shift in equilibrium to the right was determined when more soluble
sources of bicarbonate (e.g., NH
4
HCO
3
) and an alcohol cosolvent were used
[22].
HCO
−
is a strong oxidant in aqueous solutions
(
−
−
E
°
(
HCO /HCO
)
=
1 8 V vs normal hydrogen electrode [NHE]
.
4
3
.
HCO
−
has been characterized by
vibrational spectroscopy and x-ray crystallography (KHCO
4
·H
2
O
2
) [21, 66]. It
can be classified as an anionic peracid having a structural formula of
HOOCO
−
[66]. during the decomposition of
HCO
−
, chemiluminescence was observed
[67-69].
1 8 V vs normal hydrogen electrode [NHE]
.
.
)
5.1.2.1 Reactivity.
The reactivities of
HCO
−
with organic compounds in
aqueous and nonaqueous solvents have been studied [22, 24, 70-72]. The
second-order rate constants for the oxidation of amines and thiols by H
2
O
2
and
HCO
−
are given in Table 5.3 [72-74]. generally,
HCO
−
reacts about twofold
faster than H
2
O
2
. A detailed study on the oxidation of sulfides by
HCO
−
pro-
posed a solvent-assisted S
N
2 mechanism. Water as a solvent accelerates the
oxygen transfer to the nucleophilic sulfide by assisting in the displacement of
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