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possibility of reduction of Cr(VI) by catechol moieties in catecholamines
in vivo
. Furthermore, generated organic radicals may participate in further
radical reactions.
6.1.4.7 Thiols.
The reactivity of CrOO
2+
with thiols (L-Cys, GSH, and DL-
pencillamine) has been studied in aqueous perchloric acid [93]. The oxidation
of thiols by CrOO
2+
resulted in sulfinic and sulfonic derivatives. A proposed
mechanism involves the participation of the CrOO
2+
/thiol complex, CrOOH
2+
,
and CrO
2+
as intermediates of reactions. Several studies describe the kinetics
and mechanism of the reduction of Cr(VI) by Cys, GSH, and related thiols
[85, 86, 89, 94-98]. The techniques to perform measurements include kinetic
studies, uV-vis spectroscopy, electrospray mass spectrometry (ESMS), X-ray
absorption fine structure (XAFS), and X-ray absorption near-edge structure
(XANES) spectroscopy. The reactivity of Cr(VI) with various compounds
at pH 7.4 and 25°C is shown in Figure 6.6. Ascorbate, cysteine, GSH, and
pencillamine were the only reactive compounds, while other compounds
had no significant reactivity. A number of studies demonstrated the formation
of Cr(VI) complexes with reacted thiols (cysteine, GSH, N-acetyl-2-
mercaptoethylamine, and bromobenzoenethiol) [84, 94, 97, 99-101]. The for-
mation of the Cr(V) species was seen in the reaction of Cr(VI) with biological
thiols [84, 102]. The reduction of Cr(VI) in the glutathione-D-glucose solution
resulted in a stable Cr(V)-carbohydrate species, which was more stable than
the Cr(V)-thiol species [91]. Based on the stated observation above, reactions
(6.33)-(6.35) were proposed in the mechanism of the reduction of Cr(VI) by
thiols (RSH):
k
k
Cr VI RSH
(
)
+
33
33
Cr VI -SR
(
)
(6.33)
−
k
34
→
Cr VI -SR
(
)
Cr V RS
(
)
+
•
(6.34)
k
35
→
Cr VI -SR
(
)
Cr IV RSSR
(
)
+
.
(6.35)
The following rate law (Eq. 6.36) considers the Cr(VI)-thiol complex under
steady-state concentrations:
2
−
d Cr VI /dt
[
(
)]
=
(
k k
[
RSH
]
+
k k
[
RSH Cr VI /
])[
(
)] (
k
+
k
+
k
[
RSH
t
(6.36)
] ).
34
33
t
35
33
−
33
35
34
Correlations of the rates with dissociation constants of thiols suggest an
important role of the -
NH
+
group to determine the kinetics of the formation
of Cr(VI)-thioester [87].
The reaction between a water-soluble and stable aqua ethylene
diaminebis(peroxo)chromium(IV) with GSH have been carried out at a
neutral pH [103]. The EPR technique and mass spectrometry/mass spectrom-
etry (MS/MS) characterization were applied to determine the intermediates
of the reactions. The GSH molecule coordinates to the Cr(IV)-peroxo complex,
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