HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 288

coordination of Cr(V) to these molecules depends on the pH. The 1,2-diols
stabilize Cr(V) at pH 7.4, while 2-hydroxy acid ligands stabilize at pH ∼4.
Recently, the interaction of Cr(VI) with an amino sugar (2-amino-2-deoxy-D-
glucopyranose, Nglc) has been investigated [82]. The kinetics traces and EPR
measurements suggest the formation of Cr VI esters, oxo-Cr V complexes,
superoxo-Cr(III) ( CrO 2+ ), and free radicals as intermediates of the reaction.
The substitution of C 2 -OH group by C 2 -NH 2 in Nglc species inhibited the
oxidation of Nglc by Cr VI/V . This indicates the role of -NH 2 group present at
C 2 of Nglc in the oxidation reaction [82].

6.1.4.5 Ascorbic Acid. Several studies have performed the reaction of
K 2 CrO 7 with ascorbate [83-89]. The rate law for the decay kinetics of Cr(VI)
is described as

− d Cr VI /dt [ ( )] = k [ Cr VI RH ( )][

−

], (6.32)

RH

where k RH is the second-order rate constant of the reaction. [Cr(VI)] and
[RH − ] are the concentrations of Cr(VI) and ascorbate, respectively. The values
of k RH are pH dependent [90]. Cr(V), CO − , the ascorbate radical, and the
carbon-based spin-trap radical adducts were identified. The yields of the prod-
ucts depended on the ratio of ascorbate to dichromate. The formation of
Cr(IV) was indirectly observed by the reaction with Mn(II) [83]. Later work
on the reaction of Cr(VI) with ascorbic was carried out over a wide pH range
[91]. Both the Cr(V)/ascorbate complex and the ascorbate radical were
observed. The Cr(V)/ascorbate complex was most stable at a ratio of 1 : 1
([Cr(VI)] : [ascorbate]). The reaction was influenced by O 2 in the solution,
which produced Cr(V)/ascorbate/peroxo complexes with a maximum intensity
in the EPR signal at a ratio of 1 : 2. Both complexes were not stable at pH > 10
and reduced rapidly to Cr(III). In the mechanism, reactions (6.21), (6.22),
(6.25), (6.26), and (6.30) were proposed in the reduction of Cr(VI) by ascor-
bate in neutral aqueous solution [83]. Formation of Cr(V) in the reaction may
have the relevance in genotoxicty of Cr. Significantly, excess concentration of
ascorbate may rapidly eliminate high-valent Cr species and hence diminish
the potential of carcinogenicity. Workers in the industry of chromate may thus
be better off by taking high vitamin C.

6.1.4.6 Catecholamines. Catechol compounds are present in neurotrasmit-
ters and their precursors, for example, are dopamine, dihydroxyphenylalanine
(DOPA), hormones (e.g., adrenaline and noradrenaline), and melanin. The
reactions of Cr(VI) with various catecholamines were studied using the EPR
technique [92]. The reduction of Cr(VI) by catecholamines resulted in EPR
signal of Cr(V) species and organic radicals [92]. The primary species formed
from the complexation of Cr V with catechol-derived ligands with an assigned
octahedral structure ([Cr V (catechol) 2 (catechol)] + ). The complexation chemis-
try was affected by the reactant ratios and the pH. These results indicate the