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between
trans
-bis(2-ethyl-2-hydroxybutanoato(2-))oxochromate(V) and cyto-
chrome
c
II
has been studied [61]. The Cr(V) complex has several negatively
charged donor centers, which could interact with the positively charged sur-
faces of the cytochrome
c
II
and resulted in the precursor complex. A proposed
mechanism involves the outer-sphere one-electron transfer from Fe(II) to
Cr(V), which proceeds by a precursor complex.
6.1.4.2 Hydrogen Peroxide.
The Cr(VI)/H
2
O
2
reaction has been extensively
reviewed [62]. This reaction has an important role in the reduction of Cr(VI)
by biomolecules [63]. Several peroxo species such as tetraperoxochromate(V)
and oxodiperoxochromate(VI) complexes have been reported [64]. Recently,
the kinetics of the reaction between Cr(VI) and H
2
O
2
was examined in acetate
and phosphate buffer solutions as a function of pH (4.6-7.3) [65]. Cr(VI)
reduces to Cr(III) and acts as a catalyst for the dismutation of H
2
O
2
. Two
diperoxochromium(V) complexes were observed, which play an important
role in the conversion of H
2
O
2
to O
2
. Later work on Cr(VI)-H
2
O
2
spectroscopi-
cally identified four species depending on the pH of the solution and the
concentration of the reactants [63].
6.1.4.3 Carbohydrates.
A series of reactions between Cr(VI) and carbohy-
drates of biological importance under highly acidic conditions have been
examined [66-71]. The mechanisms undergo Cr
VI
→ Cr
IV
→ Cr
III
and Cr
VI
→ Cr
V
→ Cr
III
pathways [72-75]. Carbohydrates stabilize Cr
V
with the formation of
five- and six-coordinate oxo-Cr
V
complexes [76]. Oxo-Cr(V)-carbohydrate
complexes have also been characterized [77]. Some of the general conclusions
drawn using absorption and electron paramagnetic resonance (EPR) spectros-
copies were (1) the electron transfers occur in slow steps having numerous
acid and nonacid catalyzed parallel pathways and the total number of electron
transfers is always two; (2) transfer of electrons occurs intramolecularly within
the redox precursor complex and the formation of the complex is not acid
catalyzed; and (3) in slow redox steps, the highly reactive Cr
IV
is formed, which
reacts rapidly with carbohydrates [78].
6.1.4.4 Hydroxy Acids.
Reduction of chromic acid by α-hydroxy acids
has been extensively studied using kinetics and spectral measurements [74, 75].
Hydroxycarboxylato functional sites may stabilize Cr
VI,V
, and such studies
may unravel the role of chromium in biological systems. Products of the reac-
tions were formed by the cleavage of either C-C or C-H [79], depending on
the particular molecular characteristics. Importantly, the Cr(V) intermediate
was observed. Cr(V) complexed with hydroxamic acid ligands has been inde-
pendently synthesized and characterized by uV-visible (uV-vis) absorption,
EPR, and X-ray absorption techniques [80]. The Cr(V) complexes with the
naturally occurring
tert
-2-hydroxy acids, quinic acid (1R,3R,4R,5R-1,3,4,5-
tetrahydroxycyclohexanecarboxylic acid, qaH
3
), have also been obtained
[81]. Quinic acid contains both 2-hydroxy acids and 1,2-diol groups, and the
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