Oxidation of carbohydrates of biological importance by the aquachromium(IV) ion

Article abstract of DOI:10.1016/j.poly.2012.09.042

The oxidation reactions kinetics of a series of related saccharides by aqua-oxo chromium(IV) ion, (H₂O)₅Cr^IVO ²⁺, were carried out in perchloric acid aqueous solutions. These reactions yield superoxochromium(III) ion, CrO₂²⁺, providing evidence that the two-electron reduction of CrO²⁺ to Cr²⁺ occurred in a single step. In all of these reactions, Cr ²⁺ is the immediate product and could be trapped as CrO ₂²⁺ when an excess of oxygen was present. The bimolecular rate constants for different aldoses and d-glucitol are independent of [H ⁺] in the range 0.1-1.0 M. Relative reactivities of these saccharides toward CrO²⁺ reduction are 1-methyl-α-d-glucopyranose << 1-methyl-α-d-galactopyranose ～ 3-O-methyl-d-glucopyranose ～ 6-desoxi-l-galactopyranose ～ 2-desoxi-d-glucopyranose ～ d-glucopyranose << d-galactopyranose << d-glucitol. The oxidation of aldonic acid such as d-gluconic by CrO²⁺ showed the same mechanism but the redox process is strongly inhibited when [H⁺] increases. Activation parameters were also determined for selected reactions. On the basis of the kinetic result, activation parameters data and oxidized organic products, the mechanism of saccharides oxidation by CrO²⁺ is proposed to be a direct hydride-ion transfer.

Full text of DOI:10.1016/j.poly.2012.09.042

Polyhedron 49 (2013) 84–92
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Oxidation of carbohydrates of biological importance by the aquachromium(IV) ion
⇑
Juan Carlos González, María Florencia Mangiameli, Agostina Crotta Asis, Sebastián Bellú, Luis F. Sala
Área Química General, Departamento de Químico-Física, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, 2000 Rosario, Santa Fe,
Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation reactions kinetics of a series of related saccharides by aqua-oxo chromium(IV) ion, (H2-
O)5CrIVO2+, were carried out in perchloric acid aqueous solutions. These reactions yield superoxochro-
mium(III) ion, CrO22+, providing evidence that the two-electron reduction of CrO2+ to Cr2+ occurred in a
single step. In all of these reactions, Cr2+ is the immediate product and could be trapped as CrO22+ when
Received 17 August 2012
Accepted 25 September 2012
Available online 4 October 2012
an excess of oxygen was present. The bimolecular rate constants for different aldoses and
independent of [H+] in the range 0.1–1.0 M. Relative reactivities of these saccharides toward CrO2+
reduction are 1-methyl- -glucopyranose << 1-methyl- -gluco-
-galactopyranose ꢀ 3-O-methyl-
pyranose ꢀ 6-desoxi- -galactopyranose ꢀ 2-desoxi- -glucopyranose ꢀ -glucopyranose << -galacto-
pyranose << -glucitol. The oxidation of aldonic acid such as
-gluconic by CrO2+ showed the same
D-glucitol are
Keywords:
CrIVO2+
Saccharides
Kinetic
a
-
D
a
-
D
D
L
D
D
D
D
D
mechanism but the redox process is strongly inhibited when [H+] increases. Activation parameters were
also determined for selected reactions. On the basis of the kinetic result, activation parameters data and
oxidized organic products, the mechanism of saccharides oxidation by CrO2+ is proposed to be a direct
hydride-ion transfer.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
at pH 2–4 [10,29,30,34,36,37]. These acid conditions can be found
in certain cellular vacuoles like lysosomes and phagosomes, where
As been widely demonstrated, CrVI and its derivate compounds
are important pollutants because of their mammalian toxicity and
carcinogenicity [1–17].
CrVI compounds are solubilized for their intake, generating inter-
mediate oxidation states which play an important role in chro-
mium genotoxicity [10,31,36].
The interaction of CrVI/V/IV with different carbohydrates – low-
weight neutral [16–26] and acid [26–30] molecules – which are
present in biological systems has been demonstrated in previous
studies. Carbohydrate and organic compounds with 2-hydroxy-
carboxylate and/or vic-diolate functional groups are capable of
chelating CrV/IV; as a result, they become suitable ligands for intra-
cellular stabilization of these chromium oxidation states. It is
known that both CrIV and CrV species can be formed intracellularly
by reduction of CrVI [31] as well as by oxidation of CrIII with the
activated oxygen formed in enzymatic reactions [32]. Unlike CrIV,
the CrV [33] species have extensively proved its ability to form
complexes [26,29,30,34] of relative stability with biological sub-
strates such as carbohydrates. Due to CrIV instability in aqueous
media, some authors have stated for several years that this species
does not have any aqueous solution chemistry [35]. The observa-
tion that CrIV does not form complexes of significant stability with
1,2-diol moieties of carbohydrates in neutral aqueous media was
an argument against this species role in genotoxicity. However,
in the last years, many authors have showed that this species can
be stabilized by several organic substrates in acid aqueous media
Some authors established that CrIV-complexes with carboxylic
ligands like oxalic, malonic, picolinic or quinic acids [38] are being
produced in slightly acidic media and could be responsible for CrIV
toxicity. The CrIV complexes lifetimes are shorter [38] than those of
CrV, but show a higher reactivity in oxidation reactions due to
higher redox potential [5]. Intracellular concentrations of CrIV com-
plexes [39,40] are lower, compared with CrV complexes. CrIV-2-Hy-
droxy carboxylate bischelates have a higher capacity to lose one
ligand molecule than their CrV analogs [38]. This means that CrIV-
complexes can be favored in reactions with DNA. These chemical
properties of CrIV and its complexes in aqueous solutions make
probable candidates for the active species in Cr-induced genotoxi-
city. In the literature it has been reported that CrIV is a more potent
DNA damaging agent than CrV. These results are not definitive be-
cause no data on the structure, stability and kinetic behavior of CrIV
species were obtained.
The rate reaction through which CrIV oxidizes substrates and its
acidity dependence were both obtained in a few cases [41–43].
This work is the first to illustrate the kinetic behavior of CrIV with
different neutral and acidic saccharides (Table 1) in order to
determine their reactivity with CrIV and their possible contribution
in genotoxicity.
⇑
Corresponding author. Tel./fax: +54 341 4350214.
E-mail address: sala@iquir-conicet.gov.ar (L.F. Sala).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.09.042

J.C. González et al. / Polyhedron 49 (2013) 84–92
85
Table 1
was synthesized according to the method described in the litera-
ture [44]. In experiments performed at constant ionic strength, I
(1.0 M) and different hydrogen ion concentrations, mixtures of so-
dium perchlorate and perchloric acid solutions were used. The con-
centration of stock solutions of perchloric acid was determined by
titration employing standard analytical methods.
Bimolecular rate constants, k4, for non acidic and acidic substrates. [H+] = 0.3 M,
I = 1.0 M, T = 15 °C. (S1) D-gluconic acid, (P1) 2-keto-D-gluconic acid; (S2) D-glucitol,
(P2) D-gluconic acid; (S3)
glucopyranose, (P4) D-gluconic acid; (S5) 2-desoxy-
2-desoxy-D-gluconic acid; (S6) 6-desoxy- -L-galactopyranose, (P6) 6-desoxy-
galactonic acid; (S7) 3-O-methyl- -D-glucopyranose, (P7) 3-O-methyl-D-gluconic
acid; (S8) 1-methyl- -D-galactopyranose, (P8) 1-methyl- -D-galactofuranurono-
a
-D-galactopyranose, (P3) D-galactonic acid; (S4)
-D-glucopyranose, (P5)
-L-
a-D-
a
a
a
a
a
a
Caution: Hg and Cr compounds are toxic and human carcino-
gens [45]. Contact with skin and inhalation must be avoided.
6,3lactone; (S9) 1-methyl-
6,3-lactone.
a
-D-glucopyranose, (P9) 1-methyl-
a
-D-glucofuranurono-
Substrate
Product
k (Mꢃ1 sꢃ1
)
2.2. Methods
S1
S2
S3
P1
OH OH
OH
O
O
57 (6)
OH OH
O
HO
HO
HO
2.2.1. Substrate stability
OH
OH
OH
OH
OH
The stability of the organic substrates with different experimen-
tal conditions as HClO4 and oxygen concentrations – used in the
kinetics measurements – were tested by High Performance Liquid
Chromatography (HPLC). The chromatograms were obtained on a
KNK-500A chromatograph provided with a 7125 HPLC pump. The
effluent was monitored with refractive index (ERC-7522, ERMA
INC) and UV (115 UV Gilson, k 220 nm). The analysis was carried
out on Aminex HPX-87H HPLC column (300 ꢂ 7.8 mm, Bio-Rad
laboratories) using 3.2 ꢂ 10ꢃ2 M H2SO4 at 33 °C or an S5 amine re-
sin Spherisorb HPLC column with a 73:27 mixture of acetonitrile
(0.1 M) buffer phosphate at pH 6.4, as eluents. Organic saccharides
were incubated during 1.0 h under the same experimental condi-
tions used for the kinetics measurements. In all the cases, the chro-
matograms obtained from the incubated samples were identical to
those without incubating.
OH OH
OH OH
O
P2
OH OH
45 (5)
34 (4)
HO
OH OH
OH OH
OH
HO
P3
OH OH
O
H
HO
HO
H
O
O
H
CH OH
2
OH OH
OH OH
H
OH
H
S4
S5
S6
P4
18 (2)
17 (2)
16 (2)
H
H
O
HO
HO
HO
OH
OH
OH
H
CH OH
2
OH OH
OH OH
H
OH
H
P5
H
H
O
HO
HO
OH
H
H
O
CH OH
2
OH
OH OH
H
OH
H
H
2.2.2. Oxidation product analysis
The oxidation products of the aldoses/CrIV redox reaction were
determined by HPLC. The chromatograms were obtained on a Var-
ian Polaris 200 chromatograph provided with a cc Star 9000 HPLC
pump. The separation was carried out on an anion-exchange
Spherisorb S Sax HPLC column (250 ꢂ 4.6 mm) using 20 mM NaH2-
PO4 with 5% of ethanol as eluent and a flow rate of 0.5 mL minꢃ1 at
44 °C. The effluent was monitored with a UV detector (Prostar 325
P6
O
OH
H
H
OH
O
OH
H
H
O
OH OH
OH
CH
OH
3
S7
S8
P7
14 (1)
14 (1)
H
OH OCH3
H
OCH3
HO
HO
OH
OH
H
O
H
UV–Vis detector, k = 210 nm). As an example, in the D-glucopyra-
CH OH
2
OH OH
nose/CrIV system the [H+] of the reaction mixture samples was ad-
justed to 0.2 M by addition of HClO4. All the samples were filtered
through a 0.2 mm membrane prior to injection into the chromato-
OH
H
H
OH
P8
H
H
O
O
H
HO
HO
H
CH3O
H
H
O
OH
CH OH
2
graphic system. Standard solutions of
1,5-lactone and -glucopyranose were prepared individually in
0.20 M HClO4 and the chromatographic Rt were determined sepa-
rately. The mixture reaction of
-glucopyranose/CrIV shows two
peaks with Rt1 = 6.24 min and Rt2 = 7.16 min. The acid media in
the mixture reaction favored the lactonization process, and only
the lactone peak was observed in the chromatogram (Rt1). The sec-
D-gluconic acid, D-glucono-
H
H
O
D
H
OCH
H
OH
O
3
H
S9
H
H
P9
9 (1)
O
H
D
HO
HO
H
CH3O
OH
H
O
CH OH
H
2
O
H
OCH
H
OH
3
OH
2+
ond peak (Rt2) refers to the remaining CrO2 in the mixture reac-
tion, as demonstrated with a standard of this compound, prepared
2. Experimental
with H2O2 and CrVI [46]. In addition, co-chromatography was per-
formed, showing an increase of the
6.24 min.
D-glucono-1,5-lactone peak at
2.1. Materials
D
-Glucopyranose (Sigma 99.0%), 1-methyl-
(Sigma 99.0%), 2-desoxi- -glucopyranose (Sigma 99.0%),
(Sigma 99.0%), 3-O-methyl- -glucopyranose (Sigma 99.0%),
galactopyranose (Sigma 99.0%), 1-methyl- -galactopyranose
(Sigma 99.0%), 6-desoxi- -galactopyranose (Sigma 99.0%), -glu-
conic acid sodium salt (Sigma, 99.0%), -glucono-1,5-lactone (Sig-
a
-
D
-glucopyranose
2.2.3. Generation of CrII
D
D-glucitol
Zn/Hg amalgam was prepared in a 50 mL balloon by stirring a
mixture of Zn (10.0 g previously washed with 3.0 M HCl during
3.0 min) and HgCl2 (0.3 M in 0.1 M HCl) for about 30 min. After-
wards, the excess of HgCl2 was eliminated and the resulting amal-
gam washed three times with 1.0 M HClO4 and finally with
distilled water. An appropriate volume of HClO4 and distilled water
were added to the amalgam in the balloon, in order to reach pH 1.0
in a final volume of 35.0 mL. The balloon was then capped with a
rubber septum and left with H2 bubbling for, at least, 15 min to
eliminate oxygen. After that, 1.0 mL of a 0.2 M Cr(ClO4)3 was
injected and left to stir with H2 bubbling. Three hours later,
Cr(ClO4)3 was quantitatively reduced into CrII. The final [CrII] was
D
D-
a
-D
L
D
D
ma, 99.0%), 2-etil-2-hydroxibutiric acid, ehba (Sigma, 98%)
sodium perchlorate monohydrate (Fluka 98.0%), oxygen (99.99%,
Airliquid), perchloric acid (A.C.S. Baker), sodium hydroxide (Cicar-
elli, PA, ACS, ISO) Zn (Sigma–Aldrich, 99.0%), HgCl2 (Cicarelli, PA,
ACS, ISO), H2SO4 (Cicarelli, PA, ACS, ISO), Cr(ClO4)3ꢁ6H2O
(Sigma–Aldrich, 99.0%), D2O (Sigma, 99.9% D). Aqueous solutions
were prepared in milliQ water (18.0 MO/cm). [CoIII(NH3)5Cl]Cl2

86
J.C. González et al. / Polyhedron 49 (2013) 84–92
determined by treating a reaction aliquot (0.5 mL CrII) with an
aqueous solution of [CoIII(NH3)5Cl]Cl2 (4.0 mL 2.1 mM) under
anaerobic atmosphere (Ar). Then, 0.5 mL of this mixture was
poured into a septum-capped spectrophotometric quartz cell, with
a 1.0 cm path length, filled with 2.0 mL of concentrated HCl. The
[CoII] was analyzed by measuring the absorbance of [CoCl4]2ꢃ at
692 nm [44].
by CrO2+ was spectrophotometrically monitored following the
2+
appearance of CrO2 as a final redox product at 290 nm [41]. All
the saccharide/CrO2+ mixtures showed an increase in the bands
at 290 and 245 nm, with relative intensity Abs247/Abs290 = 2.2,
2+
characteristic of CrO2 [41,43]. Mixtures containing substrate,
HClO4 and NaClO4 did not show absorption at 290 nm, even after
10 min of oxygenation.
In all the kinetics measurements, [CrIV], I, [O2], and temperature
were kept constant at 0.07 mM, 1.0 M, 1.26 mM and 15–30 °C
respectively. The concentration range of every organic substrate
used was chosen in order to avoid CrO2+ disproportion. The proton
dependence of the CrIV/saccharide reaction was studied varying
2.2.4. In situ generation of aqua oxo-CrIV, (H2O)5 CrO2+
CrO2+ can be generated by rapid CrII oxygen-oxidation, accord-
ing to the following procedure: a deoxygenated solution of CrII
was injected into an acid aqueous solution of organic substrate sat-
urated with oxygen (1.26 mM). In a typical experiment, 100
l
L of
[HClO4] between 0.3 and 0.6 M. For CrIV
/D
-galactopyranose and
6.0 mM CrII were injected into a septum-capped spectrophotomet-
ric quartz cell, with a 1.0 cm path length, filled with 2.3 mL of an
O2-saturated solution containing appropriate concentrations of or-
CrIV/
D-glucopyranose mixtures the [HClO4] range was extended
to 0.01–0.6 M. The kinetic isotope effect of the O–H hydrogen
was studied performing the redox reaction in D2O and comparing
the results obtained with those in H2O.
ganic substrate, HClO4 and NaClO4 at 10–30 °C. At very low CrII/O2
ratios (<0.05) superoxochromiumIII (CrO2
)
is quantitatively
The experimental pseudo-first-order rate constants, kobs, were
obtained from nonlinear least square fits of absorbance data at
290 nm with averages of, as minimum, four determinations and
within 10% of each other. Data used to calculate the kinetic con-
stant corresponded to 80% of the exponential growth of the absor-
bance experimental values. In every case, the first-order
dependence of the rate upon [CrIV] was verified in a set of experi-
ments where the [CrIV]0 was varied while T, [organic substrate],
[O2] and I were kept constant.
2+
formed, while at intermediate CrII/O2 ratios (ꢀ0.15), the reaction
affords mixtures of CrO2+, CrIII and CrO22+ [41,43]. Under these con-
ditions, CrO2+ was immediately formed and reacted with the or-
ganic substrate to render CrII and oxidized organic products. This
CrII, formed by the CrO2+/organic substrate reaction is quantita-
tively transformed into CrO22+ (CrII/O2 ratio <0.05) and no autocat-
alytic consumption of CrO2 by CrII was observed [41,43].
2+
2.2.5. Quantification of CrO2+
The concentration of the generated CrO2+ species by the CrII/O2
reaction at ꢀ0.15 ratio can be easily determined by injection of
3. Results and discussion
100 l
L of 6.0 mM CrII into 2.3 mL of an O2-saturated solution of
Under anaerobic conditions, CrII reduces CoIII complex into CoII
according Eq. (1):
0.1 M ehba buffer at pH 3.0, 15 °C and I 1.0 M. Immediately after
mixing, the solution turned pink and the absorbance of the
½CoðNH3Þ ClꢄCl2 þ Cr2þ þ 6HCl ! CoCl42ꢃ þ 5H4NCl þ Cr3þ þ Hþ
mixture – due to [CrIV(O)(ehba)2]2ꢃ – at 512 nm ( = 2380 Mꢃ1
e -
5
cmꢃ1) was measured [47]. The percentage of CrO2+ generated
was calculated in two different ways. The first method employed
was kinetic measurements at 512 nm of a mixture containing ehba
buffer 0.1 M (pH 3.0), [CrII]0 0.3 mM and O2 1.26 mM, at I 1.0 M
and T 15 °C. In the second method, the CrIV was generated in the
same experimental conditions without substrate. The CrIV was able
to disproportion into CrIII and CrVI and the reaction could be fol-
lowed by the absorbance changes of the last species at 350 nm.
ð1Þ
The observed spectrum was in agreement with those reported
for CoCl4 in the literature [41]. The content of CrIV was deter-
2ꢃ
mined (see experimental section) as [CrIV(O)(ehba)2].
Under the experimental conditions above mentioned, reaction
between CrII and O2 rapidly produced 0.07 mM CrO2+ (29.6% aver-
age based on total [CrII]) (See Supplementary Materials Figures S1a
and S1b).
CrIV is an unstable species [41,43] with a 0.75 min-half life at
0.1 M ionic strength in acid media and room temperature [43]. This
chromium species can participate in two different and competitive
reactions: organic substrates oxidation and disproportion into CrVI
and CrIII through CrV, according to Eqs. (2)–(4):
2+
2.2.6. Characterization and stability of CrO2
One method used to characterize CrO22+ was a series of sequen-
tial spectra, between k = 200 and 800 nm. This compound presents
two intense characteristic absorption bands, at 290 nm (
e = 3100
Mꢃ1 cmꢃ1) and 245 nm ( = 7000 Mꢃ1 cmꢃ1) with a relative inten-
e
2CrIV ! CrIII þ CrV
ð2Þ
ð3Þ
ð4Þ
sity (Abs247/Abs290) of 2.2 [43]. A second method involved a specific
reaction for CrO22+ [48] with FeII. Once the redox reaction CrIV/sac-
charide was finished, FeII was added to the mixture reaction (0.8
mM). Time evolution of the mixture was recorded every 2.0 min.
In order to study the influence of temperature, [H+] and I on
CrV þ CrIV ! CrVI þ CrIII
3CrIV ! CrVI þ 2CrIII
2+
CrO2 stability, a series of experiments were carried out varying
the parameters between 10 and 25 °C, 0.30 and 0.6 M and 0.2–
1.0 M, respectively.
Since CrVI also absorbs at 290 nm, where kinetics measurements
are studied, it is important to verify that disproportion process
does not occur. For this reason, it is necessary to determine the
specific experimental conditions where substrates oxidation reac-
tions are favored over disproportion. Fig. 1 shows a comparison be-
2.2.7. CrO2+/saccharide kinetic measurements
All of the studied saccharides proved to be stable under kinetic
experimental conditions. None of them, nor inorganic reagents or
oxidation organic products absorbed at the wavelength chosen
for the kinetic measurements. Kinetic measurements were moni-
tored by absorbance changes on a Jasco V-530 spectrophotometer
with fully thermostated cell compartments ( 0.2 °C). Reactant
solutions were thermostated prior to the experiment. The reactions
were followed under pseudo-first-order conditions using excess of
organic substrate over CrIV. The oxidation of different saccharides
tween the corresponding spectra for CrO2 and CrVI. As can be
2+
seen, there is a region where both spectra are superimposed
(230–330 nm), but absorbance at 350 nm is only due to CrVI. There-
fore disproportion, if any, can be observed at 350 nm because of
CrVI formation. With the purpose of avoiding CrVI generation via
disproportion, a minimum concentration of saccharide is required.
Such minimum concentration depends on the CrIV/saccharide re-
dox reaction rate and is determined for each organic substrate.

J.C. González et al. / Polyhedron 49 (2013) 84–92
87
This implies that [H+] does not alter its stability during the redox
reaction time (See Supplementary Materials Fig. S4). Finally, at
constant [H+] and temperature, two different values of ionic
strength (0.2 and 1.0) were studied. Kinetic profiles show that
the rate decay of CrO22+ is not affected significantly, even using val-
ues five times higher of ionic strength, in the kinetics times of the
measurements (data not shown).
247 nm
0.25
0.20
0.15
0.10
0.05
0.00
256 nm
293 nm
350 nm
It must be highlighted that the effect of substrate concentration
was studied to verify any possible reaction between the CrO22+ and
saccharides present in the mixture, which would affect CrO22+ sta-
bility. The CrO22+ decay rate remained unaffected regardless of the
substrate concentration used (see Supplementary Materials
CrVI
CrO22+
2+
Fig. S5). An independent prepared solution of CrO2 decays at
the same rate as CrO2 formed in the saccharide/CrIV reactions if
2+
200 220 240 260 280 300 320 340 360 380 400
experimental conditions are the same [50].
λ (nm)
According to the mentioned above, kinetic conditions should be
accurately selected in order to guarantee that: (a) CrO2+ dispropor-
tion reaction should be negligible respect to its capacity to oxidize
Fig. 1. Comparison between CrO2 and CrVI spectra. [CrO22+] = 0.032 mM (black);
2+
[CrVI] = 0.08 mM (gray); [O2] = 1.26 mM; [H+] = 0.3 M; I = 1.0 M; T = 15 °C.
2+
the organic substrate; (b) CrO2 should be stable enough to be
considered as the final redox product of the CrO2+/saccharide reac-
tion and (c) oxidation rate should be slow enough to be measured
in a conventional spectrophotometer.
The other possible CrVI generation, which is the bimolecular
2+
decomposition of CrO2 (Eq. (5)), do not occur [49] as stated in
Section 2:
2CrO22þ þ 4H2O ! 2 HCrO ꢃ þ 6Hþ
ð5Þ
3.3. Kinetic measurements: saccharide/CrIV reactions
4
Experimental results show that CrIV disproportion is always
negligible comparable to its capacity to oxidize the organic sub-
strate. Only in absence or very low concentration of substrate, an
important increment of absorbance at 350 nm can be observed
due to the increase in the CrVI formation (data not shown). Total
CrIV disproportion occurring in the absence of substrate (Eqs. (2)–
(4)) can be used to quantify this species. According to Eq. (4), initial
concentration of CrIV was calculated from the absorbance at
350 nm. The resulting value was 0.0705 mM, which represents
28% of total chromium, in agreement with the [CrIV(O)(ehba)2] for-
mation at pH 3.0.
As mentioned before, CrO2+ is a relatively unstable species
[41,43]. With the purpose of studying CrO2+ oxidation process over
different organic substrates, it was generated in situ as described in
the experimental section. The CrO2+ generated reacts with the sub-
strate present in the reaction media to give CrII and oxidized organ-
ic substrate (Eq. (7)). The newly generated CrII is efficiently trapped
by oxygen in a fast reaction (k = 1.6 ꢂ 108 [51]) to quantitatively
2+
produce CrO2 (Eq. (8)) [41,43,49]. This reaction has been exten-
sively studied by pulse radiolysis [52], UV spectrophotometry
2+
[46] and Raman spectroscopy [53]. The CrO2 generated in this
reaction slowly decomposes into CrIII (Eq. (9)):
2+
3.1. CrO2 determination and characterization
CrIV þ S ! CrII þ Sox ðslowÞ
ð7Þ
ð8Þ
ð9Þ
For every CrO2+/substrate reaction, the CrO22+ formed was iden-
tified by analyzing its sequential electronic spectra, where its char-
acteristic absorption bands were observed, in agreement with
literature [41,43] data. The ratio Abs290/Abs245 remains constant
CrII þ O2 ! CrO
ðfastÞ
2þ
2
in time when CrO2 slowly decomposes to CrIII species (See Sup-
2+
CrO2 !!!! CrIII ðvery slowÞ
2þ
plementary Materials Fig. S2).
2+
2+
Additionally, the presence of CrO2 was verified by means of
The CrO2 species is selected as a final product redox in order
to follow the CrO2+/saccharides kinetics because: (a) it has a de-
fined and characteristic spectrum in aqueous solution [41,43]
and (b) it is stable in the kinetics times. This species rapidly comes
from the CrII formed (Eqs. (7) and (8)) and slowly decomposes into
CrIII species, Fig. 2a and Eq. (9); (c) no species present in the reac-
tion media, except for CrO22+, absorb at 290 nm and (d) no saccha-
their well-known reaction with FeII (Eq. (6)):
CrO22þ þ 3Fe2þ þ 4Hþ ! Cr3þ þ 3Fe3þ þ 2H2O
ð6Þ
2+
Every spectrum recorded was subtracted from the CrO2 spec-
trum, prior to the FeII addition. After the correction of the small
dilution, a negative absorbance around 290 nm was obtained con-
2+
2+
firming the presence of CrO2 [49] (See Supplementary Materials
ride/CrO2 reaction was observed.
Although CrO2+ generated in situ has a characteristic spectrum,
Fig. S3).
all the other species (CrO2 and CrIII) have superimposed spectra,
2+
3.2. Influence of temperature, [H+] and I on CrO2 stability
making it impossible to determine the individual contribution of
every species to the absorbance at 290 nm. For this reason, the re-
2+
The CrO2 produced in the CrO2+/saccharide mixtures slowly
dox process is followed by CrO2 formation instead of CrO2+
2+
2+
decomposes into CrIII species. However, in the time scale of kinetic
disappearance.
measurements, CrO2 is stable enough to be considered as the fi-
Moreover, the route of CrO2+ disproportion (Eqs. (2)–(4)) and
the bimolecular decomposition (Eq. (5)) are not favored under
the experimental conditions used in this work. These facts ensure
that the increase of the absorbance at 290 nm – due to generation
of CrO22+ (Eq. (8)) – is a direct reflection of the CrO2+decrease when
reacting with organic substrate.
2+
nal product of the redox reaction. The rate of CrO22+ decay is faster
2+
at higher temperatures. That means the stability of CrO2 dimin-
ishes with temperature increment. When two different [H+] were
used to perform the same reaction at constant temperature and io-
nic strength, there were no changes in the rate decay of CrO2
2+
.

88
J.C. González et al. / Polyhedron 49 (2013) 84–92
As illustrated in Table 1 and Fig. 3, the reactivity order for non-
acidic substrates is the following: 1-methyl- -glucopyranose <<
1-methyl- -glucopyranose ꢀ 6-
-galactopyranose ꢀ 3-O-methyl-
desoxi- -glucopyranose ꢀ -gluco-
-galactopyranose ꢀ 2-desoxi-
pyranose << -galactopyranose << -glucitol. Taking into account the
corresponding structure of each substrate (Table 1), we notice that
open chain saccharide, -glucitol, reacts much faster than those with
a cyclicstructure. From the experimentalresults variousobservations
can be made: (a) -galactopyranose oxidation rate is almost twice
higher than -glucopyranose, indicating that a change in the orienta-
0.42
0.40
0.38
0.36
0.34
0.32
0.30
0.28
0.26
0.24
(a)
a
D
-D
a
-
D
L
D
D
D
D
D
D
D
tion of the hydroxyl group in C4, Table 1, clearly modifies the saccha-
ride reactivity towards CrO2+; (b) the absence of hydroxyl group in C2
(2-desoxi-
methylation on C3 (3-O-methyl-
nificantly the rate constant and (d) saccharides methylated in C1 (1-
methyl- -glucopyranose and 1-methyl- -galactopyranose) are
D-glucopyranose) does not affect the rate constant; (c) the
0
50
100
150
200
D-glucopyranose) does not affect sig-
t (s)
a-D
a-D
0.42
0.39
0.36
0.33
0.30
0.27
0.24
(b)
notoriously less reactive than the corresponding aldopyranoses. In
the knowledge that the CrO2+/saccharides redox reactions yield aldo-
nic acids, it can be postulated that the most reactive site in aldopyra-
noses is C1.
The tendency observed in the redox reactivity of the saccharides
towards CrO2+ can be rationalized by considering the relative insta-
bility of the saccharide-CrO2+ chelate formed before the redox
reactions. The highest reactivity of D
-galactopyranose with CrO2+
can be a consequence of saccharide-CrO2+ complex instability in-
duced by the non-bound 1,3-diaxial interactions [18,23]. Thus,
the O4a:H2a steric interaction in the
D
-galactopyranoside stereoiso-
mer should have the highest rate accelerating effect, Scheme 1. By
the contrary, -glucopyranose with all the ring substituents in
pseudoequatorial position forms the most thermodynamically sta-
ble complex and is oxidized more slowly than -galactopyranose,
as observed. The elimination of the equatorial hydroxyl group in
C2 (2-desoxi- -glucopyranose) and the methylation of an OH in
C3 (3-O-methyl-
D
0
5
10
15
20
25
t (s)
D
Fig. 2. (a) Kinetic profile of saccharide oxidation, Abs290 vs. time. [D-Glucopyra-
nose] = 5.0 mM; [H+] = 0.3 M; [CrO2+] ꢀ 0.07 mM; [O2] = 1.26 mM; T = 15 °C;
I = 1.0 M; k = 290 nm. (b) Mathematical fitting of the experimental kinetic data;
kobs was obtained using Eq. (10).
D
D
-glucopyranose) do not essentially modify the
rate of these saccharides oxidation by CrO2+. A possible explana-
tion for such behavior is that the modified OH belongs to the axial
type. Because of this reason, there are not changes in the rings sta-
3.4. Determination of the oxidation rate, kobs
bility. This way, the saccharides,
copyranose and 3-O-methyl- -glucopyranose are oxidized by
CrO2+ practically at the same rate (see Scheme 2).
Regarding the 6-desoxi- -galactopyranose, Scheme 1, there are
two types of 1,3-diaxial interactions, as a result, this saccharide
should react faster than -galactopyranose. The obtained results
D-glucopyranose, 2-desoxi-D-glu-
D
The kinetic data oxidation of different saccharides by CrO2+
was spectrophotometrically monitored following the appearance
of CrO2 as a final redox product at 290 nm and 15 °C. These data
were set in an Abs290 versus time graphic, producing for all sub-
strates/CrO2+ mixtures, curves with a biphasic behavior, Fig. 2a.
The first part of the graphic exhibits a rapid exponential incre-
L
2+
D
cannot be explained in term of tensional effects. For this reason,
there might be additional stabilization factors in the CrO2+-6-des-
2+
ment and corresponds to the CrO2 formation (Eqs. (7) and
oxi-L-galactopyranose complex which justify this saccharide low
reactivity (Table 1). A suitable explanation would probably be a
different conformation of the aldose in the intermediate complex.
2+
(8)); while the slower second part corresponds to CrO2 decay
into CrIII species (Eq. (9)). The experimental kinetic constants, kobs
,
were calculated using a non-linear least square fits of 80% of the
experimental data exponential growth, according to Eq. (10),
Fig. 2b:
The same behavior has been observed in the 6-desoxi-
L-galacto-
pyranose oxidation by CrVI [23].
Finally, in aqueous solution, saccharides are in fast equilibrium
between their cyclic and open form. However, the low concentra-
Abs ¼ Absinf þ ðAbs0 ꢃ Absinf Þeðꢃk
where Abs0, Absinf and Abs represent respectively absorbance at
ð10Þ
obsꢁtÞ
tion of the open form [54] present in
solutions and the high reactivity of the
nose respect to
(b)-1-methyl-glucopyranose with CrVI indicate
that the saccharide reactive form is the cyclic one [18].
The activation parameters, Table 2, for CrO2+
-galactopyranose
and CrO2+
-glucopyranose reactions were calculated from the plot
of ln(k/T) versus 1/T, Fig. 4.
The isokinetic relationship plot, Fig. 5, shows that
D-galacto and
D-glucopyranose
a
(b)-1-methyl-galactopyra-
a
zero, infinite and t time and kobs is the rate constant for substrate
oxidation by CrO2+
.
/D
The effect of [substrate] on kobs for non-acidic substrates is
shown in Fig. 3. As represented in the graphic, a family of straight
lines with zero intercept is obtained; the slopes of these lines rep-
resent the bimolecular rates constants summarized in Table 1. No
dependence of the rate constant, kobs, with [H+] was observed. The
experimental rate law for these reactions can be expressed as
shown in Eq. (11):
/D
D
Hà is a linear
function of
D
Sà for the reactions of primary, secondary alcohols,
1,2-cyclohexanediols and saccharides with CrO2+. The existence
of an isokinetic relationship supports that a single mechanism
operates along the series of compounds. Certain members deviat-
ing from the correlation to a significant extent suggests that a
ꢃd½CrIVꢄ=dt ¼ d½CrO22þꢄ=dt ¼ k4½substrateꢄ½CrO2þꢄ
ð11Þ

J.C. González et al. / Polyhedron 49 (2013) 84–92
89
0.24
0.20
0.16
0.12
0.08
0.04
0.00
[S] (M)
0.000
0.002
0.004
0.006
0.008
0.010
Fig. 3. Effect of [substrate] on kobs. [CrO2+] ꢀ 0.07 mM; T = 15 °C, I = 1.0 M; [H+] = 0.3 M; [O2] = 1.26 mM.
different mechanism takes place. This deviation occurs with the
activation parameters derived from phenol/CrO2+ reaction [55].
The oxidation mechanism by CrO2+ of the linear and cyclic alco-
hols mentioned above has been accurately demonstrated by
Espenson and coworkers [41,50] as a hydride transfer from the sac-
charide to the CrO2+. Fig. 5 clearly depicts the existence of an isoki-
netic relationship and consequently makes it possible to postulate
the same mechanism for the saccharide oxidation by CrO2+. The
slope of Fig. 5 represents the isokinetic temperature, whose value
is 336 K (63 °C), in agreement with the one obtained from Fig. 4.
The kinetic isotope effect of the O–H hydrogens was negligible
H
OH
OH
H
OH
OH
HO
H
H
O
HO
HO
H
O
OH
H
OH
H
OH
H
H
H
α-D-galactopyranose
β-D-galactopyranose
H
H
H
H
OH
OH
(kH/kD = 1.2) for D
-glucopyranose/CrO2+ redox reaction. Both this
HO
H
OH
OH
HO
HO
OH
OH
O
result and the formation of CrII as a redox product suggest that
the saccharides oxidation takes place by a concerted two-electron
hydride transfer mechanism [50]. Hydride transfer is generally
O
H
H
OH
H
H
H
characterized by a positive value of
of S. The latter have been attributed to the strict orientation
requirements for hydride and achievement of the cyclic transition
value of
S. In case that organic radical and CrIII are products of the
DH and large negative values
α-D-glucopyranose
β-D-glucopyranose
D
OH
D
redox reaction, CrIII must be reduced to CrII by the organic radical.
Reaction between CrII and oxygen generate CrO22+, which is ther-
modynamically possible (E0 CrIII/CrII = ꢃ0.416 V), has been shown
not to occur because is too slow (k = 5.6 ꢂ 102 L molꢃ1 sꢃ1 [56]).
Therefore, the only way to obtain CrII (and CrO22+) in the saccha-
ride/CrIV oxygenated mixtures is by a two-electron reduction of
CrIV species.
H
H
O
H
6-desoxi-L-galactopyranose
H
OH
OH
CH3
OH
Scheme 1. Non-bound 1,3-diaxial interactions.
2+
CrO2+
H
H
HO
H
H
HO
CrOH
H
H
HO
H
OH
OH
HO
HO
O
OH
CH2OH
O
OH
CH2OH
CH2OH
H
O
H
O
H
H
OH
H
OH
H
Cr
H
HO
H2O
H+
H
H
OH
HO
OH
CH2OH
α-D-glucopyranose + CrO2+
+ H2O
D-gluconic acid + H+ + CrOH+
HO
H
HO
H
O
Scheme 2. Proposed mechanism for the oxidation of glucose.

90
J.C. González et al. / Polyhedron 49 (2013) 84–92
Table 2
120
2+
Activation parameters for the CrO2 oxidation of D-glucopyranose, D-galactopyra-
nose, diols, methanol, 2-propanol and cyclobutanol.
H = 0.10 M
H = 0.15 M
100
80
60
40
20
0
0,25
0,20
0,15
0,10
0,05
0,00
Substrate
Activation parameters
Hà (kJ molꢃ1 Sà (J molꢃ1 Kꢃ1
D
)
D
)
Ref.
34
44
41
ꢃ100
ꢃ68
This work
This work
H = 0.20 M
H = 0.25 M
0
2
4
6
8
10
12
D-Glucopyranose
1/[H+] (M-1)
D-Galactopyranose
H = 0.30 M
cis-1,2-Cyclohexanediol
trans-1,2-Cyclohexanediol 51
Methanol
2-Propanol
Cyclobutanol
ꢃ78
[50]
[50]
[41]
[41]
[41]
ꢃ48
34
33
46
ꢃ98
ꢃ112
ꢃ61
0,0000
0,0005
0,0010
0,0015
0,0020
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
[S] (M)
Isokinetic temperature
57 (6)ºC
Fig. 6. Effect of [D-gluconic acid] on kobs. Insert: D-gluconic acid rate constant, k4,
dependence with [H+]. [CrIV] ꢀ 0.07 mM; T = 15 °C, I = 1.0 M; [O2] = 1.26 mM.
determined [28–30,59]. The obtained result in this work confirms
that CrIV is a more reactive species towards saccharides than CrVI
or CrV. In fact the oxidation rate values for the CrIV/saccharide
are, at least, two orders of magnitude higher than the oxidation
rates for CrV/saccharide or CrVI/saccharide reactions.
On the other hand, as demonstrated in previous works [29,59],
acidic substrates like D-galacturonic and D-glucuronic acid, have a
completely different behavior when reacting with CrIV. The exper-
imental rate law of saccharide/CrIV reaction obtained for these sub-
strates exhibits a strong inhibition of the redox process when [H+]
0.0030
0.0032
0.0034
1/T (K-1)
0.0036
increases. Experimental results from
same behavior, as can be seen in Fig. 6. As represented in Fig. 6,
plots of kobs versus [ -gluconic acid] gave good straight lines from
D-gluconic acid shows the
Fig. 4. Plot of ln(k/T) vs. Tꢃ1 for the reaction of
D
-galactopyranose and D-
glucopyranose with CrO2+
.
[Substrate] = 1.0 mM; I = 1.0 M; [CrIV] ꢀ 0.07 mM;
[O2] = 1.26 mM; T = 10–25 °C; [H+] = 0.30 M.
D
which slope k4H values were determined (Eq. (12)). The bimolecu-
lar rate constant, k4H, varied linearly with [H+]ꢃ1 with a positive
intercept ka = 25.5 2.4 sꢃ1 Mꢃ1 and slope kb = 9.4 0.4 sꢃ1, Eq.
(13) and Fig. 6:
64
K4 ¼ k4H½d-gluconicꢄ
K4H ¼ ka þ kb½Hþꢄꢃ1
ð12Þ
trans-1,2-cyclohexanediol
56
48
40
32
24
16
ð13Þ
phenol (fast step)
cyclobutanol
The rate constant for CrIV disappearance is given by Eq. (14):
D-galactopyranose
cis-1,2-cyclohexanediol
isopropanol
K4 ¼ ðka þ kb½Hþꢄꢃ1Þ½d-gluconic acidꢄ
ð14Þ
methanol
D-glucopyranose
The bimolecular rate constant was inversely proportional with
[H+] in the range 0.1–0.3 M. The rate law was resolved using the al-
ready known acid–base equilibrium between the carboxylic acid,
HS-COOH, and the conjugate base, HS-COOꢃ, Scheme 3.
phenol (low step)
Scheme 3 shows that two species are able to react with CrO2+ in
parallel slow steps to give CrII and oxidized organic product (P). In
this scheme, k1 and k2 represent the rate constants for the oxida-
-140
-120
-100
-80
-60
-40
ΔS# (J mol-1K-1)
tion of the
D-gluconic acid (HS-COOH) and the conjugate base
Fig. 5. Plot of
D
Hà vs. Sà for the reactions of primary, secondary alcohols, 1,2-
D
(HS-COOꢃ) respectively, which leads to Eq. (15):
cyclohexanediols and saccharides with CrO2+
.
K4 ¼ k1½HS-COOHꢄ þ k2½HS-COOꢃꢄ
Taking into account that:
½Hþꢄ½HS-COOꢃꢄ
ð15Þ
As previously commented, oxidation of aldopyranose by CrIV
was independent of [H+]. Concerning the case of the
-gluco and
-galactopyranose/CrIV reaction, the experimental rate low was va-
D
D
Ka
¼
½HS-COOHꢄ
lid at pH 2. This pH can be found in subcellular compartments like
phagosomes [57].
Formation of CrV and/or CrIV intermediates in the redox reaction
with CrVI has been previously observed for the chromic oxidation
of organic substrates [58]. For all oxidation reactions of these sub-
strates with CrVI, the presence of CrIV has been demonstrated, but
only in few cases the rate constants of CrIV/substrate were
and
½HS-COOHꢄT ¼ ½HS-COOHꢄ þ ½HS-COOꢃꢄ
and making the corresponding mathematical arrangements, Eq.
(15) transforms into Eq. (16)

J.C. González et al. / Polyhedron 49 (2013) 84–92
91
Ka
References
HS-COO-
HS-COOH
CrO2+
+ H+
fast
CrO2+
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417.
2+
CrO2
O2
fast
2+
CrO2
Scheme 3. Proposed mechanism of CrO2+ reaction with
D
-gluconic acid.
ꢀ
ꢁ
½Hþꢄk1 þ k2Ka
½Hþꢄ þ Ka
k4 ¼
½HS-COOHꢄT
ð16Þ
since [H+] >> Ka [60], thus
ꢀ
ꢁ
½Hþꢄk1 þ k2Ka
k4 ¼
½HS-COOHꢄT
½Hþꢄ
¼ ðk1 þ k2Ka½Hþꢄꢃ1Þ½HS-COOHꢄT
ꢃd½CrIVꢄ=dt ¼ d½CrO22þꢄ=dt
ð17Þ
ð18Þ
¼ ðk1 þ k2Ka½Hþꢄꢃ1Þ½HS-COOHꢄT½CrO2þꢄ
The rate law for the disappearance of CrIV takes the form of Eq.
(18), which is in total agreement with the experimental rate law,
Eq. (14) where k1 = ka and k2Ka = kb.
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the formation of the precursor complex from oppositely charged
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In this paper we studied the oxidation reactions kinetics of a series
of related saccharides by aqua-oxo chromium(IV), in perchloric acid
aqueous solutions. These reactions yield superoxochromium(III) ion,
CrO22+, providing evidence that the two-electron reduction of CrO2+
to Cr2+ occurred in a single step. Relative reactivities of these saccha-
rides towards CrO2+ reduction are: 1-methyl-
1-methyl-
-galactopyranose ꢀ 3-O-methyl-
desoxi- -glucopyranose ꢀ
-galactopyranose ꢀ 2-desoxi-
nose << -galactopyranose <<
-glucitol. The CrO2+/saccharides redox
a
D
-
D
-glucopyranose <<
-glucopyranose ꢀ 6-
-glucopyra-
a
-
D
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L
D
D
D
D
reactions yield aldonic acids as products, evidencing that the most reac-
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Luis F. Sala acknowledges the Nacional Research Council of
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the Santa Fe Province Programs of Promotion for the Scientific
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.09.042.

92
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