The reduction of Cr(VI) to Cr(III) by the α and β anomers of D-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study

Article abstract of DOI:10.1016/S0008-6215(99)00282-7

The reduction of Cr(VI) by α-D-glucose and β-D-glucose was studied in dimethyl sulfoxide in the presence of pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers reduce Cr(VI) by formation of an intermediate Cr(VI) ester precursor of the slow redox step. The equilibrium constant for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer. α-D-Glucose forms the Cr(VI)-Glc ester with a higher equilibrium constant than β-D-glucose, but the electron transfer within this complex is slower than for the β anomer. The difference is attributed to the better chelating ability of the 1,2-cis-diolate moiety of the α anomer. The Cr(V) species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of Cr(VI). The EPR spectra show that the α anomer forms several linkage isomers of the five-coordinate Cr(V) bis-chelate, while β-D- glucose affords a mixture of six-coordinate Cr(V) mono-chelate and five- coordinate Cr(V) bis-chelate. The conversion of the Cr(V) mono- to bis- chelate is discussed in terms of the ability of the 1,2-cis- versus 1,2- trans-diolate moieties of the glucose anomers to bind Cr(V). (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0008-6215(99)00282-7

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Carbohydrate Research 324 (2000) 127–135
The reduction of CrVI to CrIII by the a and b anomers
of
D
-glucose in dimethyl sulfoxide. A comparative
kinetic and mechanistic study
Sandra Signorella, Rube´n Lafarga, Vero´nica Daier, Luis F. Sala *
Departamento de Qu´ımica, Facultad de Ciencias Bioqu´ımicas y Farmace´uticas, UNR, Suipacha 531,
2000 Rosario, Argentina
Received 6 July 1999; accepted 18 October 1999
Abstract
The reduction of CrVI by a-
D
-glucose and b-D-glucose was studied in dimethyl sulfoxide in the presence of
pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers
reduce CrVI by formation of an intermediate CrVI ester precursor of the slow redox step. The equilibrium constant
for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer.
a-
D
-Glucose forms the CrVI–Glc ester with a higher equilibrium constant than b-
D-glucose, but the electron transfer
within this complex is slower than for the b anomer. The difference is attributed to the better chelating ability of the
1,2-cis-diolate moiety of the a anomer. The CrV species, generated in the reaction mixture, reacts with the two
anomers at a rate comparable with that of CrVI. The EPR spectra show that the a anomer forms several linkage
isomers of the five-coordinate CrV bis-chelate, while b- -glucose affords a mixture of six-coordinate CrV mono-
D
chelate and five-coordinate CrV bis-chelate. The conversion of the CrV mono- to bis-chelate is discussed in terms of
the ability of the 1,2-cis- versus 1,2-trans-diolate moieties of the glucose anomers to bind CrV. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: Glucose anomers; Chromium; Redox mechanism
tion agents towards hypervalent chromium
1. Introduction
and can stabilize the labile oxidation states of
chromium [9–15]. For this reason, it is of
interest to look at the ability of monosaccha-
rides to reduce CrVI to CrIII, a relevant aspect
of the transport through soils of metal ions
causing environmental pollution.
Chromium is a well-known genotoxic and
carcinogenic metal [1–3]. The biological re-
duction of CrVI to lower states has been ob-
served with a wide variety of naturally
occurring cellular reductants [4–8]. Ligands
that possess two oxygen atoms able to form
five-membered rings about the metal ion, such
as 1,2-diols and a-hydroxy acids, are effective
as non-enzymatic reductants and complexa-
We have examined the reactions of CrVI
with low-molecular-weight molecules [16–25].
Our previous studies on the reduction of CrVI
by aldoses [16,26,27] and deoxyaldoses
[16,19,21,26,27] afforded information on the
relative abilities of these monosaccharides to
reduce chromate, and the steric factors influ-
encing the rate of oxidation of monosaccha-
* Corresponding author. Fax: +54-341-435-0214.
E-mail address: inquibir@satlink.com (L.F. Sala)
0008-6215/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(99)00282-7

128
S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
rides by CrVI and by the intermediate CrV
formed during the reaction course [27,28].
For all the aldoses studied, the C-1–OH
hemiacetalic function reacts faster than the
primary or any of the secondary alcoholic
groups, and the aldonic acid (and/or the cor-
responding lactone) is the only reaction
product when a ten-fold or higher aldose to
CrVI ratio is used. An important point to be
addressed in the chromic oxidation of these
monosaccharides is the reactivity of the indi-
vidual anomers. Under the acid conditions
used in all the previous studies, the a and b
anomers of the sugars equilibrate much faster
than they react with CrVI, and so all of the
reported results refer to the rates of chromic
oxidation of equilibrium solutions of the sug-
ars. In this work, we evaluate the CrVI oxida-
tion rates of the individual anomers of
generated with a Bruker 04 ER generator and
measured with a Racal-Dana frequency meter.
The magnetic field was measured with a
Bruker NMR-probe gaussmeter.
Spectrophotometric measurements.—The ki-
netics measurements were made at 350 nm, by
monitoring the absorbance changes on a Jasco
V530 spectrophotometer with fully ther-
mostatted cell compartments. Reactant solu-
tions were previously thermostatted and
transferred into a cell of 1 cm path-length
immediately after mixing, and disappearance
of CrVI was monitored until at least 80%
conversion. Experiments were performed at
30 °C unless otherwise stated. In most experi-
ments the concentration of CrVI was kept con-
stant at 8×10−4 M, while the sugar
concentration was varied from 0.05 to 0.7 M.
The first-order dependence of the rate upon
[CrVI] was verified for both anomers. The
pseudo-first-order rate constants were calcu-
lated at various [CrVI]0, but at constant tem-
perature, [Glc]0 and [PPTS]. As expected on
the basis of a −d(ln[CrVI])/dt=kobs rate law
where kobs =f([Glc][PPTS]), kobs was found to
be essentially constant with increasing [CrVI]0.
During the reaction course, the band at 350
nm decreased in intensity while a new band at
618 nm grew in (Fig. 1). At the end of the
reaction, two absorbance bands ascribed to
CrIII d–d transitions at umax =445 nm (m=
28.8 M−1 cm−1) and 618 nm (m=30.4 M−1
cm−1) were observed for both anomers. These
spectral maxima and intensities are in close
agreement with those observed for Me2SO
solutions of the Cr3+ ion.
Product analysis.—Qualitative identifica-
tion of gluconic acid as the reaction product
was carried out by HPLC. The chro-
matograms were obtained on a KNK-500 A
chromatograph equipped with a 7125 HPLC
pump and a 115 Gilson UV–Vis detector. The
separation was carried out on an S5 amine
resin Spherisorb HPLC column using a 73:27
mixture of acetonitrile–0.1 M buffer phos-
phate (pH 6.4) as eluent, and a flow rate of 1.0
mL min−1 at T=30 °C. For the reactant
concentrations used in the kinetic measure-
ments performed in Me2SO (60 times excess of
Glc over CrVI and [PPTS]=0.02 M), gluconic
acid (gluconate) was the only reaction product
detected by this technique and it was identified
D-glucose (Glc) in a medium where the mu-
tarotation of the sugar is slower than the
electron-transfer reaction. This study pro-
vides, for the first time, information on the
relative abilities of the a and b anomers to
reduce CrVI to CrIII.
2. Experimental
Materials.—Pure a-
(c 5, water); mp 145.5 °C) and b-
([h]2D0+18.9° (c 5, water); mp 149 °C) were
prepared from pure -glucose according to
D
-glucose ([h]2D0+110°
-glucose
D
D
the method described by Hudson and Dale
[29]. Water was purified by deionization, fol-
lowed by double distillation from a solution of
potassium permanganate. Dimethyl sulfoxide
(Me2SO) was purified before use [30].
Na[Cr(O)(ehba)2]·H2O was synthesized from
2-ethyl-2-hydroxybutanoic
acid
(Aldrich
grade) and sodium dichromate (E. Merck) in
acetone (E. Merck, A. R. grade) according to
the literature method [31]. Pyridinium p-tolue-
nesulfonate (PPTS) (mp=11991 °C) was
synthesized according to a previously de-
scribed method [32].
EPR measurements.—X-Band (ꢀ9.7 GHz)
EPR spectra were recorded as the first deriva-
tive of absorption, at 298 K, on a Bruker ESP
300E EPR spectrometer with 100 kHz field
modulation. The microwave frequency was

S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
129
Fig. 1. Time evolution of the UV–Vis spectrum of a mixture of a-Glc (0.555 M) and CrVI (8×10−4 M). [PPTS]=0.02 M;
T=30 °C, over a period of ꢀ60 min.
by comparison against an authentic sample.
Co-chromatography of the reaction mixture
and gluconate results in the increase of the
peaks of the reaction product appearing at
t=6%05%% and 7%40%%, which correspond to the
open acid and the 1,4-lactone forms of glu-
conate. Neither glucuronic acid nor ara-
binonic acid was observed.
Rate studies.—The mutarotation rate con-
stants were measured by monitoring the opti-
cal rotation changes on a Jena polarimeter
with thermostatted 20 cm tubes using the
sodium D line. Reactant solutions were previ-
ously thermostatted and transferred into the
cell immediately after mixing. Experiments
were performed at 25 °C. The mutarotation of
Glc obeys first-order kinetics under conditions
investigated and rate constants were deter-
mined from the slope of plots of log(rt−rꢁ)
versus time [33]. The calculated rate constants
k=ka+kb for various acid conditions are
summarized in Table 1. In multiple measure-
ments, the reproducibility of the rate con-
stants is better than 5%.
3. Results and discussion
Stability of reactants.—The CrVI stability in
Me2SO and Me2SO–PPTS solutions was
checked by UV–Vis spectroscopy. In Me2SO,
the spectrum of CrVI shows a band at 350 nm
(m=1314 M−1 cm−1) and a shoulder at 450
nm (m=175 M−1 cm−1). In 0.02 M PPTS, the
molar absorptivity coefficient of the band at
350 nm decreases to 938 M−1 cm−1. UV–Vis
spectra registered at different times after the
preparation of the solutions in 0.02 M PPTS
in Me2SO showed identical umax and molar
absorptivity coefficients as freshly prepared
solutions. The stability of Glc in Me2SO and
Me2SO–PPTS solutions was checked by
HPLC. Chromatograms registered at different
times after the preparation of the solutions
were identical to those of the freshly prepared
Glc solution.
Table 1
Values of rate constants (ka+kb) for the mutarotation of
a-Glclb-Glc in aqueous HClO4 and Me2SO–PPTS media a
104 (ka+kb) (s−1
)
0.036 M PPTS (Me2SO) b
0.08 M HClO4 (H2O) c
0.1 M HClO4 (H2O) c
0.2 M HClO4 (H2O) c
0.27(1)
7.4(4)
9.4(1)
19.5(1)
a T=25 °C.
b [Glc]=0.52 M.
c [Glc]=0.278 M.

130
S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
Fig. 2. Absorbance vs. time curve for the chromic oxidation of b-Glc, [CrVI]=8×10−4 M, [b-Glc]=0.555 M, [PPTS]=0.02 M,
T=30 °C at u=350 nm. (Õ) Experimental, (—) calculated.
6
5
The absorbance versus time curves at 350
nm for the chromic oxidation of a-Glc and
b-Glc exhibit a decrease of absorbance, which
cannot be described by a single-exponential
decay. At this wavelength, kinetic traces show
an initial deviation from first-order decay over
short time periods, which could be adequately
described by the following set of consecutive
+Abs8{1+[(k5−k6)e−2k t −k6e−k t
]
×/(2k6 −k5)}
(6)
The molar absorptivity of CrV at 350 nm (mV)
was assumed to be the same as for the com-
plex [CrV(O)(ehba)2]− in Me2SO (1390 M−1
cm−1). Parameters k6 and k5 refer to the rate
of disappearance of CrVI and CrV, respec-
tively, and were evaluated from a nonlinear
iterative computer fit of Eq. (6). Fig. 2 shows
a typical curve for one run at 350 nm and the
curve fit according to Eq. (6). The calculated
kinetic parameters, k6 and k5, for various con-
centrations of Glc in 0.02 M PPTS, are sum-
marized in Table 2. In multiple measurements,
the reproducibility of the two rate constants is
better than 10%.
first-order reactions:
slow
Glc+CrVI “gluconate+CrIV
(1)
(2)
(3)
(4)
IV
III
’
Glc+Cr “Glc +Cr
VI
V
’
Glc +Cr “gluconate+Cr
Glc+CrVs“lowgluconate+CrIII
If intermediate CrV species and the final CrIII
species superimpose CrVI absorbance, the ab-
sorbance at 350 nm, at any time during the
reaction, is given by:
CrVI oxidation of h-Glc and i-Glc.—The
oxidation of
D
-glucose by CrVI in acid
aqueous solution yields gluconic acid and
Cr3+ as final products when an excess of
sugar over CrVI is used. In 0.75 M HClO4, k6
values are in the range 1.7–7.3×10−4 s−1
(Glc to CrVI ratios of 25–250:1) at 33 °C, and
the redox reaction rate increases with increas-
ing H+ concentration [27]. The mutarotation
rates in HClO4 (Table 1) are higher than the
Abs350={mVI[CrVI]+mV[CrV]+mIII[CrIII]}l
(5)
Combining Eq. (5) with rate expressions [34]
derived from Eqs. (1)–(4), yields:
Abs350=Abs0e−k t
6
+k6mV[CrVI]0(e−k t −e−2k t)/(2k6 −k5)
5
6

S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
131
corresponding redox rates at the same H+
Table 3
Kinetic parameters independent of the [Glc] a
concentration. Thus, it is not possible to mea-
sure the redox reaction rates of the individual
anomers because they equilibrate faster than
they react with CrVI. We found that in
Me2SO, the redox rates become faster than the
mutarotation ones in the presence of PPTS as
a-Glc
b-Glc
b
k (s−1
)
1.0(4)×10−3
35(5)
1.3(1)×10−3
K (M−1
)
8(2)
b
k% (s−1
K% (M−1
k%% (M−1 s−1
)
1.8(2)×10−3
2.9(6)
c
c
)
d
)
3.3(1)×10−3
Table 2
Observed pseudo-first-order rate constants (k6 and k5) for the
a T=30 °C; [CrVI]0=8.0×10−4 M; [PPTS]=2.0×10−2 M.
b Calculated from Eq. (7).
chromic oxidation of a- and b-D
-glucose a
c Calculated from Eq. (10).
[Glc] (M)
k6 (s−1) and k5 (s−1
)
d Calculated from Eq. (11).
a-Glc
104k6
b-Glc
104k6
the proton source (Table 2).
104k5
104k5
For the two anomers, in 0.02 M PPTS,
plots of k6 versus [h-Glc] or [b-Glc] show
kinetic profiles yielding saturation curves (Fig.
3). The effect of the Glc concentration on k6 is
described by:
0.0278
0.0555
0.1388
0.2777
0.4166
0.5555
0.6944
3.88(1)
7.2(3)
8.57(7)
8.84(9)
8.8(4)
0.83(1)
2.52(4)
4.84(6)
8.5(3)
11.5(2)
11.2(1)
11.3(1)
4.64(4)
6.79(8)
8.98(4)
8.50(8)
11.7(2)
11.2(3)
1.11(5)
4.30(9)
11.1(3)
13.7(1)
17.5((1)
22.7(2)
k6=kK[Glc]/(1+K[Glc])
(7)
9.87(1)
9.20(1)
from which values of k and K were deter-
mined (Table 3). Thus, the experimental rate
law is expressed by:
a T=30 °C; [CrVI]0=8.0×10−4 M; [PPTS]=2.0×10−2 M.
−d[CrVI]/dt=k6[CrVI]T
=kK[Glc][CrVI]T/(1+K[Glc])
(8)
where [CrVI]T represents the total CrVI concen-
tration. This kinetic law indicates that any of
the anomers form an intermediate complex
precursor of the redox steps as illustrated in
Eq. (9):
K
CrVI+Glc X [CrVI(O)2(GlcH−2(Me2SO)2]
k
“gluconate+CrVI
(9)
It is known that the chromic oxidations are
preceded by the formation of a five-membered
chelate chromate (the five-membered chro-
mate esters are the most favorably formed)
within which the electron-transfer process
takes place [35,36]. In our case, this intermedi-
ate complex may be interpreted as
a
monochelate [14] with Glc acting as a biden-
tate ligand bound to CrVI at the 1,2 (or 2,3 or
3,4)-diolate moiety to yield the species
[Cr(O)2(GlcH−2)(Me2SO)2]. These three link-
age isomers might be formed by coordination
of the sugar with CrVI via any pair of vicinal
hydroxyl groups. However, considering that
Fig. 3. Effect of [Glc] on (a) k6 and (b) k5, at 30 °C and
[CrVI]=8×10−4 M, [PPTS]=0.02 M.

132
S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
CrV oxidation of h-Glc and i-Glc.—The
observation of CrV in the oxidation of Glc by
CrVI is explained as follows. CrIV formed in
the slow redox step (Eq. (1)) reacts with excess
only the hemiacetalic function is oxidized, it
seems reasonable to think that the complex
with the anomeric hydroxyl group bound to
CrVI should be the precursor of the slow redox
steps. Thus, K in the kinetic law refers to the
formation of all the three isomers, and k
should refer to the electron-transfer step from
the total complex [Cr(O)2(GlcH−2)(Me2SO)2].
It is also known that a cis-diolate binds hyper-
valent chromium stronger than a trans-diolate
and the hemiacetalic oxygen atom coordinates
CrVI stronger than the carbinolic oxygen
atoms [37]. Calculated K values for a-Glc are
4.4-times higher than for b-Glc, a difference
which account for the stronger chelation of
the cis-1,2-diolate moiety in the a anomer. In
contrast, the redox steps are faster for the b
anomer than the a anomer.
Glc to yield CrIII and the sugar radical in a
’
fast step (Eq. (2)). The formation of a sugar
radical in the reaction of CrVI with monosac-
charides has been previously demonstrated
[26,27]. The rapid reaction of the radical with
CrVI yields CrV, which further oxidizes Glc to
yield the final redox products (Eqs. (3) and
(4)).
Under the conditions used in the kinetic
measurements, a-Glc and b-Glc are oxidized
at a comparable rate by CrVI and CrV (Table
2). For the a anomer, the effect of the [a-Glc]
on k5 (Fig. 3) is described by Eq. (10):
k5=k%K%[a-Glc]/(1+K%[a-Glc])
(10)
Values of k% and K% (Table 3) reveal that the
constant for the formation of the intermediate
CrV complex is higher than the redox rate, and
the intramolecular electron-transfer process
within this CrV complex is twice as fast as for
the a-Glc–CrVI intermediate complex. In the
case of b-Glc, k5 varies linearly with [b-Glc]
(Fig. 3), probably because the intermediate
CrV species is formed slower than it reacts and
simple second-order kinetics are observed:
k5=k%%[b-Glc]
(11)
Again, k%% is twice as fast as the electron-trans-
fer within the b-Glc–CrVI complex.
Structure of intermediate CrV species.—In
the PPTS–Me2SO mixtures used in the kinet-
ics measurements, but with a higher initial
[CrVI] — required for the detection of the
EPR signal — the reaction of CrVI with excess
a-Glc generates three EPR signals at g1=
1.9820, g2=1.9785 and g3=1.9756 (Fig. 4).
The signals are broad (with a 3-5 G line
width) and prevent the resolution of the pro-
ton superhyperfine (shf) coupling. The giso val-
ues are informative on the CrV coordination
number and the ligand bound to CrV [38], and
are relatively insensitive to the nature of the
solvent [39]. The observed giso values are in the
range of those expected for five-coordinate
oxochromate(V) complexes and they corre-
spond to different linkage isomers of the bis-
Fig. 4. X-band EPR signal of a mixture of a-Glc (0.555 M)
and CrVI (0.01 M) in 0.02 M PPTS–Me2SO and T=25 °C,
modulation amplitude=0.408 G, center field=3500 G; fre-
quency=9.6947 GHz. (a) 3 min; (b) 2 h; (c) 2 days, after the
beginning of the reaction.

S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
133
nate oxochromate(V) and the Aiso =
15.8×10−4 cm−1 observed for the remaining
CrV signal (g1) confirms the assignation. How-
ever, the g4 value is significantly lower than
the values observed for the five-coordinate
oxochromate(V) complexes and, since the g
values of the CrV species are very sensitive to
coordination [38], it may be assigned to a
six-coordinate
oxochromate(V)
complex
[CrV(O)(GlcH-2)(Me2SO)3]+, with only one
Glc acting as a bidentate ligand via any pair
of vicinal alcoholate groups. This six-coordi-
nate species is not observed in the reaction of
a-Glc with CrVI. Interestingly, under condi-
tions used in the kinetic measurements
(PPTS:CrVI ratios higher than used in the
EPR measurements), the CrVI is totally re-
duced to CrIII (no CrV is observed after the
total CrVI consumption). However, in the ex-
periments performed under the EPR condi-
tions (2:1 PPTS:CrVI ratio), 2 days after the
beginning of the reaction, CrIII is not formed
in a measurable quantity. The EPR results
provide us with two interesting points of evi-
dence. The first is that the electron-transfer
reaction within the [CrV(O)(GlcH−2)2]− inter-
mediate complexes requires protons to take
place. The second is that PPTS favors the
formation of the six-coordinate CrV species
[CrV(O)(GlcH−2)(Me2SO)3]+ and that this
complex disappears when PPTS is consumed.
In previous work performed in water, we had
observed that the six-coordinate complex is
only detected when high [HClO4] is used, but
this species disappears and the five-coordinate
CrV species stabilize at higher pH. Now we
have additional information on this species: it
is observed only when the b anomer is used.
This means that the a anomer does not form
this intermediate species, or else it forms but
disappears faster than it forms and it cannot
be detected. The difference may account as
evidence for the binding of the trans-1,2-dio-
late moiety of the b anomer versus the cis-1,2-
diolate moiety of the a anomer. The latter is
expected to afford a bis-chelate easier than the
former because of the enhanced ability of the
cis-1,2-diolate to bind CrV.
Fig. 5. X-band EPR signal of a mixture of b-Glc (0.555 M)
and CrVI (0.01 M) in 0.02 M PPTS–Me2SO and T=25 °C,
modulation amplitude=0.408 G, center field=3500 G; fre-
quency=9.6945 GHz. (a) 6 min; (b) 80 min; (c) 2 days, after
the beginning of the reaction.
chelate [CrV(O)(GlcH-2)2]− formed with two
Glc molecules bound to CrV via the 1,2(2,3-
and/or 3,4-)-diolate moieties [38]. The relative
intensity of the three CrV intermediate species
varies with time (Fig. 4(b)) and after a few
hours the species giving rise to the signal at g1
is the major one and remains in solution for
several days (Fig. 4(c)). For this signal (g1), it
is possible to observe the four weak 53Cr
(9.55% abundance, I=3/2) hyperfine peaks at
15.9×10−4 cm−1 spacing, typical of five-co-
ordinate oxochromate(V) complexes [38,40].
In the reaction of CrVI with b-Glc, the EPR
spectrum consists of four signals at g1=
1.9820, g=1.9789, g3=1.9767 and g4=
1.9700. Again, these signals evolve differently
with time and the species at g1 accumulates in
the reaction mixture while the other species
decrease but with different rates (Fig. 5). Sig-
nals at g1–3 have values typical of five-coordi-
Scheme 1 summarizes the EPR results. In
the scheme, B represents the mixture of five-

134
S. Signorella et al. / Carbohydrate Research 324 (2000) 127–135
Foundation for Sciences (IFS), the National
Agency for Sciences Promotion and the AN-
TORCHAS Foundation for financial support.
Thanks are also due to J.C. Gonzalez for
HPLC measurements.
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require PPTS to afford the final redox prod-
ucts, but the intramolecular electron-transfer
steps within B are favored by the presence of
PPTS. In the absence of PPTS, the redox
reaction is extremely slow and the five-coordi-
nate CrV isomers (B) convert to the CrV spe-
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case of the a anomer, the equilibrium AlB is
displaced towards the formation of the CrV
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The two anomers of Glc reduce CrVI and
CrV at similar rates. However, these rates de-
pend on the equilibrium constant (K) for the
formation of the intermediate Glc–CrVI (and
Glc–CrV) complex and the rate of the subse-
quent intramolecular electron-transfer (k),
which differ from one anomer to the other.
The higher K(K%) value for a-Glc reflects that
in this anomer the 1,2-cis-diolate moiety fa-
vors the CrVI (CrV) chelation, and conse-
quently, the activation barrier for the redox
step is higher than for the intermediate chelate
formed with the b anomer. Both effects com-
pensate each other to afford the observed
kinetic constants (k5 and k6) of the same
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The authors thank the National Research
Council of Argentina (CONICET), the Third
World Academy of Sciences (TWAS), the Na-
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