Reactivity of some sugars and sugar phosphates towards gold(III) in sodium acetate-acetic acid buffer medium

Article abstract of DOI:10.1016/S0008-6215(00)00181-6

The kinetics of the oxidation of some aldoses and aldose phosphates have been studied spectrophotometrically in sodium acetate-acetic acid buffer medium at different temperatures. The reactions are first order with respect to [Au(III)] and [substrate]. Both H⁺ and Cl^- ions retard the reaction. The reactions appear to involve different gold(III) species, viz. AuCl₄/^-, AuCl₃(OH₂) and AuCl₃(OH)^-. The results are interpreted in terms of the probable intermediate formation of free radicals and Au(II). Aldoses react with gold(III) in the order: triose > tetrose > pentose > hexose. The sugar phosphates react with gold(III) at a faster rate than the parent sugars except glucose-1-phosphate, which reacts at slower rates than glucose. A tentative reaction mechanism leading to the formation of products has been suggested.

Full text of DOI:10.1016/S0008-6215(00)00181-6

www.elsevier.nl/locate/carres
Carbohydrate Research 330 (2001) 115–123
Reactivity of some sugars and sugar phosphates towards
gold(III) in sodium acetate–acetic acid buffer medium
Kalyan Kali Sen Gupta,* Biswajit Pal, Bilkis Ara Begum
Department of Chemistry, Jada6pur Uni6ersity, Calcutta 700 032, India
Received 8 November 1999; accepted 6 June 2000
Abstract
The kinetics of the oxidation of some aldoses and aldose phosphates have been studied spectrophotometrically in
sodium acetate–acetic acid buffer medium at different temperatures. The reactions are first order with respect to
+
−
[
Au(III)] and [substrate]. Both H and Cl ions retard the reaction. The reactions appear to involve different
−
−
gold(III) species, viz. AuCl , AuCl (OH ) and AuCl (OH) . The results are interpreted in terms of the probable
4
3
2
3
intermediate formation of free radicals and Au(II). Aldoses react with gold(III) in the order: triose\tetrose\pen-
tose\hexose. The sugar phosphates react with gold(III) at a faster rate than the parent sugars except glucose-1-
phosphate, which reacts at slower rates than glucose. A tentative reaction mechanism leading to the formation of
products has been suggested. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Kinetics; Sodium acetate–acetic acid; Gold(III); Sugars and sugar phosphates
1
7
sion of rheumatoid arthritis. However, the
mechanism of the reactions involving gold(III)
with biologically important organic molecules
is yet to be understood. The present investiga-
tion on the reduction of gold(III) by aldoses
and sugar phosphates in sodium acetate–ace-
tic acid medium was carried out in order to
throw light upon the reactivity of aldoses and
aldose phosphates towards gold(III).
1
. Introduction
Trivalent gold are mostly complexes and are
powerful oxidising agents. The redox potential
3
+
+
for the system (Au +2e X Au ) is about
1
1
.29 V at 298 K. There have been compara-
tively few reports on the kinetics of oxidation
by gold(III). Although the kinetics of oxida-
2
–13
tion of some aldoses
by some oxidants
have been published, there are no literature
data involving the oxidation of aldoses and
sugar phosphates by gold(III). Gold com-
2. Experimental
14,15
pounds have been used
in medicine for
centuries, an application known as chrysother-
apy. However, complexes of gold have also
been used most successfully to treat arthritic
disorders in humans and other animals. Au(I)
compounds are currently the only class of
pharmaceuticals known to halt the progres-
Reagents.—Aldoses were obtained from
BDH, E. Merck, or Sigma.
phate, -glucose 6-phosphate,
phosphate, -erythrose 4-phosphate and
D
-Glucose 1-phos-
1
6
D
D
-ribose 5-
D
DL-glyceraldehyde 3-phosphate were of Sigma
grade. Gold(III) was prepared using HAuCl4
Johnson Matthey) in a pyrex stoppered bottle
and the concentration of gold(III) was esti-
(
*
Corresponding author.
0
008-6215/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(00)00181-6

1
16
K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
18
mated gravimetrically
as follows: To a
Stoichiometry and product analysis.—The
known volume (5 mL) of the standard gold
solution in a 150 mL beaker, was added
dropwise with stirring, a 1 N solution of
KOH until the yellow color was just dis-
charged. A further 2 mL of KOH solution
was added, followed by dilution to ca. 75
mL with distilled water. Then 5–6 mL of 1
N oxalic acid solution was added dropwise
with stirring, and the contents were kept on
a boiling water bath. The solution turned
purple with a violet tinge when gold sepa-
rated out. The beaker was kept on the water
bath for about an hour with occasional stir-
ring. The solution was filtered through a
Whatman No. 42 (7 cm) filter paper. The
precipitate was washed with distilled water,
dried, ignited in a silica crucible and weighed
as metal. The solution after estimation was
stored in the dark. The other materials em-
ployed were of the highest purity available.
All solutions were made in doubly distilled
reactions occur according to the following
stoichiometric equation:
RCHO+Au(III)+H O
2
+
“RCOOH+Au(I)+2H
(1)
where, R=ꢀ(CHOH) CH OH
4
2
After the kinetic experiments, the reaction
mixtures of the aldoses and the aldose phos-
phates were acidified and treated with 2,4-dini-
trophenylhydrazine
hydrochloride.
The
absence of any yellow precipitate (except in the
case of glucose 1-phosphate) indicated that
neither the ꢀCH OH nor the ꢁCHOH group is
2
oxidized. On the other hand, iron(III) chloride
solution that had been colored violet with
phenol and the violet-colored compound when
added to the reaction mixture gave a bright
2
yellow coloration. This indicates that aldonic
acids are formed in the oxidation of the aldoses.
Similar yellow coloration has also been ob-
tained in the oxidation products of aldose
phosphates (other than glucose 1-phosphate)
19
water. Buffer solutions were prepared from
a standard solution of sodium acetate and
acetic acid. The pH of the solution was
checked against standard buffer solution
with an Elico (LI 120) pH meter (India).
Kinetic measurements.—Gold(III) has a
strong absorption in the 280–355 nm region
with an absorption maximum at 313 nm
1
3
with the above-mentioned reagents. The
C
NMR spectrum of the oxidation product with
glucose in D O was recorded on a Bruker
2
DPX-300 Spectrometer (75 MHz), and the
2
1
appearance of a peak at 178.8 ppm, confirmed
the presence of the ꢀCOOH group in the
oxidation product. The oxidation product of
glucose 1-phosphate when treated with 2,4-
DNP gave a yellow precipitate. The yellow
derivative was filtered, washed several times
with water, dried and then analyzed: mp 175–
3
−1
−1
(
‡max=4.86×10
M
cm ).
When
4
aqueous solutions of AuCl or HAuCl are
exposed to UV light, colloidal gold is
3
2
0
formed, which may be enhanced in the
presence of reducing species. Consequently,
the reactions were monitored at u=400 nm
using higher gold(III) concentrations in or-
der to minimize error due to colloidal gold
formation. The kinetic investigations were
carried out in a Systronics (India) UV–Vis
spectrophotometer using a thermostated cell
of 1 cm path length. The water was circu-
lated from the bath maintained at the re-
2
2
178 °C, lit. mp 175–180 °C, Anal. Calcd for
C H N O P: C, 32.9; H, 3.42; N, 12.78.
1
2
15
4
12
Found: C, 33.1; H, 3.47; N, 12.8. These results
indicate that the ꢀCH OH group of glucose
2
1-phosphate is oxidized to ꢀCHO. After the
experiments, part of the reaction mixture was
treated with excess NaOH when a greyish-
2
3
violet solid was obtained that was insoluble in
H SO but soluble in HCl. On the other hand,
quired
temperature
(90.1 °C).
The
2
4
pseudo-first-order rate constants (k ) were
the addition of NaOH to gold(III) failed to give
any such greyish-violet precipitation. Gold(I)
was found to be product of reduction of
gold(III).
Test for free radicals.—Involvement of
free radicals in the reaction mixture has been
tested by the following experiments.
obs
obtained from linear log A (A=absorbance)
versus time plots. The linearity further indi-
cated that the redox reaction is not compli-
cated by the formation of colloidal gold.
The kobs values were reproducible to
within95%.

K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
117
1
. Using HgCl : When mercuric chloride so-
Blank experiments in which either the oxi-
dant or the substrates was excluded gave
no polymeric suspension. Cationic and an-
ionic polymerizations have been dis-
counted under the conditions at which the
reactions were studied. All these indicate
that free radicals are generated during the
reactions, and a one-step two-electron
transfer process in the rate-determining
step has been ruled out.
2
lution was added to the reaction mixture,
an immediate precipitation of mercurous
chloride occurs. This is to be expected if a
reducing intermediate like free radicals is
2
4
generated. HgCl failed to give Hg Cl
2
2
2
when added to the substrate solutions.
2
. Using acrylamide: Acrylamide (40% w/v)
was added during the course of the reac-
tions. An immediate haziness appeared
during the oxidation of the substrates by
gold(III). When an excess of methanol was
added to the reaction mixture, a thick pre-
cipitate of polyacrylamide was formed.
3
. Results and discussion
Table 1
Effect of reactant concentrations.—The re-
a
Effect of [Au(III)] on the rate of oxidation at 298 K
actions were studied at varying concentrations
−
of Au(III) but at constant [substrate], [Cl ],
3
−1
Substrate
kobs×10 (s
)
pH and temperature. The pseudo-first-order
rate constant is independent of oxidant con-
centration in each reaction (Table 1).
D
D
D
-Glucose
-Ribose
-Erythrose
1.8890.06
2.4090.08
3.3590.1
4.5690.2
1.4890.05
6.0290.2
7.0490.3
9.4290.1
12.690.07
The reactions were also studied at four dif-
ferent temperatures with variation of [sub-
DL-Glyceraldehyde
D
D
D
D
-Glucose 1-phosphate
-Glucose 6-phosphate
-Ribose 5-phosphate
-Erythrose 4-phosphate
−3
−3
strate] from 0.6×10
to 5.0×10
mol
−
3
−
dm , keeping [Au(III)], [Cl ] and pH con-
−
4
−3
−3
stant at 3.0×10 , 1.96×10
and 3.72, respectively. The plot of kobs against
substrate] was found to be linear, passing
through the origin at each temperature (Fig.
), indicating that kinetic evidence for inter-
mol dm
DL-Glyceraldehyde 3-phosphate
[
a
−3
−3
[
Substrate]=2.0×10
mol dm , [Cl ]=1.96×10 mol dm , pH 3.72.
mol dm , [Au(III)]=(0.5–3)×
−
4
−3
−
−3
−3
10
1
mediate complex formation between the reac-
tants is insignificant.
+
Effect of [H ].—The rate of the reaction
was studied at 298 K but at different pH
values (3.42–4.45) using NaOAc–AcOH
buffer while keeping [Au(III)], [substrate] and
−
[
Cl ] constant. No attempt was made to keep
ionic strength constant as the values of kobs
remain unchanged at different ionic strengths,
−
1
−3
(
0.3–2.0)×10
mol dm , varied by the
addition of sodium perchlorate. The oxidation
+
rate decreases with an increase in [H ]. The
+
plot of kobs versus 1/[H ] is linear, making an
intercept on the y-axis (Fig. 2).
−
Effect of [Cl ].—The reaction was studied
varying [NaCl] but at constant [Au(III)], [sub-
Fig. 1. Variation of the reaction rate constant with [substrate].
−
4
−
4
strate], pH and temperature of 3×10 , 2×
−3 −3
Plots of k
vs. [substrate] at [Au(III)]=3.0×10
mol
pH 3.72 and tempera-
-ribose; (c) -erythrose; (d)
-glucose 1-phosphate; (f) -glucose
-ribose 5-phosphate; (h) -erythrose 4-
phosphate; (i) DL-glyceraldehyde 3-phosphate.
obs
−
−
3
−3
−3
dm , [Cl ]=1.96×10
ture 298 K. (a) -Glucose; (b)
DL-glyceraldehyde; (e)
-phosphate; (g)
mol dm
10
mol dm , 3.72 and 298 K. The
D
D
D
oxidation rate decreases with an increase of
D
D
−
−
[
Cl ]. A plot of k versus 1/[Cl ] is linear
6
D
D
obs
making an intercept on the y-axis (Fig. 3).

1
18
K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
(
1/T) (Fig. 4), followed by entropy of activa-
"
tion (DS ) as mentioned in an earlier commu-
nication.
1
1,12
The activation parameters are
recorded in Table 2.
The enthalpy of activation is linearly related
2
5
with the entropy of activation (r=0.9857,
Fig. 5) and the isokinetic temperature is 325
2
6
K. The isokinetic behaviour supported by
the linear plot of log k% versus log k (r=
0
.9794), where k% and k are the second-order
rate constants at the temperatures 298 and 293
K, respectively (Fig. 5). The isokinetic temper-
ature was calculated from the relation i=
T T (1−f )/(T −T f ), where f is the slope of
1
2
1
2
+
Fig. 2. Dependence of reaction rate constant on [H ]. Plots of
+
−3
−3
kobs vs. 1/[H ] at [substrate]=2.0×10
mol dm
,
−
4
−3
−
−3
[
Au(III)]=3.0×10
mol dm , [Cl ]=1.96×10
and temperature 298 K. (a) -Glucose; (b) -ribose; (c)
-glucose 1-phos-
-ribose 5-phosphate;
-erythrose 4-phosphate; (i) DL-glyceraldehyde 3-phos-
phate.
mol
−
3
dm
D
D
D
-erythrose; (d) DL-glyceraldehyde; (e)
phate; (f) -glucose 6-phosphate; (g)
h)
D
D
D
(
D
Fig. 4. Influence of temperature on second-order rate con-
stant. Plots of log (k/T) vs. 1/T. (a) -Glucose; (b) -ribose;
c) -erythrose; (d) DL-glyceraldehyde; (e) -glucose 1-phos-
phate; (f) -glucose 6-phosphate, (g) -ribose 5-phosphate,
h) -erythrose 4-phosphate, (i) DL-glyceraldehyde 3-phos-
D
D
(
D
D
D
D
(
D
phate.
Table 2
Values of activation parameters of the oxidations of some
aldoses and sugar phosphates
−
Fig. 3. Dependence of reaction rate constant on [Cl ]. Plots
−
−3
−3
"
"
−1
of kobs vs. 1/[Cl ] at [substrate]=2.0×10
mol dm
mol dm , pH 3.72 and temperature
-Glucose; (b) -ribose; (c) -erythrose; (d) DL
glyceraldehyde; (e) -glucose 1-phosphate; (f) -glucose 6-
phosphate; (g) -ribose 5-phosphate; (h) -erythrose
-phosphate; (i) DL-glyceraldehyde 3-phosphate.
,
Substrate
DH (kJ
DS (JK
−
4
−3
[
2
Au(III)]=3.0×10
98 K. (a)
−1
−1
mol
)
mol
)
D
D
D
-
D
D
D
D
D
-Glucose
-Ribose
-Erythrose
8892
8292
7993
6796
9694
5593
4692
4392
3692
5097
3297
23910
D
D
4
DL-Glyceraldehyde
−14920
73913
−52910
−8097
−8697
−10797
Acti6ation parameters.—The second-order
D
D
D
D
-Glucose 1-phosphate
-Glucose 6-phosphate
-Ribose 5-phosphate
-Erythrose 4-phosphate
rate constants (k) for the oxidation of aldoses
and sugar phosphates by gold(III) were deter-
mined at different temperatures. The enthalpy
DL-Glyceraldehyde
-phosphate
"
of activation (DH ) for the reactions were
3
calculated from the plots of log (k/T) against

K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
119
exists mainly in the cyclic hemiacetal form.
The apparent ionisation constants of -glu-
cose 6-phosphate are known. The pK values
D
3
2
a
(
pKa1 0.94, pKa2 6.11) indicate that it exists
preponderantly in the b-pyranose form, and
the unusually strong ionisation of the first
acidic hydrogen has been attributed to the
formation of a hydrogen bond between the
ring oxygen of the sugar moiety and the sec-
ond acid hydrogen of the phosphate group.
3
2
3
3
Disodium
D
-ribose 5-phosphate exists as a
dinegative anion in aqueous solution, but un-
−
3
der highly acid conditions (2.5 mol dm ),
fast diprotonation to yield -ribose 5-phos-
phate is believed to occur, which also exists
D
3
4
mainly as the furanose form where in-
tramolecular hydrogen bonding involving the
ring oxygen atom and the phosphate group
can occur. Because of intramolecular hydro-
gen bonding between HO-2 and O-1, the a
anomer is expected to be the preponderant
form.
Fig. 5. Isokinetic plots for the oxidations of aldoses and
aldoose phosphates by gold(III) in NaOAc–AcOH buffer
"
"
medium. Plots of (i) DS vs. DH (ii) log k% vs. log k. (1)
-Glucose; (2) -ribose, (3) -erythrose; (4) DL-glyceralde-
hyde; (5) -glucose 1-phosphate; (6) -glucose 6-phosphate;
7) -ribose 5-phosphate; (8) -erythrose 4-phosphate; (9)
DL-glyceraldehyde 3-phosphate.
D
D
D
D
D
(
D
D
D-glucopyranose 1-phosphate (pKa1 1.10,
the Exner plot and i is 329 K. Since the value
of i is above the experimental temperature
pK 6.13) suggests the formation of a hydro-
a2
gen bond between the oxygen of the sugar
moiety in the b configuration and the second
acidic hydrogen atom of the phosphate
(
295), it can be concluded that enthalpy is a
8
controlling factor of the reaction.
3
3
Aldohexoses exist mainly in the pyranoid
group. Again,
D-glucopyranose 1,6-diphos-
2
7,28
form,
whereas in aldopentoses the pyra-
phate practically does not undergo substantial
oxidation under comparable conditions, indi-
cating that C-1 and C-6 are the reactive sites.
The reactive site of glucose 1-phosphate is
believed to be C-6 since C-1 is blocked by the
phosphate group.
noid form remains in equilibrium with the
furanoid form. Though erythrose is known to
29
exist in acyclic form, literature evidence indi-
3
0
cates that it can also exist in furanoid form.
In aqueous solution, the composition of
31
D
-
Dilute solution of tetrachloroauric acid is
erythrose is 25% a-furanose, 63% b-furanose,
3
5–37
known
to involve the following equilibria
ꢀ
2% aldehyde and 10% aldehydrol form.
K
1
Again, crystalline form of DL-glyceraldehyde
is dimeric with a 1,4-dioxane structure, but in
aqueous solution it dissociates into the
monomeric form, which remains in equi-
librium with hydrated and nonhydrated
forms. Since the concentration of water is
constant, the proportions of aldehyde and hy-
drated form are also constant. Consequently,
the reaction has been shown to occur between
the hydrated form and oxidant.
+
−
HAuCl X H +AuCl
(2)
(3)
4
4
K
2
−
−
AuCl +H O X AuCl (OH )+Cl
4
2
3
2
K
3
−
+
AuCl (OH ) X AuCl (OH) +H
(4)
3
2
3
−6
where K , K and K are 1.0, 9.5×10
and
1
2
3
0
.25, respectively, at 298 K. Consequently
−
4
four different species, viz. HAuCl , AuCl ,
4
−
AuCl (OH) and AuCl (OH) , may oxidize
3
2
3
the substrate under the present experimental
+
Monosodium
D-glucose 6-phosphate exists
conditions. HAuCl dissociates to give H
4
−
4
as a mononegative anion in aqueous solution,
but under conditions of high acidity (2.5 mol
and AuCl in the pH range studied. But in a
+
−4
−3
solution of [H ] of ꢀ10
mol dm
,
−
3
dm ), it is rapidly protonated to yield
D
-glu-
[HAuCl ] is practically insignificant compared
4
− −
cose 6-phosphate (Robinson’s ester), which
with [AuCl ], and further aquation of AuCl
4
4

1
20
K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
electron transfer
’
may be prevented if the reactions occur at
RꢀCꢁO+AuCl (OH ) ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢂ RCO H
3
2
2
−
−2
−3
followed by hydrolysis
high [Cl ] of 3×10
mol dm , which has
3
8
−
+
been noticed in the oxidation of a-hydroxy
+AuCl (OH ) +H
(8)
3
2
compounds by gold(III). Since the present re-
k
3
’
−
−
actions have been studied at [Cl ] of 1.96×
RCHO+AuCl (OH) “RꢀCꢁO
3
−
3
−3
slow
10
mol dm , it is suggested that all the
−
4
2−
+
three species, namely AuCl , AuCl (OH ) and
+AuCl (OH) +H
(9)
3
2
3
−
AuCl (OH) participate during the reaction.
electron transfer
3
’
−
RꢀCꢁO+AuCl (OH) ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢂ
This is in keeping with the observations made
3
followed by hydrolysis
3
9
earlier in the oxidation of glycolaldehyde by
gold(III). It may therefore be proposed that
2
−
+
RCO H+AuCl (OH) +H
(10)
2
3
−
4
−
AuCl , AuCl (OH ) and AuCl (OH ) oxidize
the substrates.
3
2
3
Au(II) undergoes fast disproportionation into
Au(I) and Au(III).
36
Gold(III) is known to behave as a one- or
two-electron transfer oxidant, depending upon
the nature of the substrate and the experimen-
tal conditions. In the present investigation, the
reaction mixture gave a polymeric suspension
in the presence of acrylamide, indicating the
involvement of free-radical intermediates dur-
ing the reaction. The formation of unstable
gold(II) as an intermediate has been predicted
fast
2
4
−
−
2
−
4
−
2
2
AuCl “AuCl +AuCl +2 Cl
(11)
fast
−
−
−
AuCl (OH ) “AuCl +AuCl +2 H O
3
2
2
4
2
(
12)
fast
2
−
+
−
2
−
4
2
AuCl (OH ) +2 H “AuCl +AuCl
3
2
40–43
+2 H O
(13)
2
by a number of workers.
Consequently
reduction of gold(III) to gold(I) in a one-step,
two-electron process has been ignored. A free-
radical intermediate may also be produced by
the reaction between substrates with gold(I).
However such a possibility has been ruled out,
since neither colloidal gold nor any precipitate
of gold(0) was detected under kinetic condi-
tions. Therefore gold(II) appears to be more
reactive than gold(I). The reaction steps in-
volving substrates and different species of
gold(III) may be shown as mentioned below
According to the suggested mechanism, the
rate of disappearance of [Au(III)] may be
expressed as
−
=
d[Au(III)]/dt
−
{k [AuCl ]+k [AuCl (OH )]
1
4
2
3
2
−
+
k [AuCl (OH) ]} [S]
(14)
3
3
If C
[AuCl
=[Au(III)], x=[AuCl
3
(OH
)] and y=
0
3
2
−
(OH) ], Eq. (14) may be written as
(
where R=ꢀ(CHOH) CH OH and n=4, 3, 2
n
2
−
d[Au(III)] /dt
t
and 1 for glucose, ribose, erythrose and glyc-
eraldehyde, but R=ꢀ(CHOH) CH OP-
O)(OH)O when n=4, 3, 2 and 1 for six-,
n
2
={k C +(k −k )x+(k −k )y} [S]
(15)
1
0
2
1
3
1
−
(
five-, four- and three-membered rings, respec-
tively).
Again, the value of K
so that
2
indicates that C
0
ꢁx,y,
−
−
k
1
x[Cl ]
x[Cl ]
’
−
2−
4
+
K =
:
RCHO+AuCl “RꢀCꢁO+AuCl +H
2
4
C −x-y
C0
slow
0
(
5)
K C
[Cl ]
2
0
−
electron transfer
’
ƒ x=
−
4
RꢀCꢁO+AuCl ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢂ RCO H
2
followed by hydrolysis
and
2
−
+
+
AuCl +H
(6)
(7)
4
+
−
k
2
y[H ][Cl ]
K K C
[H ][Cl ]
’
2
+
3
0
−
K =
3
or y=
RCHO+AuCl (OH) “RꢀCꢁO
3
2
K C
slow
2
0
−
+
+
AuCl (OH ) +H
Substitution of x and y in Eq. (15) gives
3
2

K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
121
Table 3
Rate-determining steps for the oxidations of the aldoses and sugar phosphates by different gold(III) species at 298 K
a
3
−1 −1
3
−1 −1
3
−1 −1
Substrate
k (dm mol
s
)
k (dm mol
s
)
k (dm mol
s
)
1
2
3
D
D
D
-Glucose
-Ribose
-Erythrose
0.467
0.667
0.867
1.067
0.266
1.30
1.53
1.86
2.03
0.494
0.694
0.935
1.13
0.294
1.50
1.74
2.17
2.44
0.543
0.770
1.00
1.27
0.337
1.52
1.80
2.25
2.61
DL-Glyceraldehyde
D
D
D
D
-Glucose 1-phosphate
-Glucose 6-phosphate
-Ribose 5-phosphate
-Erythrose 4-phosphate
DL-Glyceraldehyde 3-phosphate
a
−4
−3
−3
−3
−
−3
−3
[
Au(III)]=3.0×10 mol dm , [substrate]=2.0×10 mol dm , [Cl ]=1.96×10 mol dm pH 3.72.
1
d[Au(III)]
different experimental conditions. The calcu-
lated and experimental values (Table 4), which
are not widely different, justify the involvement
of three different gold(III) species in the reac-
tions with sugars and sugar phosphates.
k = −
obs
[
Au(III)]
dt
!
(k −k )K
(k −k )K K
1
"
2
1
2
3
1
2
3
=
k1+
+
·
−
−
+
[
Cl ]
[Cl ]
[H ]
(16)
×
[S]
Table 4
Measured and calculated pseudo-first-order rate constants of
the oxidations of some aldoses and sugar phosphates by
Eq. (16) predicts a linear plot of kobs against [S]
passing through the origin at constant [Au(III)],
a
gold(III) at 298 K
+
−
[
H ] and [Cl ]. This has been verified experi-
−
3
−3
mentally (Fig. 1). Eq. (16) further indicates that
the addition of Cl decreases the concentra-
Substrate
k
×10
)
k
(s
×10
cal
obs
−1
−
−1
(
s
)
−
tions of AuCl (OH ) and AuCl (OH) , thereby
3
2
3
D
D
D
-Glucose
-Ribose
-Erythrose
1.87
2.45
3.39
4.59
1.46
5.98
7.02
9.39
1.90
2.65
3.46
4.72
1.43
5.41
6.42
8.63
increasing the concentration of the less reactive
−
4
species, AuCl , and, hence, the retarding effect
−
of the Cl ion on the reaction rate. From the
linear plot of kobs against 1/[Cl ] at a constant
temperature and from the intercept of the plot
Fig. 3), the values of k at 298 K for different
DL-Glyceraldehyde
−
D
D
D
D
-Glucose 1-phosphate
-Glucose 6-phosphate
-Ribose 5-phosphate
-Erythrose 4-phosphate
(
1
substrates were evaluated (Table 3)+.
DL-Glyceraldehyde
3-phosphate
12.3
11.4
The reactions are inhibited by H ions at a
−
constant [Cl ], indicating the fact that
a
−4
−3
−3
[
Au(III)]=3.0×10
mol dm , [substrate]=2.0×10
−
AuCl (OH) is the most reactive amongst dif-
−3
−
−3
−3
3
mol dm , [Cl ]=1.96×10 mol dm pH 3.72.
ferent Au(III) species. The linear plot of k
versus 1/[H ] at 298 K (Fig. 2) conforms to Eq.
obs
+
(
16)). From the intercept and slope of the
straight line and using the value of k obtained
1
earlier, the values of k and k have been
2
3
evaluated at 298 K (Table 3). Of the three
−
different gold(III) species, AuCl (OH) ap-
3
pears to be the most reactive species, and
−
4
AuCl is the least reactive species. Reactivity
of AuCl (OH ) lies between these two species,
3
2
4
4
in keeping with the observation made earlier.
Substituting the values of k , k and k into Eq.
1
2
3
(16) the values of kobs were calculated under
Scheme 1.

1
22
K.K. Sen Gupta et al. / Carbohydrate Research 330 (2001) 115–123
low the order:
D
-glyceraldehyde\
D
-ery-
throse\
D-ribose\
D
-glucose. On the other
hand, the sugar phosphates react with
gold(III) at a faster rate than the parent sug-
ars, except for glucose 1-phosphate, which
reacts at slower rate than glucose, thereby
indicating that sugars are not stronger ligands
for gold(III) in comparison with sugar phos-
phates. The aldose and aldose phosphates (ex-
cept glucose 1-phosphate) are oxidized to give
aldonic acids and phosphoaldonic acids by
similar mechanisms. The reaction product ob-
tained in the oxidation of glucose 6-phosphate
failed to give any 2,4-DNP derivative, indicat-
ing that C-2, C-3 and C-4 are unreactive
towards gold(III) under the kinetic conditions
of these experiments. On the other hand, when
C-1 is blocked by phosphate group, the oxida-
tion product gave a 2,4-DNP derivative. The
higher rate of oxidation of glucose 6-phos-
phate than glucose 1-phosphate is to be ex-
pected since C-1 has a potential ꢀCHO group
Scheme 2.
whereas C-6 possesses a ꢀCH OH group. Thus
2
the oxygen atom of the ring exerts a −I
4
5
effect, which is more pronounced on the
nearer C-1 position than that at C-6.
Scheme 3.
Acknowledgements
Thanks are due to C.S.I.R for financial
assistance to K.K.S.G.
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