H _,__E;_,.:_::_,: z = :5 5, mm

 

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3 1293 00572 5779
i L I B R A R Y
Michigan State
University ,

 

THE METAL ION CATALYSIS OF THE
MUTAROTATION OF dbD-GLUCOSE

By

Irene H. Kochevar

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Biochemistry

1966

ABSTRACT
THE METAL ION CATALYSIS OF THE
MUTAROTATION OF chin-GLUCOSE

by Irene H. Kochevar

Catalysis of the mutarotation ofIXJD-glucose by metal
ions in acetate buffer was studied. The metal ion concentra-
tions varied from 0.025 M'to 0.20 M. Polarimetric readings
were converted to rates of reaction. Calcium, manganous,
nickelous and cupric ions and the monoacetate complex of
cupric ion were all found to enhance the rate of mutarota-
tion. The catalyst concentrations were calculated taking
into account the formation of metal acetate complexes.
Catalytic coefficients were determined for the metal ions
and for the cupric monoacetate complex. The order of catal-
ytic efficiency is: cupric monoacetate complex > cupric >
nickelous>manganousrcalcium. The results were discussed

in terms of both stepwise and concerted mechanisms.

‘9‘

 

 

 

The writer gratefully acknowledges the assistance,
criticism, and many suggestions given by Professor J. C.

Speck, Jr. throughout the course of this study.

ii

TABLE OF CONTENTS

INTRODUCTION00OOOOOOOOOOOOCOOOOOOOOOOOOOOOOOOOOOOOOOC

Mutarotation of Glucose.........................
Metal Ion CatalySiSOOOOOOOOOOOOOOOOOOOO00.0.0...

EXPERIEILENTALOOOOOQOOOOIOOOOOCOOIOOOOOOOOOOOOOOOOOOOOO

Reagents and Solutions..........................
Equ1pmentsooosocoosooooso...coo.0.00.00.00.00...
Procedure............0.....0....................

Data....o....o......o............o.............o

DISCUSSIONQOOOO0.000000COOCOOOOOQOOOOOOOOOOOOOOOOOOOO

REFERENCESOOOQOOOO0OOOOOOOOOOOOOOO0.0.0.0....0.0.0...

iii

Page

13
13

13
i3
29

35

Q's-0.000.000... coo-00.9.-

OOOOOOOOOOOecoaoono..-o,..

00.0.0000cocoooovoosostnoo-

Cocooooooooooooooc-oo-.o..o

O O O O O O I C U I O 3 . . I O I O O I O O ‘ I O . O

II.
III.

IV.

TABLES

Page

Mica]. Dataoosc000000000000000.000000000000000 15
Uncorrected Data on the Effects of Metal Ions.. 1?

Calculated Molarities of Catalytic Species in
Solution..............................oo..... 21

Effects of Uncomplexed Metal Ions.............. 2h

Effects of Uncomplexed plus Complexed Metal

IOMOOOOOOOOOOOOOCOOOO00......O...00.0.0.0... 27

iv

O...so-oe-- ‘0

O .
I .
O O O
‘ |
O O .

0.00.0.0

FIGURES

Page

I. Typical Data................................... 16
II., Uncorrected Data on the Effects of Metal Ions.. 18

III. Effects of Uncomplexed Metal Ions.............. 23
IV. Effects of Uncomplexed plus Complexed Metal

Ions.....OOOOOOOOO00....COCOOOOOOOOOOOOOOOOOO 25

INTRODUCTION

1. Mutarotation of Glucose

The mutarotation of glucose has been known to be a gen-
eral acid-base catalyzed reaction for almost forty years.
Three mechanisms have been proposed to account for the experi-
mental results.

The first of these mechanisms, the stepwise or bimole-
cular mechanism, requires prior proton donation by an acid
to the bridge oxygen of glucose or prior proton abstraction
by a base from the C-1 hydroxyl group of glucose before the
rate-determining ring opening step. The experimental expres-
sion is a sum of second order terms. The general rate

expression for the stepwise mechanism is:

rate: gkiuiflc] + gkj [stG]

where: hi

2
21

The second mechanism is called the concerted or termole-

catalytic constant characteristic of A
catalytic constant characteristic of B
any Bronsted acid

any Bronsted base

glucose

cular mechanism. In this case proton donation to the bridge
oxygen, proton abstraction from the C-1 hydroxyl and ring
opening occur simultaneously. The rate expression, as shown

below, contains all third order terms.

rate = {gki [A1]} {231:3 [33]} [G]

-2-

The notation is the same as the first rate expression that
appears on page 1, and it is assumed that the effectiveness
of an acid does not depend on the base it acts in concert
with.

The development of the first two mechanisms can be fol-
lowed historically. Bronsted and Guggenheim in 1927 were
the first to propose that mutarotation was general acid-base
catalyzed.1 Their experimental expression contained only
second order terms. Several years later Pedersen reported
that if a concerted mechanism was assumed, discrepancies
appeared in the relative catalytic power of various pairs of
acid and bases in the experimental expression. He concluded
that only the stepwise mechanism was important in aqueous
solution.2

Swain, in 1950, showed that Pedersen's interpretation
was invalid because some of the terms were actually due to a
combination of two indistinguishable acid-base pairs.3
Swain factored the rate expression for the concerted mecha-
nism, inserted experimental values, multiplied, and found
that the coefficients for the cross terms agreed well with
the coefficients in the original Brdnsted expression. In
addition, Swain's expression contained a small catalytic term
due to the combination of acetic acid and acetate ion. Swain
concluded that all catalysis of the mmtarotation of glucose
in acetate buffer was termolecular with.water serving as an

acid or a base.

c
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a
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.

I.

-3-

Soon afterwards Hill and Thumm carried out mutarotation
experiments with glucose in methanol-water solvent with ace-
tate ion as a catalyst.“ They varied the concentration of
water and looked for third order kinetics; but found that
their data indicated second order kinetics.

Other arguments against the concerted mechanism in gen-
eral acid-base catalyzed reactions in aqueous medium were
presented by Bell in 1953.5 He showed that neither the pre-
sence of a third order term in a catalytic expression nor
its size was sufficient proof that all other terms in the
expression were third order. Bell's elegant arguments were
accepted as convincing evidence that the reaction in aqueous
solution is catalyzed by a stepwise mechanism.

The case for a concerted mechanism has been revived
recently by Eigen.6t7 Using rapid reaction techniques he
has experimentally shown that the Br8nsted catalysis law,
kA = GAR: , is valid only over a limited range of dissoci-
ation constants, MA. The constant!“ approaches zero as the
ApKA is increased for a number of reactions which follow the
Bronsted catalysis law in the time range accessable to
classical kinetic methods. At ak= 0 the rate of these
reactions is diffusion controlled. Proton switching is no
longer the rate determining step and the rate is independent
of RA.

The mutarotation of glucose appeared to be an exception

since A remained constant over a considerably larger AplfA

(x

-14...

range. To account for this behavior, Eigen proposed a con-
certed mechanism. The rate of this three centered reaction
would be so far below the diffusion controlled rate that
even large KA differences between substrate and catalyst
could not make d.go to zero.

The third mechanism was proposed by Schmid in 1963.8
From values for AH: 48’ and 56" for the formation of the
activated complex, he formulated a mechanism which includes
a water molecule between the glucose molecule and the catal-
ytic species. He proposed that during base catalysis a
water molecule in the solvation shell of the base accepts
the C-1 hydroxyl proton from glucose. Simultaneously, the
base accepts a proton from the water molecule. This proton
switching requires exact positioning of the molecules.
Schmid proposed that the process of aligning the molecules
is the rate determing step. The mechanism of base catalysis

is shown onipage 5.

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H “\o“ a 6-— (.__ “ ‘Ou ..
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OH H OH 0“ an

Mutarotation experiments were carried out in aqueous
solutions containing various acids and bases as catalysts.
Schmid pointed out that the values calculated from the data
for All“ are nearly identical while the values for AS‘ vary.
From the similarity of the AH‘ values he concluded that the
same molecule is involved in the proton transfer to or from
glucose regardless of the identity of the catalyst. He

correlated the values of AIS" with the degree of salvation

-1

.6.

of ths catalyst. The contribution made to the rate constant
mods by 48* in the equation....

k a I?" sxp["“H sxp [9.13;]
far outweighs that made bybafii.

Schmid's data and mechanism are not consistent with
classical general acid-bass catalysis theory. First of all,
tbs rate constant in Schmid’s treatment is related to the
degree of solvation of the catalyst rather than to the acid
or base strength as required by the Bronsted catalysis law.
Secondly, because the rats determining step postulated for
nondiffusion controlled general acid-bass catalysis is a
proton transfer, ths rats constant should vary with Ali“
rather than aS”. Finally. Schmid's data gives rates at only
two or three temperatures ‘ for each catalyst. The largest
range is from 5° c. to 25‘ 0. Since ths‘graph of the natural
logarithm versus the reciprocal of ths absolute temperature
required to obtain 1311* and 48* values would have a maximum
of three points, the conclusions from this type of data are
questionable.

- Ths-availabls evidence does not permit a judgment upon
the W validity of the stepwise and concerted mecha-
nisms. Both mechanisms will be discussed in connection with
the present study. The mschanism proposed by Schmid will

19..

-7-

not be discussed further because it does not appear to

result from reliable experimental findings.

11. thal Ion Catalysis

‘A general introduction to metal ion catalysis will be
presented first as background for the discussion of metal
ion catalysis of the mutarotation of glucose.

The effects of metal ions in aqueous solution upon the
rate of a reaction can be grouped as follows:

A. primary salt effects

B. secondary salt effects

C. changes in the catalyst '

D. changes in the ground state of the reactants

E. changes in the transition state

This order will be followed in discussing metal ion
catalysis.

.A primary salt effect changes the ionic atmosphere of
the reacting species and results in a shift in the equili-
brium between reactants and the activated complex. By know-
ing the charge on each reactant involved in the activated
complex, the direction of change produced by the primary
salt effect can be predicted.

.A secondary salt effect does not change the catalytic
constant. Rather, it affects the rate by changing the
amount of catalyst present. An increase in the ionic
strength changes the activity coefficients thereby disturb-
ing the equilibrium of weekly dissociated species. In order

to return to equilibrium the concentrations of the ions and

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-8-

the undissociated acid.must change.

A.meta1 ion can change the rate by acting upon (a) the
catalyst, (b) the ground state of the reactants, or (c) the
transition state.

(a) The catalyst: If the metal ion merely changes the
concentration of the catalyst by associating with a certain
number of the catalytic molecules, the rate of the reaction
is changed. For example, in the mutarotation of glucosamine
the C-2 amino group catalyzes the reaction.9 Ni‘" and Cd“-
slow down the reaction by associating with the free electron
pair of the amino group nitrogen thereby destroying its

catalytic power.

 

In other cases the metal ion-catalyst complex is a better
catalyst than the uncamplexed catalyst.

(b) The ground state of the reactant: Association
between.metal ion and the ground state of a reactant can
increase the rate constant by decreasing the activation energy.
An example is the decarboxylation.of dimethyloxaloacetic acid.11

The metal ion associates with two points on the molecule and

Polarizes the carbonyl bond, thus facilitating the electron

Shifts

-9-

o‘ 0 ah
i u \
O=C"C'—CH
‘
CA4:

Metal ions could conceivably decrease the rate constant
by forming a stable chelate with the ground state in which
the electron density is unfavorable for reaction. In effect,
formation of the chelste would increase the activation
energy.

(c) Transition state: Acceleration due to metal ion
association with the transition state has been postulated
for several reactions including the bromination of ethyl
acetoacetate.12 The metal ion begins to co-ordinate with
the oxygens during the transition state. as the drawing
below shows the transition state resembles the final product.
This metal-organic complex is more stable than the free
transition state; therefore, a lowered activation energy is

required and the rate constant is increased.

(HA:- C .cnrzc— o— Caz-CH3
o '0
\cu/

-10...

A reaction which requires a smaller increase in entropy
during the formation of the transition state will have a
larger rate constant. As shown below, stabilization of a
ring into a rigid conformation which has the desired stereo-

chemistry for a l,2-elimination of water fits this

description.
9
n* " GD
M: ~ a
Q." R
H:
H on

This brief outline attempted to place a certain order
upon the many possible ways in which a metal ion could influ-
ence the rate of a reaction.

In the particular case of mutarotation of glucose very
little investigation of metal ion effects has been carried
out. Bransted and Guggenheim demonstrated the absence of
primary salt effects up to an ionic strength of 0.20}- Secon-
dary salt effects are expected if a week acid or weak base
is used as a catalyst.

The first demonstration of metal ion catalysis was
reported in the original article by Bronsted and Guggenheim
on the mutarotation of glucose.1 While studying catalysis
by various bases, they found that the basic cations
00(NH3)59HT*’ and Cr(OH2)50H+*'behaved as general bases in
the catalysis of mutarotation. The neutral cation
00(NH3)6+++ did not catalyze the reaction.

 

 

-11-

In 1950 Delaney, Kleinberg and Argersinger observed an
increase in the rate of mutarotation of dLglucose when FeCl3
was added to the reaction mixture.13 The authors attributed
catalysis to preferential complex formation between p-glucose
and Fe+++ although the equilibrium rotation was not affected

by the presence of FeClj. Other evidence indicated that com-

plex formation does occur. The rate data could be interpreted

as a case of general base catalysis by Fe+++e

Metal ion catalysis was also reported by Neely in 1952 3"“
This researcher found increases in the rate constant in the
presence of Mg**, and Cu++ in unbuffered solutions. Fe++*,
Bi’, Mg“, and Be+ were shown to increase the rate in perch-
loric acid solution and in acetate buffer, Li+ and Mg** acted
as catalysts. The resultant change in rate in acetate buffer
was due to two factors: (1) association between acetate ion
and the metal ion which produced a change in catalyst concen-
tration and (2) specific acceleration by the metal ion. .At
low metal ion concentrations the first factor outweighed the
second, resulting in a net decrease in the rate. No evidence
was found for the formation of a complex between Li+ or Mg++
and the sugar. Specific acceleration by the metal ion was
attributed to stabilization of the transition state by the
metal acting as an electrophilic agent. The proposed mecha-

nism is shown on page 12.

 

! V“ ’a -4‘\

 

-12..

1
J

69.0“ M'“ ch10“ or!

   

 

The effects of alkali metal and alkaline earth metal
ions were investigated by Nicolle and weisbuch in 1957.15
These researchers used high concentrations of salts, l to
h M, and unbuffered solutions. They found retardation of
the rate of mutarotation by the alkali metal salts and no
effect or slight increase in rate by the alkaline earth
salts. RbCl had the largest decreasing effect and MgCl had
the largest enhancing effect of the salts employed.

At about the same time Grandchamp-Chaudin investigated
the effects of 2'M solutions of NaCl, KCl, NHhCl and LiCl
upon the rate of mutarotation and the equilibrium rotation
glucose.16 Her results correlated well with those of Nicolle

and weishbuch.
Because Nicolle and weishbuch and Grandchamp~Chaudin
used high salt concentrations in unbuffered solutions the

reliability of their date makes any interpretation
questionable.

In summary, one can see that only indications of the
possible role of metal ions in the catalysis of the mutarota-
tion of glucose have been demonstrated. The purpose of this
study is to further delineate these effects.

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EXPERIMENTAL

Reagents and Solutions: cn-D-glucose was obtained from the
National Bureau of Standards, Lot numbers 5052 and 5250.

The cupric nitrate, nickelous sulfate, manganous sulfate,
and calcium nitrate were reagent grade chemicals. The
glacial acetic acid from the J. T. Baker Company was 99.9%
pure. Stock metal ion solutions were prepared by weighing
the anhydrous salt or the hydrate on an analytical balance
and then diluting to the correct volume with glass distilled
water in a volumetric flask. The correct concentrations
were later determined by standard quantitative procedures.
The 0.1 M 1:1 stock acetate buffer was prepared from standard
sodium hydroxide and glacial acetic acid. The sample solu-
tions were prepared by pipetting the required volume of
stock metal ion solution and 10 ml. stock buffer solution
into a 50ml. volumetric flask and diluting with glass

distilled water.

Equipment: Rotation measurements were taken at 5780.X with
a Zeiss Photoelectric Polarimeter using a mercury lamp. The
one decimeter cell was water-jacketed and kept at lB.9°:0.l° C.

with a circulating constant terperature water bath.

Procedure: For each run, 1.125 g. (0.00625 mole) oan-Dm
glucose was washed into a 25 m1. volumetric flask with the
appropriate buffer-metal ion solution which had been previ-

ously prepared in a 50 ml. volumetric flask and equilibrated

-13-

.
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was
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ti<

-19-

to 19° C. AAll experiments were run at 18.90 C. The glucose
concentration was 0.25 M. The acetic acid and the acetate

ion concentrations were both 0.02 M. Elapsed time was meas-
ured from the initial mixing of glucose and solvent. The
first reading was usually at 250 seconds. Subsequent measure-

ments were made at 250-second intervals.

Data: The equilibrium rotation was 2.h65°. Initial rotation

was calculated to be 5.0h90. The rate was determined by

de-do
dc‘“
tion, 4,: initial rotation, and o( = rotation at time, t.)

plotting values of log vs. time (de.= equilibrium rota-
The unit of time was the second.

A typical set of data is given in Table I and the corres—
sponding graph in Figure 1. Experiments were replicated
either once or twice. The rate at each concentration was
taken to be the average of the observed values. The observed
values were never outside the experimental error, i2%.

Figure II shows the observed rates versus the concentra-
tion of metal ion in the sample solution. The data for this
graph is given in Table II.

The metal ions complexed with the acetate ion; therefore,
the actual concentrations of metal ion, metal acetate complex,
acetate ion and acetic acid were obtained by calculation.

The initial metal ion concentration, the dissociation con-
stant for acetic acid, and the association constants for the

metal acetate complexes were known.

4
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-15-

TABLE I TYPICAL DATA
(“WP 0.175. [mac-r] = 0.0337, [G] = 0-25. T = 18-96 C

 

 

 

 

cs -

Time, Seconds Rotation, Degrees 108$?
250 8.710 0.0611
500 “.385 0.1290
750 h.100 0.1988
1000 3.860 0.2677
1250 3.660 0e33u9

1500 3.uso 0.h058 '
1750 3.335 0.h728
2000 3.200 0.5h60
2250 3.100 0.6151
2500 3.005 0.6799
2750 2.920 0.7549
3000 2.855 0.8212

 

-16-

 

0.80 '-

        

[Cu‘rf = 0.175

0.20 .—

 

 

+ A L A A A ‘ I A A A J

 

 

 

1000 3000 3000

time, seconds

FIGURE I TYPICAL DATA

TABLE II UNCORRECTED DATA.0N THE EFFECTS OF METAL IONS

 

Metal ion Molarity Observed rate x 10“
Ca++* 0.0088 1.08
0.0732 1.09
0.0976 1.11
0.1167 1.11
0.1000 1.12
Cu++** 0.0258 1.61
0.0516 1.80
0.0770 2.07
0.1032 2.20
0.1290 2.05
0.1508 2.61
0.1806 2.78
Mh++* 0.0202 1.09
0.0080 1.10
0.0726 1.17
0.0968 1.20
0.1210 1.23
0.1052 1.25
0.1690 1.29
Ni++** 0.0200 1.08
0.0080 1.12
0.0720 1.10
0.0960 1.21

 

*Rates for this ion are the average of three values.
**Rates for this ion are the average of two values.

0.20

3.80

l

'33.10

8

3.00

Observed rate x 10h
m
E»
o

1.80

1.00

-18-

 

 

 

 

 

0 Calcium
0 Cupric
0 Manganous and
Nickelous
I
I
(
I
(If 4‘0———4
o O
‘ *‘CF—v 14.11
1 l 1 1 1 I
0.06 0.12 0.18

Total metal ion concentration

FIGURE II UNCORRECTED DATA ON THE EFFECTS OF METAL IONS

 

-19-

Activity of hydrogen ion as measured by a Beckman pH
meter was converted to hydrogen ion concentration.17 The
change in the dissociation constant for acetic acid due to
the increase in ionic strength.was corrected for.18

The association constants used for the metal acetate

complexes we re :19

log K1 log K2
Calcium. 0.53
Cupric I . 67 o .98 K1 = 0A6
Manganous O . 61 c"
Nickelous 0.67
K2 3. 0A0)
c O c

In the following calculations it was assumed that only
a monoacetate complex was formed in sufficient quantities to

be catalytically important.

005*“: concentration of metal ion free in solution

[My]: total concentration of metal ion added

[HOAch = total concentration of acetic acid and acetate ion
added

[HOAcFI I final concentration of acetic acid

Kl: g0A 0:! Equation 1
KA” @" I [OAc‘ 1 Equation 2
HOAcl

IMF)“ EM"? (MOM?

[HOAc l = [301 OAc
T mongfi ++ gone]

333":- [M F":

“Q

-20-

[HOACF] : [HOAcTJ - [0A5] - [MF'] +[MF”]
Substituting for [M0Ao1 into Equation 1 yields:

Kl = IM+ _ +41
[13%; [OAcq

Rearranging this expression yields:

[0A6] = "‘1 -- [M “'1
%1 F

Substituting this value for [0A6] and the expression for [HOAcF]
into Equation 2 yields:

++ - E+
K = + 4* *
A [HOAcd .. 3W ‘1 ‘JM 1 —[M$"'] 4M; ]

The value of [Mfi41was determined by approximation
after substituting values for KA’ K1, [Mf‘l [HOAcTI, and [Hi

The values of [MOAd'Il [0A6],'and [HOAcF] were calculated

from the above equations. The results of these calculations

are tabulated in Table III.
Two possibilities for metal ion catalysis exist:

(1) the free (uncomplexed) metal ion, [MfifL is the only
catalytic species or (2) both the metal ion and its acetate
complex are catalytically active. If only the uncomplexed

metal ion catalyzes the reaction, the experimental rate

expression is:

rate a 8.8 x 10"5 .- LnLi x 10"” [end] 4- 1 x 102 [011‘]
b, x 10-5 [HOAcF] + 2.1; x 10-3 [H3o“ + kM [Mfi‘l

“uh-Aw“ °“'

yr

w

-21..

Table III Calculated Molarities of Calalytic
Species in Reaction Mixtures

 

 

Metal ion MF mAc OAc HOAc H30 1: 105
Calcium 0.005 0. 000 0. 018 0.023 3.10
0.069 0. 005 0. 016 0.020 3.70
0.091 0. 007 0. 016 0.021 3.96
0.110 0. 012 0.015 0.018~ 0.29
0.133 0.013 0.010 0.018 0.20
Cupric 0. 018 0. 008 0.006 0.012 7.95
0. 039 0. 013 0. 005 0.021 13.7
0. 060 0. 010 0. 000 0.021 17.0
0. 089 0. 010 0. 003 0.022 22.2
0.110 0.015 0.003 0.021 26.8
0.100 0.015 0. 002 0.021 33.0
0.165 0. 016 0. 002 0.020 38.6
Manganous 0.022 0.002 0. 018 0.020 3.50
0.000 0.000 0. 016 0.022 0.60
0.069 0.000 0. 013 0.026 6.23
0.090 0. 007 0. 010 0.020 6.56
0.111 0. 013 0.008 0.016 6.85
0.130 0. 013 0.007 0.017 8. 30
0.157 0.013 0.006 0.018 10. 27
Nickelous 0.023 0.001 0.018 0.021 3.01
0.005 0.003 0.010 0.022 3.97
0.068 0.000 0.013 0.022 0.36
0.091 0.005 0.013 0.022 0.58

 

f“,

E“

.-A‘

H

"1
"a.

-22-

This expression is made up of the original Brbnsted and
Guggenheim.expression with a term for metal ion catalysis
added. The difference between the observed rate and the
rate calculated from the terms of the original Br6nsted-
Guggenheim expression was attributed to participation of the
uncomplexed metal ion. The rate due to metal ion catalysis
is plotted versus the uncomplexed metal ion concentration in
Figure III. The slopes of the lines were taken to be the
catalytic constants for CaffI, MnF*+} and NiF++L The cata-
lytic constants are given in Table IV along with the data for
Figure III. No value for Cup‘* is given because of the non-
linearity of the plot.

If both the metal ion and its acetate complex are cata-

lytically active the following expression is appropriate.

rate 8.8 x 10'”5 + 0.0 x lO'u (OAdj + l x 102 [OH'I-r
b, x 10-5 [HOAcF] + 2.1;, x 10-3 [1130‘] + kM [Mgfl +
kMOAC. [MOACJ]

The combined rate due to Mg? plus MOAe* is plotted versus

the total metal ion concentration,[Mqu, in Figure IV. The
lines for Cu§* are curved indicating that at least two cata-
lytically active species exist. The catalytic coefficients
for the two species in.Cu*+ catalysis were determined by sub-
stitution of the appropriate rates and concentrations into

the following expression at two cupric ion concentrations:

1.60

|-'
O

N
0

Rate due to metal ion catalysis x 10h
.0
oo
o

0.00

-23-

 

 

 

 

 

 

D
0 Calcium ’
0 Cupric
. Manganous
O Nickelous ,
D
D
D
0
L
/
1 4 ‘
L I - L . L
0.06 0.12 0.18

Uncomplexed metal ion concentration

FIGURE III EFFECTS OF UNCOMPLEXED METAL IONS

-20-

TABLE IV EFFECTS OF UNCOMPLEXED METAL IONS

 

 

 

0 0
Metal ion ‘Molarity gagetg g++ RM:x 10
Calcium 0.006 0.11 0.59
0.070 0.13
0.093 0.15
0.117 0.16
0.100 0.17
Cupric 0.019 0.70 *
0.003 0.93
0.067 1.16
0.092 1.33
0.117 1.50
0.103 1.70
0.169 1.87
ManganoUs 0.023 0.12 1.07
0.006 0.18
0.070 0.22
0.093 0.27
0.120 0.31
0.103 0.33
0.165 0.37
Nickelous 0.023 0.11 2.32
0.006 0.17
0.070 0.19
0.093 0.26

 

*No catalytic constant could be determined because of the non-

linearity of the curve.

Rate due to metal ion catalysis x 101+

-25-

 

 

 

 

 

 

 

 

9
0 Calcium .
O Cupric——experimental
1,60 - 3’ Cupric--calculated
. Manganous .,
O Nickelous
' £3
D
1020 ""
t
0.80 -
D
F
0.00 -
- 1 g; 1 A _J
0060 0.12 0.18

Total metal ion concentration

FIGURE IV EFFECTS OF UNCOMPLEXED PLUS COMPLEXED METAL IONS

 

-26-
velocity due to metal ion: km [Cufiq + KMOAc [CuOAc‘]

The two equations were then solved simultaneously for kM

and kMOAc° The rates calculated using these catalytic coef-
ficients are compared with the experimental values in Figure IV.
Table V presents the data for Figure IV and a summary of the
catalytic coefficients.

-27-

TABLE V EFFECTS OF UNCOMPLEXED PLUS COHPLEXED METAL IONS

 

 

Metal ion. Molarity Rate x 10h Calc'd km, kMOAc
of due to rates “x110 0h
Mf+ 11016" MF“+ 11010" 1 1
0.3+ 0.0088 0.113 0.63
0.0732 09131
0.0976 0.156
0.1220 0.160
0.0770 1.15 1.12
0.1032 1.30 1.00
0.1290 1.55 1.56
0.1508 1.70 1. 0
-0.1806 1.89 1. 9
Mn“ 0.02 0.11—I. 1.00
0.00 g 0.19
0.072 0.20
0.0968 0.27
0.1200 0.30
0.1070 0.32
0.1700 0.36
01** 0.02 0 0.12 2.09
0.00 0 0.17
0.0720 0.22
0.0960 0.27

 

DISCUSSION

Mutarotation of D-glucose in acetate buffer has been
studied polarimetrically in the presence of metal ions. An
increase in the rate of mutarotation resulted from the addi—
tion of Ca(N03)2, Cu(N03)2, MnSOu or NiSOu to the reaction
mixture. The total increases cannot be ascribed to primary
or secondary salt effects for two reasons. First of all,
variation in rate occured between solutions containing dif-
ferent metal ions at identical ionic strengths. Secondly,
experiments using salts such as NaN03 and Nazsoh indicated
that only small variations in rate occur with increasing
ionic strength. .At the maximum ionic strength used in the
metal ion catalysis experiments, the variation due to salt
effects was less than 0%. For these reasons metal ions
appear to be specific catalysts for the mutarotation reaction.

Before an attempt is made to elucidate the mechanism of
this catalysis, the catalytic species must be identified.
Each of the metal ions forms a complex with acetate ion and
'the concentrations of the metal ion-acetate ion complexes in
the reaction mixtures have been calculated.

The appearance of+hcplot of total metal ion concentra-
tion, [MT++] , versus rate due to metal ion catalysis depends
upon which species, the free metal ion (Mf++) or the metal

ion-acetate ion complex (MDAc+), is catalytically active.

-29-

 

 

-30-

Three possibilities exist: (1) If both MF++ and MDAo+ are
catalysts and have the same catalytic coefficient, the curve
would be linear. (2) If only the free metal ion catalyzes
the reaction, the slope of the line would decrease at high
concentrations due to removal of the metal ion by formation
of the acetate complex. (3) If both species are catalysts
but have different catalytic coefficients, the shape of the
curve would depend on the relative catalytic efficiencies of
M?*+ and MDAc+.

Ca**, Mn++, and Ni++ show a linear relationship between
rate and concentration of total metal in Figure IV, indicat-
ing that MOAc+ and MF‘H' have the same catalytic coefficient.
However, a linear relationship also exists between‘MF++ and
the rate as shown in Figure III, indicating that only MF++
is catalytically active. Apparently a slight curvature
exists in one of these sets of lines which cannot be detected
due to the small slopes of the lines. Because of these contra-
dictory pieces of evidence no conclusion can be drawn concern-
ing the identity of the catalytic species for these three
metal ions. Catalytic coefficients for CaF++, MnF++, and
NiF++ were calulated from the slopes of the lines in Figure
111. These values must be considered approximations since
it could not be determined whether the acetate complex of
these metals had any catalytic power.

The plot of CuT++ versus rate in Figure IV shows a defi-

nite decrease in slope with increasing concentrations. The

 

-31-

same is true in Figure 111 for the concentration of CuF++
versus rate. Two possible explanations are: (l)more than
one species is responsible for the change in rate, or (2) only
CuF++ is active but the reaction is not first order with
respect to CuF'H' .

The first explanation requires that the two catalytic
species have different catalytic coefficients. If they had
had the same catalytic coefficient, the plot of CuT++ versus
rate would have been linear. Catalytic coefficients for
Cup++ and CuOAo have been calculated as shown in the experi~
mental section. The catalytic coefficient for CuOAo+ is
larger then that for CuF+*. The rates determined from these
rate constants fit the date very well as shown in Figure IV,
indicating that this explanation is consistent with the
experimental results.

If CuF++ is considered the only catalyst as postulated
by the second reason above, the rate must be less than first
order with respect to CuF*+ because of the shape of the curve
in Figure III. A.reacti0n order greater than unity would
have led to a curve with increasing slope at high concentra~
tions, and with a reaction order of unity the line would have
been straight. A reaction order between zero and unity is
also consistent with the date. Only the first explanation
was employed in calculating catalytic coefficients.

Three conclusions concerning the identity of the catal-

ytic species can be drawn. First of all, the identity of the

 

-31..

same is true in Figure 111 for the concentration of CuF++
versus rate. Two possible explanations are: (l) more than
one species is responsible for the change in rate, or (2) only
CuF++ is active but the reaction is not first order with
respect to CuF‘H' .

The first explanation requires that the two catalytic
species have different catalytic coefficients. If they had
had the same catalytic coefficient, the plot of CuT++ versus
rate would have been linear. Catalytic coefficients for
CuF++ and CuOAe have been calculated as shown in the experin
mental section. The catalytic coefficient for CuOAc+ is
larger then that for CuF+*. The rates determined from these
rate constants fit the date very well as shown in Figure IV,
indicating that this explanation is consistent with the
experimental results.

If CuF++ is considered the only catalyst as postulated
by the second reason above, the rate must be less than first
order with respect to Cup++ because of the shape of the curve
in Figure 111. A.reacti0n order greater than unity would
have led to a curve with increasing slope at high concentra-
tions, and with a reaction order of unity the line would have
been straight. A.resction order between zero and unity is
also consistent with the date. Only the first explanation
was employed in calculating catalytic coefficients.

Three conclusions concerning the identity of the catal-

ytic species can be drawn. First of all, the identity of the

-32..

catalytic species in solutions of Mn++, Ca++ and Ni++ in ace-
tate buffer cannot be decided. Secondly, both Gui” and CuOAc+
are catalytically active. Thirdly, CuOAo+ is a much better
catalyst than CuF++.

The mechanism of metal ion catalysis will be discussed
in terms of both the stepwise and the concerted mechanisms.
The stepwise mechanism will be shown first.

In nondiffusion controlled general acid catalysis, the

rate determining step as shown in the following sequence is

the (animals: initiation,

 

 

 

 

“4,08 1' 08,90 0
H4 1
H ' i ' '\
\ 0,. on 1 T H u 0,, °'
0 + a4 s
OH O“ OH on
can»: A
K
I “ ‘3“ o' +-I‘1+* “ ‘I
on R on
* +
saga. a“
0‘ 1| \ 0‘ R

-33-

A water molecule could be included between the C-5
oxygen and the metal ion without changing this scheme.

The metal ion behaves as a general acid and lowers the
activation energy by stabilizing the transition state through
binding with the C-5 oxygen. The monoacetate complex of
cupric ion could catalyze the reaction by means of this
mechanism.also. No explanation of its greater catalytic
efficiency can be made with this mechanism.

The mechanism.for a concerted reaction would be the same
as that proposed by Neely.1u Again a water molecule could

be included between the 0-5 oxygen and the metal ion.

 

¢H3°H O“ CHIP” [.A'Fr H

9
H a ““~C>

 

O“ on

-3u-

The proton switching determines the rate of the reaction.
proceeding by this mechanimn.

The relative catalytic coefficients of the metal ions
cannot be quantitatively related using the Bronsted catalysis
law. because the dissociation constants of the metal ions
acting as acids have not been determined.

In.comolusion one can state that metal ions do catalyze
the mutarotation of glucose in acetate buffer with character-
istic rates. The evidence presented does not allow a decision

to be made between the two mechanisms which were discussed.

11.

12.
13.

10.
15.
16.
17.

18.

19.

REFERENCES
J. N. Bronsted and E. A. Guggenheim, J. Am. Chem. Soc.,
02, 2550 (1927).
K. J. Pedersen, J. Phys. Chem., 38, 581 (1930).
C. C. Swain, J. em. Chem. Soc., 12, 0578 (1950).

D. G. Hill and B. A. Thumm, J. Am. Chem. Soc., 20, 1380,
(1952).

R. P. Bell and P. Jones, J. Chem. Soc., 88(1953).

M. Eigen, Pure and Appl. Chem., 2, 97(1963).

M. Eigen, Angew, Chem. (Intern. Ed. Engl.), 3, 1(1960).
H. Schmid, Montach. Chem., 22, 1206(1963).

 

P. Tang and N. C. Li, J. Inorg. Nucl. Chem., 26, 1606(19641
J. I. Hoppe and J. E. Prue, J. Chem. Soc., 1775(1957).

R Steinberger and F. H. weatheiler, J. Am. Chem. Soc., 12,

'029(1951).
K. J. Pedersen, Acts Chem. Scand., g, 252(1908).

J. C. Delaney, J. Kleinberg and W. J. Argersinger, Jr.,
J. Am. Chem. Soc., 13, 0257(1950).

W. B. Neely, Thesis, Michigan State University, 1952.
J. Nicole and F. Weisbuch, Comp. rend., 200, 2006(1957).
A. de Grandchamp-Chaudin, Comp. rend., 200, 1560(1957).

"Handbook of Analytical Chemistry," L. Meites, ed., MbGraw-

H. S. Harned and B. B. Owen, "Physical Chemistry of Elec-
trolyte Solutions," Reinhold Publishing Corporation, New
York, N. Y., 1958, p. 667.

Stability Constants of Metal-Ion Complexes. Special Publi-

cation no. 17 London: The Chemical Society, Burlington
House, W. 1, I960.

-35..

.1

r‘

("I

I.

I.

I"

1‘.

4“

11111111111“