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The sorption of glycine by a strong anion exchange resin
SURFACE
SCIENCE
10 (1968)327-331 0 North-Holland
THE SORPTION
OF GLYCINE
ANION EXCHANGE JAMES
Publishing Co., Amsterdam
BY A STRONG RESIN
L. HAYNES*
Department of Biochemistry, University of Minnesota, St. Paul, Minnesota 55101, U.S.A. Received 7 December
1967
The sorption of the amino acid glycine by a strong anion exchange resin was studied. It was found that the sorption behavior of this amino acid could be described in terms of the existence of a Donnan equilibrium, which was definable in terms of the pKz of the amino acid. Experimental results are presented in support of some theoretical considerations.
1.
Introduction
The selective uptake of ions by various types of ion exchange materials has been extensively investigatedi). The physico-chemical nature of an ion exchange resin system has been described in detail by Gregor 2, and otherss). A strong anion exchange resin may be pictured as consisting of a flexible hydrocarbon network to which is attached chemically quaternary ammonium groups which are electrically compensated by anions which are free to move between the internal solvent of the resin phase and the external volume of the ambient solution. These conditions describe a system in which a Donnan equilibrium might be established. The selective sorption of amino acids by resin systems has been studied for application to the separation of mixtures of amino acids. Seno4) and Feitelsons) have applied a more fundamental approach to the study of the interaction between amino acids and resins. Recently it has been shown that the sorption of amino acids by strong cation exchange resins can be described by equations derived on the basis of the existence of a Donnan equilibriums). In this paper it will be shown that the sorption of glycine from aqueous solution by a strong anion exchange resin is governed primarily by the existence of a Donnan equilibrium which can be described in terms of the pK, of the amino acid. * Present address: Litton Systems, Inc., 2003 East Hennepin Ave., Minneapolis, 55413, U.S.A. 327
Minnesota
328
J. L. HAYNES
2. Experimental The resin used in this study was AGl-X8
(Bio-Rad-Corp.).
The exchange
capacity of the resin was determined by displacing the hydroxyl ions from one gram of dry resin with NaCl and titrating the liberated base with standard acid. The exchange capacity of the resin was 3.5 millimoles per gram dry resin. Resin volumes were determined by the method of Gregor7). The resin was converted to the hydroxyl form by use of 10% NaOH solutions. Ninhydrin and the amino acids were obtained from the Nutritional Biochemicals Corp. The sorption experiments were carried out batchwise by adding 50 ml of the amino acid solution to one gram of dry resin-OH. The mixture was kept at 20 “C and stirred for 72 hours before the concentration of the ambient amino acid solution was determined. The amino acid concentration was determined by the ninhydrin method of Moore and Steins), pH measurements were made with a Beckman Zeromatic glass electrode pH meter, and temperature was maintained constant by use of a constant temperature air cabinet. 3. Theory The interaction between a neutral amino acid molecule in aqueous solution at its isoelectric point and a strong anion exchange resin in the hydroxyl form, and assumed to be completely dissociated, can be represented by the equation R+OH-
+ R’-CH-COO-
= R+ -OOC-CH-R’
+ H,O.
I NH2
I +NH3
(1)
R+ represents the fixed cationic groups within the resin which are initially electrically compensated by OH- groups. The OH- groups are initially present in VI at a concentration of C,, the total number of which are C, VI =n. The amino acid zwitterion R’-CH-COO-, and anion R’-CH-COO-, I I +NH, NH, will be represented by the symbols A* and A-, respectively. The subscripts 1 and 2 refer to the resin and external volume phases, respectively. The requirements for a Donnan equilibrium are
(OH-h/(OH-), = @-),/(A-), . The required
concentrations
in VI at equilibrium
are (OH-)
1 = C, -X/V,
SORPTION
and (A-) =X/V,,
329
OF GLYCINE
where X refers to the number
of millimoles
of amino
acid
present as anion in the resin phase at equilibrium. The dissociation of the amino acid zwitterion is represented as A+ =A- + H+, and the constant governing the dissociation is K, =(A*) (H+)/(A*). K, is the second dissociation constant of the amino acid. Making rearrangements in (2) leads to the equation x
where K,,, is the ion product and simplifying gives x
=
=
appropriate
substitutions
CI VI @+I,(A’), &+ (H+)dA*h’ constant
of water.
and
(3) Substituting
CI~I (K&v) (A*), = n(WE;,) (A*), 1 + &/Kv) (A+), 1 + (K,/Kv)(A’), ’
K, into
(3)
(4)
In the absence of electrolytes other than the amino acid and the resin, the pH in V, remains essentially constant during the course of the reaction and essentially all of the amino acid in V, is the zwitterion form. Therefore (A*), may be replaced by C,, the concentration of total amino acid in V, at equilibrium, and K,/K, may be replaced by Kb. These substitutions lead to the relationship X=---_ n&C, 1 + K&’
(9
Experimental values of the constants were evaluated by rearranging (5) to the linear form X/C,=nK,-KJ and plotting the experimental values of X/C, versus X. The slope of the line is -K,, and the intercept of the X/C, axis is nK,. The value of n determined in this manner should theoretically be equal to the exchange capacity of the resin. The constant Kb is related to the free energy change of the reaction (eq. (1)) by the expression LIFO= - nRTln Kb(H20) = - cnRT In K,,, where c is the concentration
of water in
4. Results and discussion The isotherm for the sorption of glycine by one gram of resin- OH is shown in fig. 1. Fig. 2. is a plot of X/C, versus X, from which the constants n and K,, of (5) are evaluated. The experimental isotherm corresponds very well to the derived equation (5). The constants were found to be in good agreement with those predicted from the theory. The value of n was found to be 3.45 millimoles/gram dry resin as compared to a value of 3.50 millimoles/gram as determined by titration of the liberated OH- from one gram of dry resin. The value of K,, was found to be 25500 which corresponds to
J. L. HAYNES
I
0
0.0002
I
I
I
0.0004
0.0006
0.0008
I
0.001
II
I
‘1
0.01
I
I
0.03
=;t
Fig. I.
Isotherm
Fig, 2.
Plot of X/C, vs. Xfor the determination of the constants n and Kb for the sorption of glycine by one gram of AGI-X8 (OH-Form)
a value reported
for the sorption of glycine from aqueous AGl-X8 (OH-Form)
of 2.55x lo-” in the literature
for K,. This is in good for the K, of glycine.
solution
agreement
by one gram of
with
values
In order for the reaction (eq. (1)) to go essentially to completion, i.e., the replacement of all hydroxyl groups initially present in the resin volume VI, the pH in VI must remain well above the pK, of the amino acid. This permits the basic titration of the protonated amino group of the amino acid so that there can be very little electrostatic repulsion between the amino acid and the positively charged quaternary ammonium group of the resin. That this condition is maintained throughout the course of the reaction can be shown by calculating the pH in the solvent of the resin
SORPTION
phase
by using
reasonable
values
331
OF GLYCINE
for n and
V,. If V, is taken
as 1.0 ml
and IZ as 3.0 millimoles/gram, the calculated pH in I’, is 11.5 when 99% of the OH- has been replaced. There are two regions of an isotherm described by eq. (5) which are of interest because of the contrast in the rate at which X changes with C,. The first region is that of very low C,, where K,C,< 1 and the sorption equation becomes X=nK,C,. The second region is that of higher C,, where K,C, 9 1. In the first case a small increase in C, is accompanied by a relatively large increase in X due to the large value of K,. Conversely, in the second case large increases in C, are accompanied by small changes in X as the exchange capacity of the resin is approached. A comparison of the sorption of a monoamino-monocarboxylic acid by equivalent amounts of a strong anion exchange resin-OH and a strong cation exchange resin-H from dilute aqueous solution, in the absence of other electrolytes, and in the vicinity of the isoelectric point of the amino acid can be made. The equations which apply are X = nC,IK,
(6)
for the cation exchange resin-H and X=nK,C, for the anion exchange resin-OH. It is seen from these equations that the greater sorption should take place with the anion exchange resin-OH.
References 1) Review sources: (a) G.E. Boyd, Ann. Rev. Phys. Chem. 2(1951) 309. (b) F.A. Kitchner, Ion Exchange and Its Application (Sot. Chem. Ind., London, 1955) p. 24. 2) H.P. Gregor, J. Am. Chem. Sot. 73 (1951) 642. 3) G.E. Boyd, S. Lindebaum and G.E. Meyers, J. Phys. Chem. 65 577 (1961) 577. 4) M. Seno and T. Yamabe, Bull. Japan. Chem. Sot. 33 (1960) 1532. 5) J. Feitelson, Biochem. Biophys. Acta 66 (1963) 229. 6) J.L. Haynes, J. Colloid Interface Sci., to be published. 7) H.P. Gregor, F. Gutoff and J.I. Bergman, J. Colloid Sci. 6 (1951) 245. 8) S. Moore and W.H. Stein, J. Biol. Chem. 211 (1954) 895.
SCIENCE
10 (1968)327-331 0 North-Holland
THE SORPTION
OF GLYCINE
ANION EXCHANGE JAMES
Publishing Co., Amsterdam
BY A STRONG RESIN
L. HAYNES*
Department of Biochemistry, University of Minnesota, St. Paul, Minnesota 55101, U.S.A. Received 7 December
1967
The sorption of the amino acid glycine by a strong anion exchange resin was studied. It was found that the sorption behavior of this amino acid could be described in terms of the existence of a Donnan equilibrium, which was definable in terms of the pKz of the amino acid. Experimental results are presented in support of some theoretical considerations.
1.
Introduction
The selective uptake of ions by various types of ion exchange materials has been extensively investigatedi). The physico-chemical nature of an ion exchange resin system has been described in detail by Gregor 2, and otherss). A strong anion exchange resin may be pictured as consisting of a flexible hydrocarbon network to which is attached chemically quaternary ammonium groups which are electrically compensated by anions which are free to move between the internal solvent of the resin phase and the external volume of the ambient solution. These conditions describe a system in which a Donnan equilibrium might be established. The selective sorption of amino acids by resin systems has been studied for application to the separation of mixtures of amino acids. Seno4) and Feitelsons) have applied a more fundamental approach to the study of the interaction between amino acids and resins. Recently it has been shown that the sorption of amino acids by strong cation exchange resins can be described by equations derived on the basis of the existence of a Donnan equilibriums). In this paper it will be shown that the sorption of glycine from aqueous solution by a strong anion exchange resin is governed primarily by the existence of a Donnan equilibrium which can be described in terms of the pK, of the amino acid. * Present address: Litton Systems, Inc., 2003 East Hennepin Ave., Minneapolis, 55413, U.S.A. 327
Minnesota
328
J. L. HAYNES
2. Experimental The resin used in this study was AGl-X8
(Bio-Rad-Corp.).
The exchange
capacity of the resin was determined by displacing the hydroxyl ions from one gram of dry resin with NaCl and titrating the liberated base with standard acid. The exchange capacity of the resin was 3.5 millimoles per gram dry resin. Resin volumes were determined by the method of Gregor7). The resin was converted to the hydroxyl form by use of 10% NaOH solutions. Ninhydrin and the amino acids were obtained from the Nutritional Biochemicals Corp. The sorption experiments were carried out batchwise by adding 50 ml of the amino acid solution to one gram of dry resin-OH. The mixture was kept at 20 “C and stirred for 72 hours before the concentration of the ambient amino acid solution was determined. The amino acid concentration was determined by the ninhydrin method of Moore and Steins), pH measurements were made with a Beckman Zeromatic glass electrode pH meter, and temperature was maintained constant by use of a constant temperature air cabinet. 3. Theory The interaction between a neutral amino acid molecule in aqueous solution at its isoelectric point and a strong anion exchange resin in the hydroxyl form, and assumed to be completely dissociated, can be represented by the equation R+OH-
+ R’-CH-COO-
= R+ -OOC-CH-R’
+ H,O.
I NH2
I +NH3
(1)
R+ represents the fixed cationic groups within the resin which are initially electrically compensated by OH- groups. The OH- groups are initially present in VI at a concentration of C,, the total number of which are C, VI =n. The amino acid zwitterion R’-CH-COO-, and anion R’-CH-COO-, I I +NH, NH, will be represented by the symbols A* and A-, respectively. The subscripts 1 and 2 refer to the resin and external volume phases, respectively. The requirements for a Donnan equilibrium are
(OH-h/(OH-), = @-),/(A-), . The required
concentrations
in VI at equilibrium
are (OH-)
1 = C, -X/V,
SORPTION
and (A-) =X/V,,
329
OF GLYCINE
where X refers to the number
of millimoles
of amino
acid
present as anion in the resin phase at equilibrium. The dissociation of the amino acid zwitterion is represented as A+ =A- + H+, and the constant governing the dissociation is K, =(A*) (H+)/(A*). K, is the second dissociation constant of the amino acid. Making rearrangements in (2) leads to the equation x
where K,,, is the ion product and simplifying gives x
=
=
appropriate
substitutions
CI VI @+I,(A’), &+ (H+)dA*h’ constant
of water.
and
(3) Substituting
CI~I (K&v) (A*), = n(WE;,) (A*), 1 + &/Kv) (A+), 1 + (K,/Kv)(A’), ’
K, into
(3)
(4)
In the absence of electrolytes other than the amino acid and the resin, the pH in V, remains essentially constant during the course of the reaction and essentially all of the amino acid in V, is the zwitterion form. Therefore (A*), may be replaced by C,, the concentration of total amino acid in V, at equilibrium, and K,/K, may be replaced by Kb. These substitutions lead to the relationship X=---_ n&C, 1 + K&’
(9
Experimental values of the constants were evaluated by rearranging (5) to the linear form X/C,=nK,-KJ and plotting the experimental values of X/C, versus X. The slope of the line is -K,, and the intercept of the X/C, axis is nK,. The value of n determined in this manner should theoretically be equal to the exchange capacity of the resin. The constant Kb is related to the free energy change of the reaction (eq. (1)) by the expression LIFO= - nRTln Kb(H20) = - cnRT In K,,, where c is the concentration
of water in
4. Results and discussion The isotherm for the sorption of glycine by one gram of resin- OH is shown in fig. 1. Fig. 2. is a plot of X/C, versus X, from which the constants n and K,, of (5) are evaluated. The experimental isotherm corresponds very well to the derived equation (5). The constants were found to be in good agreement with those predicted from the theory. The value of n was found to be 3.45 millimoles/gram dry resin as compared to a value of 3.50 millimoles/gram as determined by titration of the liberated OH- from one gram of dry resin. The value of K,, was found to be 25500 which corresponds to
J. L. HAYNES
I
0
0.0002
I
I
I
0.0004
0.0006
0.0008
I
0.001
II
I
‘1
0.01
I
I
0.03
=;t
Fig. I.
Isotherm
Fig, 2.
Plot of X/C, vs. Xfor the determination of the constants n and Kb for the sorption of glycine by one gram of AGI-X8 (OH-Form)
a value reported
for the sorption of glycine from aqueous AGl-X8 (OH-Form)
of 2.55x lo-” in the literature
for K,. This is in good for the K, of glycine.
solution
agreement
by one gram of
with
values
In order for the reaction (eq. (1)) to go essentially to completion, i.e., the replacement of all hydroxyl groups initially present in the resin volume VI, the pH in VI must remain well above the pK, of the amino acid. This permits the basic titration of the protonated amino group of the amino acid so that there can be very little electrostatic repulsion between the amino acid and the positively charged quaternary ammonium group of the resin. That this condition is maintained throughout the course of the reaction can be shown by calculating the pH in the solvent of the resin
SORPTION
phase
by using
reasonable
values
331
OF GLYCINE
for n and
V,. If V, is taken
as 1.0 ml
and IZ as 3.0 millimoles/gram, the calculated pH in I’, is 11.5 when 99% of the OH- has been replaced. There are two regions of an isotherm described by eq. (5) which are of interest because of the contrast in the rate at which X changes with C,. The first region is that of very low C,, where K,C,< 1 and the sorption equation becomes X=nK,C,. The second region is that of higher C,, where K,C, 9 1. In the first case a small increase in C, is accompanied by a relatively large increase in X due to the large value of K,. Conversely, in the second case large increases in C, are accompanied by small changes in X as the exchange capacity of the resin is approached. A comparison of the sorption of a monoamino-monocarboxylic acid by equivalent amounts of a strong anion exchange resin-OH and a strong cation exchange resin-H from dilute aqueous solution, in the absence of other electrolytes, and in the vicinity of the isoelectric point of the amino acid can be made. The equations which apply are X = nC,IK,
(6)
for the cation exchange resin-H and X=nK,C, for the anion exchange resin-OH. It is seen from these equations that the greater sorption should take place with the anion exchange resin-OH.
References 1) Review sources: (a) G.E. Boyd, Ann. Rev. Phys. Chem. 2(1951) 309. (b) F.A. Kitchner, Ion Exchange and Its Application (Sot. Chem. Ind., London, 1955) p. 24. 2) H.P. Gregor, J. Am. Chem. Sot. 73 (1951) 642. 3) G.E. Boyd, S. Lindebaum and G.E. Meyers, J. Phys. Chem. 65 577 (1961) 577. 4) M. Seno and T. Yamabe, Bull. Japan. Chem. Sot. 33 (1960) 1532. 5) J. Feitelson, Biochem. Biophys. Acta 66 (1963) 229. 6) J.L. Haynes, J. Colloid Interface Sci., to be published. 7) H.P. Gregor, F. Gutoff and J.I. Bergman, J. Colloid Sci. 6 (1951) 245. 8) S. Moore and W.H. Stein, J. Biol. Chem. 211 (1954) 895.