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A new molecular model for complexation between carboxymethylcellulose and alkaline—earth metal ions in aqueous systems
Food Hydrocolloids Vol.6 noA pp .379-386, 1992
A new molecular model for complexation between carboxymethylcellulose and alkaline-earth metal ions in aqueous systems Takayoshi Matsumoto and Hirofumi Zenkoh Department of Polymer Chemistry, Kyoto University, Kyoto 606, Japan Abstract. A new molecular model for the complexat ion between carboxymeth ylcellulose (CMC) and alkaline-earth metal ions has been discussed through the change in relaxation time and radius of gyrat ion of the polymer. Elongat ion of the relaxati on time of the CMC main chain due to the complexat ion is inedependent of the choice of alkaline-earth metal ion , but depends only on the num ber s of metal io ns bo und to CMC . The radius of gyrati on of CMC in aqueo us systems is elong ated by complexation . In the new model for the complexation , there are two possibilities for th e complex site. The rotational angle around the bond between the neighbouring residues decreases with complex formation from - 60 to 21°.
Introduction
Certain mono- and oligosaccharides form complexes with alkaline and alkalineearth metal ions and the combining ratios of saccharide to metal ions are mostly 1:1 or 2: 1. However, there has been no clear evidence for the mode of complexation , though much experimental data exist on solubility , optical rotation and electrophoresis (1-4). The complex formation between polysaccharides and alkaline- earth metal ions has been investigated in terms of gelation , and a crosslink model and a pendant model for complexation have been developed (5). On the other hand, our recent studies on complexation between carboxymethylcellulose (CMC) and alkaline-earth metal ions have revealed that (i) complex formation is an intramolecular rather than intermolecular process and elongates the relaxation times of the CMC main chain, (ii) the site of complex formation is near a carboxyl group , and (iii) the complexation site does not act as a crosslink point (6,7) . Furthermore we proposed a new model for complexation, using data for complex formation between carboxymethylchitin (CMCh) and alkaline-earth metal ions through NMR, small angle X-ray scattering (SAXS) and rheological measurements (8). In the new complex model, an alkaline-earth metal ion is bound in a cavity between the carboxyl group and the hydroxyl group attached to the carbon C3 of the neighboring residue. In the present paper we discuss the applicability of the new complex model for the complexation between CMC and alkaline-earth metal ions and also discuss the degree of restriction of the molecular mobility by complexation. Material and methods
The CMC (sodium salt) employed was kindly provided by Daiichi Kogyo Seiyaku Co . (Kyoto , Japan). The weight average molecular weight Mw measured by light scattering, the weight average degree of polymerization Pw 379
T.Matsumoto and H.Zenkoh
Table I. Characteristics of carboxymethylcellulose emplo yed Sampl e
M../IO"
CMC18
22.2
880
1.13
Table II. Ratio of alkaline-earth metal ions bound to CMC18 Ion
Rat io
Mg Ca Sr Ba
0.529 0.972 0.714 0.915
and the degree of substitution D; are shown in Table I. The method of sample characterization and sample preparation are described in detail in a previous paper (7). SAXS was measured with a 6 m point focusing SAXS camera at the High Intensity X-ray Laboratory, Kyoto University (9). The X-ray wavelength was 1.542 A (CuKa radiation). The thickness of the sample solution was 1.5 mm. The ratio of alkaline-earth metal ions bound to CMC was estimated by the following method. A 1% solution of CMC (15 ml) in -0 .05 N alkaline-earth metal chloride aqueous solution was dialysed in 1 drrr' distilled water using a cellophane tube until the electroconductivity of the outer (surrounding) liquid became almost constant (for 5-7 days). EDTA (ethylene diaminetetraacetic acid) was used as the chelating agent with EBT (Erochrome Black T) as indicator. Table II shows the ratio of bound metal ion , f (metal ion bound to CMC chain/added metal ion) calculated. The affinity for binding of metal ions to CMC is rather larger than that in CMCh systems (8). However the order of affinity Ca-s- Ba >Sr > Mg is similar to that in CMCh systems. Results and discussion
Influence of binding metal ion on relaxation time Elongation in the relaxation times by complex formation has been estimated through the dynamic viscoelastic measurements, as has been sufficiently discussed (7,8). Dynamic modulus shifts to a longer time-scale region with complexation and the shift factor as (=TITO) (where T and TO are the relaxation times of the salt-added system and the salt-free system) increases with addition of the salt , i.e. with complexation (7,8). In Figure 1a,b the logarithm of the shift factor for 15% aqueous solution of CMC18 is plotted against c" the concentration of various alkaline-earth metal ions added to the system and against Cb (=cJ) , the concentration of the metal ions bound to the CMC chain. It should be noted that the shift factor, i.e. the relaxation time, becomes large with addition of the metal ion and that the shift factor apparently depends on the choice of the metal ion. However the dependence can be almost reduced by plotting as versus Cb ' In Figure la, log as decrease with increasing Cs for the 380
A model for complexation between
C~C
and alkaline-earth metal ions
2.----------------,
cscu ~ 1 IXI
o
10
ao
20 Cs
J
N (0 )
Cb/N 0.7 1.4
20
2.1
CMCI8.15"1. 40·C
III
01
Mg
01
o
O~--.L----.L------I
o
0.5
1.0
1.5
Cb I mol/mol residue (b)
Fig. 1. (a) Log as plotted against the concentration of added salt, c" for various metal salts. (b) Log as plotted against Cb, the concentration of metal ion bound to the CMC chain.
CaCl 2-added system at relatively large c" shown by a dotted line. This is likely to be attributed to the influence of salting-out. This kind of region is omitted in the present consideration. From the molecular dynamic point of view, the relaxation time of p-mode, T p , is theoretically represented by (10,11) b2n2~
6Tr 2p 2kT
(p = 1,2,3, ...)
(1)
where n is the degree of polymerization, ~ is the monomeric friction coefficient, b is the root mean-square end-to-end distance per monomeric unit, k is the Boltzmann constant and T is the absolute temperature. The shift factor as is given by the ratio of Tp of a salt-free system and that of a salt-added system, i.e. Tp s
as = - -
(2)
TpO
381
T.Matsumoto and H.Zenkoh
Here subscripts 0 and s mean the values for salt-free systems and salt-added systems, respectively. This relationship can also be obtained by the modified Rouse theory (12), the Graessley theory (13) and the Doi-Edwards theory (14). The change in relaxation time can be related to the change in b and ~ with concentration of metal ions. According to Ferry, the monomeric friction coefficient depends greatly upon the free volume and steric hindrances to rotations around the main chain (15). From the experimental results on CMCh, the contribution of the change in ~ to as is -70-80% in as (8). Influence of binding metal ion on molecular extension
The radius of gyration, R G , can be estimated from SAXS data by Guinier approximation, (3)
Here I is the scattering intensity and q is the scattering vector (4'lT/A)sin(e/2), 4
CMe 18
3
o
0
Q
0.4'10
0
0
3 o
2
0
0
00067
00 °0
0
0°0
000 00
3
en 2
0
o
0.01 o
0
0
o
0
3
2 o
0.02 0
0
o
0
Fig. 2. Log I plotted against q2 for SAXS data of 0.4% CMC aqueous systems at various concentration of CaCI 2 •
382
A model for complexation between CMC and alkaline-earth metal ions
where A is the X-ray wavelength and 6 is the scattering angle. Figure 2 shows Guinier plots (log I versus q2) for the 0.4% CMC18 aqueous systems at various concentrations of CaCl 2 from 0 to 0.2 N. However, the data points rather scatter, the plot can be approximated by a straight line in the extremely small angle region and the radius of gyration can be obtained from the slope of the straight line with a certain experimental error. Table III shows the radii of gyration obtained, which increase with increasing concentration of CaCI 2 . The values are reasonable compared with the data for other cellulose derivatives (16). In Figure 3, log as and log (b s2Ibo2 ) are plotted against concentration, Cb, of calcium ion bound to CMCI8. The relationship between log (b s2Ibo2 ) versus Cb which can be represented by a straight line, almost coincides with the data Table III. Radius of gyration of the CMC molecule in aqueous solution at various concentrations of CaCI 2 CaC I2 cone. N
mol/mol residue
o
o
0.0067 0.01 0.02
0.206 0.309 0.618
157 168 174 199
± ± ± ±
7 7 3 6
2 a
IJI
......
III
IJI Cl
s
~o
......
NIIl .0
..
Cl
.9 III
b~ I b ~
e
Ol 0
0.5 c b / mo ll m o l residue Fi~. 3. Logarithm of the shift factor as ( 6) , the ratio of the monomeric end-to-end distance b// bo (0) and the monomeric friction coefficient ~Ao (dotted line) plotted against the concentration Cb for CMC system . The ratio of b//b o2 is also plotted by the closed circle for the CMCh system.
383
T.Matsumoto and H.Zenkoh
(closed circles) of the CMCh systems described in a previous paper (9). It should be noted that the change in the end-to-end distance due to the complexation with calcium ions is almost equal in the systems of CMC and CMCh. There is no information on the influence of polymer concentration on the ratio b/bo. Here, we assume that the ratio is approximately independent of the polymer concentration. According to equation (2), the change in ~j~o with Cb can be estimated and represented by a dotted line in Figure 3. The contribution of ~Ao to as is much larger than that of bJbo. The relationship between log (b s2J b 0 2 ) and Cb can be given by (4) for the systems of both CMC and CMCh. Restriction of molecular rotation by complexation
In the new complex model proposed for complexation between the CMCh and an alkaline-earth metal ion, the alkaline-earth metal ion is bound in a cavity between the carboxyl group and the hydroxyl group attached to the carbon C3 of neighboring residue and this cavity, which is schematically shown by an area 51 in Figure 4, has a relatively high electronegative atmosphere , because it is surrounded by four oxygen atoms designated by a superscript asterisk in the chemical formula in Figure 4. In the case of complexation between the CMC and an alkaline-earth metal ion there is another cavity which is designated by an area 52 in Figure 4. The area 52 has sufficient room for complexation between the carboxyl group and the hydroxyl group attached to the carbon C2 of neighboring residue, and this area also has a relatively high electronegative atmosphere because four oxygen atoms, designated by a subscript asterisk, surround the area. The metal ion forms a clamp between the neighboring residues by complexation and the clamp structure restricts the molecular rotation around the bond between the neighboring residues.
Fig. 4. Chemical formula of CMC. The areas 51 and 52 are the complex sites between CMC and an alkaline-earth metal ion.
384
A model for complexation between CMC and alkaline-earth metal ions
Assuming a symmetric rotational potential , the mean-square end-to-end distance r2 can be represented by (5) where & is the bond angle and value of cose and given by (cos
=
is the rotational angle and (coso) is an average
I::1T cos
I
(6)
Here U(
385
T.Matsumoto and H.Zenkoh
11. Zimm,B.H. (1956) J. Chern. Phys., 27, 673. 12. Ferry,J.D., Landel,R.F. and Wiliiams,M.L. (1955) J. Appl. Phys., 26, 359. 13.Graessley, W.W. (1971) J. Chern. Phys., 54, 5143. 14. Doi,M. and Edwards,S.F. (1978) J. Chern. Soc. Faraday Trans., II, 74,1789,1802. 15. Ferry,J.D. (1980) Viscoelastic Properties of Polymers, 3rd edn, Wiley, New York, p. 332. 16. Kratky,a., Leopold,H. and Puchwein,G. (1967) Koll-Z. Z. Polyrn., 216/217, 255. 17. Matsumoto,T. and Zenkoh,H. (1989) J. Soc. Rheol. Japan, 17,43. 18. Matsumoto,T., Kawai,M. and Masuda,T. (1992) Biopolyrn., 32,1721.
386
A new molecular model for complexation between carboxymethylcellulose and alkaline-earth metal ions in aqueous systems Takayoshi Matsumoto and Hirofumi Zenkoh Department of Polymer Chemistry, Kyoto University, Kyoto 606, Japan Abstract. A new molecular model for the complexat ion between carboxymeth ylcellulose (CMC) and alkaline-earth metal ions has been discussed through the change in relaxation time and radius of gyrat ion of the polymer. Elongat ion of the relaxati on time of the CMC main chain due to the complexat ion is inedependent of the choice of alkaline-earth metal ion , but depends only on the num ber s of metal io ns bo und to CMC . The radius of gyrati on of CMC in aqueo us systems is elong ated by complexation . In the new model for the complexation , there are two possibilities for th e complex site. The rotational angle around the bond between the neighbouring residues decreases with complex formation from - 60 to 21°.
Introduction
Certain mono- and oligosaccharides form complexes with alkaline and alkalineearth metal ions and the combining ratios of saccharide to metal ions are mostly 1:1 or 2: 1. However, there has been no clear evidence for the mode of complexation , though much experimental data exist on solubility , optical rotation and electrophoresis (1-4). The complex formation between polysaccharides and alkaline- earth metal ions has been investigated in terms of gelation , and a crosslink model and a pendant model for complexation have been developed (5). On the other hand, our recent studies on complexation between carboxymethylcellulose (CMC) and alkaline-earth metal ions have revealed that (i) complex formation is an intramolecular rather than intermolecular process and elongates the relaxation times of the CMC main chain, (ii) the site of complex formation is near a carboxyl group , and (iii) the complexation site does not act as a crosslink point (6,7) . Furthermore we proposed a new model for complexation, using data for complex formation between carboxymethylchitin (CMCh) and alkaline-earth metal ions through NMR, small angle X-ray scattering (SAXS) and rheological measurements (8). In the new complex model, an alkaline-earth metal ion is bound in a cavity between the carboxyl group and the hydroxyl group attached to the carbon C3 of the neighboring residue. In the present paper we discuss the applicability of the new complex model for the complexation between CMC and alkaline-earth metal ions and also discuss the degree of restriction of the molecular mobility by complexation. Material and methods
The CMC (sodium salt) employed was kindly provided by Daiichi Kogyo Seiyaku Co . (Kyoto , Japan). The weight average molecular weight Mw measured by light scattering, the weight average degree of polymerization Pw 379
T.Matsumoto and H.Zenkoh
Table I. Characteristics of carboxymethylcellulose emplo yed Sampl e
M../IO"
CMC18
22.2
880
1.13
Table II. Ratio of alkaline-earth metal ions bound to CMC18 Ion
Rat io
Mg Ca Sr Ba
0.529 0.972 0.714 0.915
and the degree of substitution D; are shown in Table I. The method of sample characterization and sample preparation are described in detail in a previous paper (7). SAXS was measured with a 6 m point focusing SAXS camera at the High Intensity X-ray Laboratory, Kyoto University (9). The X-ray wavelength was 1.542 A (CuKa radiation). The thickness of the sample solution was 1.5 mm. The ratio of alkaline-earth metal ions bound to CMC was estimated by the following method. A 1% solution of CMC (15 ml) in -0 .05 N alkaline-earth metal chloride aqueous solution was dialysed in 1 drrr' distilled water using a cellophane tube until the electroconductivity of the outer (surrounding) liquid became almost constant (for 5-7 days). EDTA (ethylene diaminetetraacetic acid) was used as the chelating agent with EBT (Erochrome Black T) as indicator. Table II shows the ratio of bound metal ion , f (metal ion bound to CMC chain/added metal ion) calculated. The affinity for binding of metal ions to CMC is rather larger than that in CMCh systems (8). However the order of affinity Ca-s- Ba >Sr > Mg is similar to that in CMCh systems. Results and discussion
Influence of binding metal ion on relaxation time Elongation in the relaxation times by complex formation has been estimated through the dynamic viscoelastic measurements, as has been sufficiently discussed (7,8). Dynamic modulus shifts to a longer time-scale region with complexation and the shift factor as (=TITO) (where T and TO are the relaxation times of the salt-added system and the salt-free system) increases with addition of the salt , i.e. with complexation (7,8). In Figure 1a,b the logarithm of the shift factor for 15% aqueous solution of CMC18 is plotted against c" the concentration of various alkaline-earth metal ions added to the system and against Cb (=cJ) , the concentration of the metal ions bound to the CMC chain. It should be noted that the shift factor, i.e. the relaxation time, becomes large with addition of the metal ion and that the shift factor apparently depends on the choice of the metal ion. However the dependence can be almost reduced by plotting as versus Cb ' In Figure la, log as decrease with increasing Cs for the 380
A model for complexation between
C~C
and alkaline-earth metal ions
2.----------------,
cscu ~ 1 IXI
o
10
ao
20 Cs
J
N (0 )
Cb/N 0.7 1.4
20
2.1
CMCI8.15"1. 40·C
III
01
Mg
01
o
O~--.L----.L------I
o
0.5
1.0
1.5
Cb I mol/mol residue (b)
Fig. 1. (a) Log as plotted against the concentration of added salt, c" for various metal salts. (b) Log as plotted against Cb, the concentration of metal ion bound to the CMC chain.
CaCl 2-added system at relatively large c" shown by a dotted line. This is likely to be attributed to the influence of salting-out. This kind of region is omitted in the present consideration. From the molecular dynamic point of view, the relaxation time of p-mode, T p , is theoretically represented by (10,11) b2n2~
6Tr 2p 2kT
(p = 1,2,3, ...)
(1)
where n is the degree of polymerization, ~ is the monomeric friction coefficient, b is the root mean-square end-to-end distance per monomeric unit, k is the Boltzmann constant and T is the absolute temperature. The shift factor as is given by the ratio of Tp of a salt-free system and that of a salt-added system, i.e. Tp s
as = - -
(2)
TpO
381
T.Matsumoto and H.Zenkoh
Here subscripts 0 and s mean the values for salt-free systems and salt-added systems, respectively. This relationship can also be obtained by the modified Rouse theory (12), the Graessley theory (13) and the Doi-Edwards theory (14). The change in relaxation time can be related to the change in b and ~ with concentration of metal ions. According to Ferry, the monomeric friction coefficient depends greatly upon the free volume and steric hindrances to rotations around the main chain (15). From the experimental results on CMCh, the contribution of the change in ~ to as is -70-80% in as (8). Influence of binding metal ion on molecular extension
The radius of gyration, R G , can be estimated from SAXS data by Guinier approximation, (3)
Here I is the scattering intensity and q is the scattering vector (4'lT/A)sin(e/2), 4
CMe 18
3
o
0
Q
0.4'10
0
0
3 o
2
0
0
00067
00 °0
0
0°0
000 00
3
en 2
0
o
0.01 o
0
0
o
0
3
2 o
0.02 0
0
o
0
Fig. 2. Log I plotted against q2 for SAXS data of 0.4% CMC aqueous systems at various concentration of CaCI 2 •
382
A model for complexation between CMC and alkaline-earth metal ions
where A is the X-ray wavelength and 6 is the scattering angle. Figure 2 shows Guinier plots (log I versus q2) for the 0.4% CMC18 aqueous systems at various concentrations of CaCl 2 from 0 to 0.2 N. However, the data points rather scatter, the plot can be approximated by a straight line in the extremely small angle region and the radius of gyration can be obtained from the slope of the straight line with a certain experimental error. Table III shows the radii of gyration obtained, which increase with increasing concentration of CaCI 2 . The values are reasonable compared with the data for other cellulose derivatives (16). In Figure 3, log as and log (b s2Ibo2 ) are plotted against concentration, Cb, of calcium ion bound to CMCI8. The relationship between log (b s2Ibo2 ) versus Cb which can be represented by a straight line, almost coincides with the data Table III. Radius of gyration of the CMC molecule in aqueous solution at various concentrations of CaCI 2 CaC I2 cone. N
mol/mol residue
o
o
0.0067 0.01 0.02
0.206 0.309 0.618
157 168 174 199
± ± ± ±
7 7 3 6
2 a
IJI
......
III
IJI Cl
s
~o
......
NIIl .0
..
Cl
.9 III
b~ I b ~
e
Ol 0
0.5 c b / mo ll m o l residue Fi~. 3. Logarithm of the shift factor as ( 6) , the ratio of the monomeric end-to-end distance b// bo (0) and the monomeric friction coefficient ~Ao (dotted line) plotted against the concentration Cb for CMC system . The ratio of b//b o2 is also plotted by the closed circle for the CMCh system.
383
T.Matsumoto and H.Zenkoh
(closed circles) of the CMCh systems described in a previous paper (9). It should be noted that the change in the end-to-end distance due to the complexation with calcium ions is almost equal in the systems of CMC and CMCh. There is no information on the influence of polymer concentration on the ratio b/bo. Here, we assume that the ratio is approximately independent of the polymer concentration. According to equation (2), the change in ~j~o with Cb can be estimated and represented by a dotted line in Figure 3. The contribution of ~Ao to as is much larger than that of bJbo. The relationship between log (b s2J b 0 2 ) and Cb can be given by (4) for the systems of both CMC and CMCh. Restriction of molecular rotation by complexation
In the new complex model proposed for complexation between the CMCh and an alkaline-earth metal ion, the alkaline-earth metal ion is bound in a cavity between the carboxyl group and the hydroxyl group attached to the carbon C3 of neighboring residue and this cavity, which is schematically shown by an area 51 in Figure 4, has a relatively high electronegative atmosphere , because it is surrounded by four oxygen atoms designated by a superscript asterisk in the chemical formula in Figure 4. In the case of complexation between the CMC and an alkaline-earth metal ion there is another cavity which is designated by an area 52 in Figure 4. The area 52 has sufficient room for complexation between the carboxyl group and the hydroxyl group attached to the carbon C2 of neighboring residue, and this area also has a relatively high electronegative atmosphere because four oxygen atoms, designated by a subscript asterisk, surround the area. The metal ion forms a clamp between the neighboring residues by complexation and the clamp structure restricts the molecular rotation around the bond between the neighboring residues.
Fig. 4. Chemical formula of CMC. The areas 51 and 52 are the complex sites between CMC and an alkaline-earth metal ion.
384
A model for complexation between CMC and alkaline-earth metal ions
Assuming a symmetric rotational potential , the mean-square end-to-end distance r2 can be represented by (5) where & is the bond angle and value of cose and given by (cos
=
is the rotational angle and (coso) is an average
I::1T cos
I
(6)
Here U(
385
T.Matsumoto and H.Zenkoh
11. Zimm,B.H. (1956) J. Chern. Phys., 27, 673. 12. Ferry,J.D., Landel,R.F. and Wiliiams,M.L. (1955) J. Appl. Phys., 26, 359. 13.Graessley, W.W. (1971) J. Chern. Phys., 54, 5143. 14. Doi,M. and Edwards,S.F. (1978) J. Chern. Soc. Faraday Trans., II, 74,1789,1802. 15. Ferry,J.D. (1980) Viscoelastic Properties of Polymers, 3rd edn, Wiley, New York, p. 332. 16. Kratky,a., Leopold,H. and Puchwein,G. (1967) Koll-Z. Z. Polyrn., 216/217, 255. 17. Matsumoto,T. and Zenkoh,H. (1989) J. Soc. Rheol. Japan, 17,43. 18. Matsumoto,T., Kawai,M. and Masuda,T. (1992) Biopolyrn., 32,1721.
386