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Adsorption of Glucose and Fructose Containing Disaccharides on Different Faujasites
J. Weitkamp, H.G. Karge, H. Pfeifer and W. HBlderich (Eds.) Zeolires and Related Microporous Marerials: &are of rhe A n 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.
1363
Adsor tion of Glucose and Fructose Containing Disaccharides on Di erent Faujasites
8
C. Buttersack, W. Wach and K. Buchholz Sugar Institute, Technical University P.O.Box 4636,38036 Braunschweig, Germany The Henry constants of glucose, fructose, trehalulose, sucrose, turanose, leucrose and isomaltulose on Y-zeolites in the predominant Na+-, K+-,S P - and Ca++-form and CaXzeolites were compared with corresponding data for a dealuminated NaY-zeolite. Linear free energy relationships have demonstrated that the fructose moiety determines both the interaction with K+-ions inside the Y-zeolite and the dealuminated Y-zeolite. The nearly reciprocal correlation of the Henry constants suggests the preferential adsorption of the same isomer and the same conformation in both microenvironments. The complexation by Ca++ and Sr++ is similar, but quite different to that by K+. The complexation of divalent cations by the fructose residue of turanose seems to be masked. 1
Introduction
The chromatographic separation of carbohydrates on zeolites has been established on an industrial scale for more than 10 years [1,2]. Naturally, the shape selectivity must be taken into consideration as an essential phenomenon of the separation process. Furthermore, analogous to the most applied separation with ion-exchange resins, the interaction of the sugar with the zeolite is also specifically influenced by various multivalent ions (3-61 following the strength of complexation in solution [7-91.However, strong affinities induced by K+-ions were reported for the interaction of sugars with Yzeolites which obviously can only occur in the special microenvironment of the zeolite supercage [3,4]. Beside the complexation of hydroxyl groups with cations, the nonpolar CH-moieties can contribute to hydrophobic interactions. This was proven by the increased solubility of nonpolar solvents in concentrated sugar solutions [l 0-121, by chromatography on alkylmodified silica [13-151 or copolymers of polystyrene-divinylbenzene [10,16] and by adsorption on activated carbon [17]. Computer simulations have shown that the "surface" of a carbohydrate molecule is characterized by a specific pattern of hydrophilic and hydrophobic regions [18,19]. Hydrophobic interactions are obviously the basis of the adsorption of saccharides by strongly dealurninated zeolites. An intensive contact of the adsorbed molecule was necessary to develop these interactions resulting in the fact that the selectivity of adsorption of different saccharides was significantly higher in case of disaccharides compared to monosaccharides [20]. The following study mainly concentrates on isomeric disaccharides composed of a glucose and a fructose moiety but linked by different glycosidic bonds and the influence of the counterion and the dealumination on the strength of sugar-zeolite interaction.
1364
2
Experlmental sectlon
The following Y- and X-zeolites (SVAI = 2.4 and 1.5 resp.) were obtained by Bayer: Nay, BOKNaY, BOCaNaY, 65SrNaY and 75CaNaX. The first number designates the percental degree of ion exchange with K+, Ca++ or Sr++ respectively. After washing, the zeolite powder was dried at 11OW16h, activated at 450'C/4h, and then stored over P4Ol0. A 1.O-g portion of the preconditioned zeolite was placed in a 10-mt glass vessel, and 3 mL of a solution containing 10 g/L of the respective saccharide was added. After shaking (20 hI25.C) the sugar concentration was analyzed by HPLC (22 'C; stationary phase: LiChrosorb-1OONH:! (Merck); mobile phase: acetonitrile/water (80/20,v/v); flow: 0.8 rnl/min). The accuracy ofthe concentration measurementwas improved to 0.3 % by referring each peak area prior to adsorption to that after adsorption. The difference was used to calculate the excess isotherm.
3
Results and Dlscusslon
Saccharides in the crystalline state are sterically fixed, e.g. D-glucose crystallized from water solution is identified as a-D-glucopyranoside. However by dissolving in water mutarotation occurs leading to an equilibrium of 5 physically different isomers (Figure 1). In case of disaccharides and oligosaccharidesthe number of isomeric compounds may be restricted. When the hydroxyl group in 5 or 6 position of the fructosyl residue is blocked by the glycosidic linkage, only the pyranose or the furanose respectively can occur, and the glycosidic linkage to the reducing position (sucrose) does not allow any tautomerisation. Table 1 shows the equilibrium composition of the isomers in bulk water.
/
6-fructopyranoside
6-fructofuranoside
HO
OH OH
OH
efructopyranoside Figure 1.
Tautomeric forms of fructose
OH
OH efructofuranoside
1365
Table 1 Tautomeric equilibrium of the saccharides containing glucose (G) and fructose (F) moieties saccharide (D-form)
glycosyl linkage
glucose fructose trehalulose sucrose turanose leucrose isomaltulose
G(1-l)F G(1-2)F G(1-3)F G(1-5)F G(1-6)F
pyranose a
furanose
B
a
I3
0.15 25.0
38.8 2.5
60.9 65.0
0.14 6.5
2.3
70.9
5.7
1.4 1.9
47.3 98.1
-
-
14.5
21.1 100.0 36.8
19.7
80.3
acyclic
ref.
0.007 0.8
[21] P21 1231 ~ 3 1 1231 ~ 3 1
When the adsorption equilibrium between water and zeolite is determined only by measuring the change of the bulk concentration, the calculated amount of adsorbed substance is due an unkown mixture of isomers in the zeolite pore because each isomer is characterized by its own affinity to the zeolite. Furthermore, the situation is complicatedby the fact that each pyranoide configuration can exist in two different conformations. However, it is proven by NMR measurements of carbohydrate solutions that the complexation with a metal cation can strongly influence the equilibrium distribution [7]. Therefore, in contrast to the approach presented by Schbllner et al. [3], the distribution of isomers known from measurements in aqueous solution is not considered here. 3.1
Analysts of the origlnal data
Table 2 shows the excess Henry constants of the saccharides investigated towards different types of zeolites. Glucose war adsorbed only in case of the dealuminated zeolite. In all other cases the concentration of glucose inside the zeolite pores was found to be lower than those in bulk water (negative values of KE). Remarkable is the positive contribution of the K+- and SF-ions, which confirms data previously published by Schollner et al. [3]. The well-known interaction of fructose with divalent cations [7] is reflected by the positive KE-values for Ca++ and SP+ containing zeolites. Although K+ions show only weak interactions with monosaccharides [8], in aggreement with results recently published [3], within the microenvironment of the Y-zeolite a significant interaction was found to exist. The main question of interest was to find out wheather these effects are additive or not. As expected, the interaction of the glucose and fructose containing disaccharides with the NaY-zeolite was low. However, the data for the Ca++ containing zeolites show that the fructosyl residues cannot develop their intrinsic complexation in the same extent compared with the monosacharide. In case of the CaX-zeolite the KE-valuescome near to those found for glucose, maltose, maltotriose and maltotetraose, which are assumed to be totally excluded from the zeolite pores. In case of the CaY-zeolite the affinity and also the
1366
selectivity was generally increased. Obviously, the interaction of the fructosyl residue depends on the position of the attached glucose moiety. The affinity due to this position was found to decrease according to: 5>6.;1>2>3 Table 2 Excess Henry constants KE [mug] at 25'C for a dealuminated NaY-zeolite (Si/AI=I lo), for Nay-, 80KNaY-, 65SrNaY and 8OCaNaY-zeolites (SilAL2.4) and a 75CaNaX-zeolite (Si/AI=I 5).
Ilq
k.
0.20 1.03
-0.30 -0.03
trehalulose sucrose turanose leucrose isomaltulose
6.26 22.0 1.3 23.0 6.5
maltose maltotriose maltotetraose
0.40 0.3 1 -0.06
saccharide (D-form) glucose fructose
Na
zeolite 2.4 K
2.4 Sr
24 , Ca)
1.5 Ca
-0.05 0.30
-0.29 0.52
-0.27 0.43
-0.28 0.02
-0.22 -0.25 -0.20 -0.20 -0.17
0.25 -0.26 0.51 -0.25 -0.01
-0.13 -0.24 -0.21 0.00 -0.21
-0.08 -0.11 -0.27 0.01 -0.08
-0.27 -0.30 -0.30 -0.22 -0.29
-0.32 -0.30 -0.32
-0.28 -0.25
-0.28 -0.28
-0.27 -0.32 -0.33
-0.27 -0.28 -0.27
*) taken from ref. [20]
It is generally accepted that an axial-equatorial-axial (ax-eq-ax) arrangement of three neighbouring OH groups in the pyranose ring or a cis-cis arrangement of three OH groups in the furanose ring are best suited for the complexation of Ca++ ions [7-91. None of these sequences can be formed by glucose and fructose. However, weak interactions with two OH-groups in an eq-ax or cis-cis arrangement resp. have been discussed [24-271. These positions are located at C2 and C3 or C4 and C5 in the p-fructopyranoside or the pfructofuranosidemoieties. Therefore the following hypothesis is presented: The complexationtakes place at the oxygen atoms of hydroxyl groups at C, and C+. A glycosidic linkage to the glucose residue at these positions weakens the complexationmost strongly. The strength of complexation increases with increasing distance of the glycosidic bond from the complexationsites. Due to this rule one can derive the following orders of affinity: C5 > C4 > C,for the pfructopyranoide moiety, c6 > C4 > C3 and C1 > C2 for the P-fructofuranoide moiety, and C5 > c6. In the latter case both positions have the same distance to the complexing sites but differ by the fact that the linkage at C5 or C6 are only possible in either the pyranoide or furanoide form respectively.
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This is in aggreement with the experimental finding. However, the adsorption of the disaccharide with a 1-4 glycosidic bond (maltulose) has not yet been measured. (This substance is not readily available). As expected, a similar effect was found for the adsorption on a SF+ containing Yzeolite. Again, the attachment of the glucose residue at C5 of the P-fructopyranoside (leucrose) yields the greatest Henry constants, while the linkage at C2 or C3 (sucrose or turanose) is unfavourable. Remarkable is the low value for isomaltulose. In this molecule the fructosyl residue is fixed in the furanoide form. Therefore, it may be assumed that the general effect of the less favourable furanoid form is more pronounced for the SF+-ion. The hypothesis is supported by the relatively low affinity of sucrose, the fructosyl residue of which can also solely exist in the furanoide form. The Y-zeolite exchanged with K+-ions shows quite another interaction with the disaccharides. The affinity due to the position of the glycosidic linkage at the fructosyl moiety was quite another: 3>1>6>5=2
Remarkable is the extraordinary high complexationof K+-ions by turanose compared with the extreme low affinity towards divalent cations. As the interaction of monosaccharides with K+-ions in solution was shown to be very low [8], a cooperative mechanism of K+ions inside the supercage and an interaction with the inner surface of the zeolite pore may be imaginated. It must be noted that Na+-ions do not exhibit such properties (Table 2). Probably, the K+ ion, which is isoelectronic with the Ca++-ion, can easier loose its hydration shell thus getting contact with the hydroxyl groups of the saccharides. The interaction of the disaccharides with the KY-zeolite can be compared with the affinities towards a strongly dealuminated Y-zeolite 1201 (Table 2). As the most surprising result the respective row of affinity due to the position of the glucosidic linkage at the fructosyl moiety is reversed when compared with the KY-zeolite: 2=5>6=1>3
One is tempted to postulate the preferential adsorption of the same isomers and a similar conformational state of the disaccharides in both types of zeolites. Considering the dealuminated zeolite, a high interaction of the sugar with the inner surface of the pores by the apolar CH-groups must be compared with the energy contribution of the remaining hydroxyl groups. According to current models of hydrophobic effects [28] this energy should be minimized by hydrogen bridges between these hydroxyl groups plus water molecules. The high selectivity (e.9. glucose: KE = 0.2 and sucrose: @ = 22) is obviously due to the ability of some molecules to change their conformation resulting in a high energy contribution by dispersive forces between the CH-moieties and the inner SiQ surface plus an optimal hydrophilic interaction located in the inner part of the zeolite pore (induced fitting). It seems that K+-ions present in a usual Y-zeolite interact with the same isomer and induce the same conformation of the guest-molecules.
1368
3.2
Analysls of the real Henry constants
The excess measures used so far cannot be directly interpreted as thermodynamic quantities. The zeolites are characterized by a defined pore volume. The true adsorption isotherm can be obtained by a pore-filling model [29]. Regarding low degrees of adsorption respective pores filled mainly with water, the true Henry constant is approximatelygiven by K=e+KE where e is the void volume of the pores completely filled with water. Usually this amount is measured by equilibration of the dried zeolite in an environment of constant humidity p]. However, in case of hydrophobic dealuminated zeolites this method fails [30]. To overcome this problem, the value of e was taken from the excess Henry constant for those saccharides which are assumed to be totally excluded from the pores:
0 = e + KEmin, With reference of the data in Table 2 an average value of KEmin= - 0.315 was assumed for all zeolites investigated. The free energy of adsorption is proportional to In K. Thus free energy relationships can be established between the adsorption on different types of zeolites. Figure 2 shows linear relations for the adsorption on the Ca++ containing Y- and X-zeolites and the Yzeolite exchanged with Sr++. The slope (2.2) is significantly greater than 1 indicating that the weakening effect of glucose bound on the fructosyl residue is increased for the interaction with Sr++-ion or within the less hydrophobic environment of the X-zeolite. Glucose cannot fit this straight line because of the absence of complexation. The exceptional state of turanose indicates that this disacharide is most similar to glucose; the complexation ability of the fructosyl residue seems to be masked. Figure 3 demonstrates the total different states of the saccharides in the presence of Ca++- or K+-ions. Figure 4 shows that decreasing complexation ability of fructose correlates with increasing hydrophobicity. The small slope (-0.3) reflects the significant higher selectivity caused by hydrophobic interactions. Again turanose is shown to be very similar to glucose. Figure 5 demonstrates the inverse affinity of fructose and all fructose plus glucose containing disaccharides (without maltulose). The slope is about - 1.O and shows that both K values are correlated by a pure reciprocal relationship. As expected, only glucose is located out of this line.
1369
iru
d
leu r
u
suc 0 imalt 0 glu 0 glu
-4
tur
0
-5
suc I
I
-3.5
-3.0
-2.5
1
1
1
-2.0
In
-1.5
-1.0
1
-0.5
0.0
KCaY
Figure 2. Linear free energy relationship of the adsorption on the 80CaNaY-with that on the 75CaNaX- (D)and the 65SrNaY-zeolite (0). In KCaX= -0.27 + 2.2 In kav , r = 0.95 In KSrY= 0.45 + 1.7 In Kcay , r = 0.94
0 tur -0.5 - 1 .o
imalt 0
0 glu
-1.5 In
fru 0
tre 0
KKY
-2.0
-2.5 SUC
-3.0
-3.5
1 -3.5
I
-3.0
I
-2.5
0
1
-2.0
In
1
-1.5
0 leu
1
-1.0
1
-0.5
0.0
KCaY
Figure 3. Comparison of the relative free energies of adsorption on the 80KNaY- and the 8OCaNaY-zeolite.
I370
,
0.0
In
-0.5
-
-1.0
-
-1.5
-
-2.0
-
-2.5
-
-3.0
-
KCaY
0 fru
0 leu suc
I
-3.5
0 tur
0 glu
-1
I
I
I
I
0
1
2
3
4
Figure 4. Linear free energy relationship of the adsorption on a dealuminated Nay-zeolite (Si/AI = 1 10) and that on the 80CaNaY-zeolite (SVAL2.4). In Kcay = -0.48 - 0.33 In Ksi , r = 0.75 0.0 tur
\a,
fru 0
-0.5 -
i
In ,K
-3.0 -3.5
O glu
1 f
-1
0 SUC 0 leu I
I
I
I
0
1
2
3
4
In K,, Figure 5. Linear free energy relationship of the adsorption on a dealuminated Nay-zeolite (Si/AI=llO) and that on the 80KNa-zeolite. In Kw = 0.24 - 0.88 In Ksi , r = 0.91
1371
Llterature 451 1. J. A. Johnson and A. R. Oroskar, Stud. Surf. Sci. Catal., 46 (1989) 2. D. M. Ruthven, Chern. Eng. Progr. 84 (2) (1988) 42 3. R. Schollner, W.-D. Einicke and B. Gltiser, J. Chern. SOC.,Faraday Trans. 89 (1 993) 1871 4. R. Schollner, S. Unverricht and W.-D. Einicke, Proc. 9th Int. Zeol. Conf., Montreal 1992 (Eds.: R. von Ballrnoos, J. 8. Higgins, M. M. Tracy), Vol 11, p. 3-11 5. T. M. Wortel and H. van Bekkurn, J. Roy. Netherl. Chern. SOC.97 (1978) 156 6. J. D. Sherman and C. C. Chao, Stud. Surf. Sci. Catal. 28 (1986) 1025 7. S. J. Angyal, Adv. Carbohydr. Biochern. 47 (1989) 1 8. J. P. Morel, J. Chern. SOC.,Faraday Trans 1, 84 (1988) 2567 9. J. P.Morel-Desrosiersand J. P. Morel, J. Chern. SOC.,Faraday Trans 1, 84 (1989) 3461 10. Y. Yano and M. Janado, Trends Glycosci. Glycotechnol. 2 (1 990) 156 11. Y. Yano and M. Janado, Bull. Chern. SOC.Jpn. 58 (1985) 1913 12. C. Sivakarna Sundari, B. Rarnan and D. Balasubrarnanian, Biochirn. Biophys. Acta 1065 (1991) 35 13. N. W. H. Chetharn and G. Teng, J. Chrornatogr. 336 (1984) 161 14. L. A. T. Verhaar, B. F. M. Kuster and H. A. J. Claesens, J. Chrornatogr. 284 (1984) 1 15. P. Vratny, J. Coupek, S.Vozka and Z. Hostornska,J. Chromatogr. 254 (1983) 143 16. Y. Yano and M. Janado, J. Solut. Chern. 14 (1985) 891 17. 1. Abe, K. Hayashi and M. Kitagawa, Carbon 21 (1983) 189 18. K. Miyajirna, K. Machida and M. Nakagaki, Bull. Chern. SOC.Jpn. 58 (1985) 2595 19. F. W. Lichtenthaler, S.lrnrnel and U. Kreis, Starch 43 (1991) 121 20. C. Buttersack, W. Wach and K. Buchholz, J. Phys. Chern. 97 (1993) 11861 21. S. Angyal, Adv. Carbohydr. Chern. Biochern. 49 (1 991) 19 22. S. Angyal, Adv. Carbohydr. Chern. Biochern. 42 (1984) 15 23. F.W. Lichtenthaler and S. Ronninger, J. Chern. SOC.Perkin Trans. (1990) 1489 24. M. C. R. Syrnons, J. A. Benbow and M. Pelrnore, J. Chern. SOC.,Faraday Trans. 1, 80 (1984) 1999 25. J. P. Morel, Can. J. Chern. 64 (1986) 996 26. J. P.Morel, Can. J. Chern. 65 (1987) 2656 27. R. W. Goulding, J. Chrornatogr. 103 (1975) 229 28. W. Blokzijl and J. B. F. N. Engberts, Angew. Chern. 105 (1993) 1610 29. S.Sircar, Surf. Sci. 148 (1 984) 489 30. M. W. Anderson and J. Klinowski, J. Chern. SOC.,Faraday Trans. I , 82 (1 986) 3569
1363
Adsor tion of Glucose and Fructose Containing Disaccharides on Di erent Faujasites
8
C. Buttersack, W. Wach and K. Buchholz Sugar Institute, Technical University P.O.Box 4636,38036 Braunschweig, Germany The Henry constants of glucose, fructose, trehalulose, sucrose, turanose, leucrose and isomaltulose on Y-zeolites in the predominant Na+-, K+-,S P - and Ca++-form and CaXzeolites were compared with corresponding data for a dealuminated NaY-zeolite. Linear free energy relationships have demonstrated that the fructose moiety determines both the interaction with K+-ions inside the Y-zeolite and the dealuminated Y-zeolite. The nearly reciprocal correlation of the Henry constants suggests the preferential adsorption of the same isomer and the same conformation in both microenvironments. The complexation by Ca++ and Sr++ is similar, but quite different to that by K+. The complexation of divalent cations by the fructose residue of turanose seems to be masked. 1
Introduction
The chromatographic separation of carbohydrates on zeolites has been established on an industrial scale for more than 10 years [1,2]. Naturally, the shape selectivity must be taken into consideration as an essential phenomenon of the separation process. Furthermore, analogous to the most applied separation with ion-exchange resins, the interaction of the sugar with the zeolite is also specifically influenced by various multivalent ions (3-61 following the strength of complexation in solution [7-91.However, strong affinities induced by K+-ions were reported for the interaction of sugars with Yzeolites which obviously can only occur in the special microenvironment of the zeolite supercage [3,4]. Beside the complexation of hydroxyl groups with cations, the nonpolar CH-moieties can contribute to hydrophobic interactions. This was proven by the increased solubility of nonpolar solvents in concentrated sugar solutions [l 0-121, by chromatography on alkylmodified silica [13-151 or copolymers of polystyrene-divinylbenzene [10,16] and by adsorption on activated carbon [17]. Computer simulations have shown that the "surface" of a carbohydrate molecule is characterized by a specific pattern of hydrophilic and hydrophobic regions [18,19]. Hydrophobic interactions are obviously the basis of the adsorption of saccharides by strongly dealurninated zeolites. An intensive contact of the adsorbed molecule was necessary to develop these interactions resulting in the fact that the selectivity of adsorption of different saccharides was significantly higher in case of disaccharides compared to monosaccharides [20]. The following study mainly concentrates on isomeric disaccharides composed of a glucose and a fructose moiety but linked by different glycosidic bonds and the influence of the counterion and the dealumination on the strength of sugar-zeolite interaction.
1364
2
Experlmental sectlon
The following Y- and X-zeolites (SVAI = 2.4 and 1.5 resp.) were obtained by Bayer: Nay, BOKNaY, BOCaNaY, 65SrNaY and 75CaNaX. The first number designates the percental degree of ion exchange with K+, Ca++ or Sr++ respectively. After washing, the zeolite powder was dried at 11OW16h, activated at 450'C/4h, and then stored over P4Ol0. A 1.O-g portion of the preconditioned zeolite was placed in a 10-mt glass vessel, and 3 mL of a solution containing 10 g/L of the respective saccharide was added. After shaking (20 hI25.C) the sugar concentration was analyzed by HPLC (22 'C; stationary phase: LiChrosorb-1OONH:! (Merck); mobile phase: acetonitrile/water (80/20,v/v); flow: 0.8 rnl/min). The accuracy ofthe concentration measurementwas improved to 0.3 % by referring each peak area prior to adsorption to that after adsorption. The difference was used to calculate the excess isotherm.
3
Results and Dlscusslon
Saccharides in the crystalline state are sterically fixed, e.g. D-glucose crystallized from water solution is identified as a-D-glucopyranoside. However by dissolving in water mutarotation occurs leading to an equilibrium of 5 physically different isomers (Figure 1). In case of disaccharides and oligosaccharidesthe number of isomeric compounds may be restricted. When the hydroxyl group in 5 or 6 position of the fructosyl residue is blocked by the glycosidic linkage, only the pyranose or the furanose respectively can occur, and the glycosidic linkage to the reducing position (sucrose) does not allow any tautomerisation. Table 1 shows the equilibrium composition of the isomers in bulk water.
/
6-fructopyranoside
6-fructofuranoside
HO
OH OH
OH
efructopyranoside Figure 1.
Tautomeric forms of fructose
OH
OH efructofuranoside
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Table 1 Tautomeric equilibrium of the saccharides containing glucose (G) and fructose (F) moieties saccharide (D-form)
glycosyl linkage
glucose fructose trehalulose sucrose turanose leucrose isomaltulose
G(1-l)F G(1-2)F G(1-3)F G(1-5)F G(1-6)F
pyranose a
furanose
B
a
I3
0.15 25.0
38.8 2.5
60.9 65.0
0.14 6.5
2.3
70.9
5.7
1.4 1.9
47.3 98.1
-
-
14.5
21.1 100.0 36.8
19.7
80.3
acyclic
ref.
0.007 0.8
[21] P21 1231 ~ 3 1 1231 ~ 3 1
When the adsorption equilibrium between water and zeolite is determined only by measuring the change of the bulk concentration, the calculated amount of adsorbed substance is due an unkown mixture of isomers in the zeolite pore because each isomer is characterized by its own affinity to the zeolite. Furthermore, the situation is complicatedby the fact that each pyranoide configuration can exist in two different conformations. However, it is proven by NMR measurements of carbohydrate solutions that the complexation with a metal cation can strongly influence the equilibrium distribution [7]. Therefore, in contrast to the approach presented by Schbllner et al. [3], the distribution of isomers known from measurements in aqueous solution is not considered here. 3.1
Analysts of the origlnal data
Table 2 shows the excess Henry constants of the saccharides investigated towards different types of zeolites. Glucose war adsorbed only in case of the dealuminated zeolite. In all other cases the concentration of glucose inside the zeolite pores was found to be lower than those in bulk water (negative values of KE). Remarkable is the positive contribution of the K+- and SF-ions, which confirms data previously published by Schollner et al. [3]. The well-known interaction of fructose with divalent cations [7] is reflected by the positive KE-values for Ca++ and SP+ containing zeolites. Although K+ions show only weak interactions with monosaccharides [8], in aggreement with results recently published [3], within the microenvironment of the Y-zeolite a significant interaction was found to exist. The main question of interest was to find out wheather these effects are additive or not. As expected, the interaction of the glucose and fructose containing disaccharides with the NaY-zeolite was low. However, the data for the Ca++ containing zeolites show that the fructosyl residues cannot develop their intrinsic complexation in the same extent compared with the monosacharide. In case of the CaX-zeolite the KE-valuescome near to those found for glucose, maltose, maltotriose and maltotetraose, which are assumed to be totally excluded from the zeolite pores. In case of the CaY-zeolite the affinity and also the
1366
selectivity was generally increased. Obviously, the interaction of the fructosyl residue depends on the position of the attached glucose moiety. The affinity due to this position was found to decrease according to: 5>6.;1>2>3 Table 2 Excess Henry constants KE [mug] at 25'C for a dealuminated NaY-zeolite (Si/AI=I lo), for Nay-, 80KNaY-, 65SrNaY and 8OCaNaY-zeolites (SilAL2.4) and a 75CaNaX-zeolite (Si/AI=I 5).
Ilq
k.
0.20 1.03
-0.30 -0.03
trehalulose sucrose turanose leucrose isomaltulose
6.26 22.0 1.3 23.0 6.5
maltose maltotriose maltotetraose
0.40 0.3 1 -0.06
saccharide (D-form) glucose fructose
Na
zeolite 2.4 K
2.4 Sr
24 , Ca)
1.5 Ca
-0.05 0.30
-0.29 0.52
-0.27 0.43
-0.28 0.02
-0.22 -0.25 -0.20 -0.20 -0.17
0.25 -0.26 0.51 -0.25 -0.01
-0.13 -0.24 -0.21 0.00 -0.21
-0.08 -0.11 -0.27 0.01 -0.08
-0.27 -0.30 -0.30 -0.22 -0.29
-0.32 -0.30 -0.32
-0.28 -0.25
-0.28 -0.28
-0.27 -0.32 -0.33
-0.27 -0.28 -0.27
*) taken from ref. [20]
It is generally accepted that an axial-equatorial-axial (ax-eq-ax) arrangement of three neighbouring OH groups in the pyranose ring or a cis-cis arrangement of three OH groups in the furanose ring are best suited for the complexation of Ca++ ions [7-91. None of these sequences can be formed by glucose and fructose. However, weak interactions with two OH-groups in an eq-ax or cis-cis arrangement resp. have been discussed [24-271. These positions are located at C2 and C3 or C4 and C5 in the p-fructopyranoside or the pfructofuranosidemoieties. Therefore the following hypothesis is presented: The complexationtakes place at the oxygen atoms of hydroxyl groups at C, and C+. A glycosidic linkage to the glucose residue at these positions weakens the complexationmost strongly. The strength of complexation increases with increasing distance of the glycosidic bond from the complexationsites. Due to this rule one can derive the following orders of affinity: C5 > C4 > C,for the pfructopyranoide moiety, c6 > C4 > C3 and C1 > C2 for the P-fructofuranoide moiety, and C5 > c6. In the latter case both positions have the same distance to the complexing sites but differ by the fact that the linkage at C5 or C6 are only possible in either the pyranoide or furanoide form respectively.
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This is in aggreement with the experimental finding. However, the adsorption of the disaccharide with a 1-4 glycosidic bond (maltulose) has not yet been measured. (This substance is not readily available). As expected, a similar effect was found for the adsorption on a SF+ containing Yzeolite. Again, the attachment of the glucose residue at C5 of the P-fructopyranoside (leucrose) yields the greatest Henry constants, while the linkage at C2 or C3 (sucrose or turanose) is unfavourable. Remarkable is the low value for isomaltulose. In this molecule the fructosyl residue is fixed in the furanoide form. Therefore, it may be assumed that the general effect of the less favourable furanoid form is more pronounced for the SF+-ion. The hypothesis is supported by the relatively low affinity of sucrose, the fructosyl residue of which can also solely exist in the furanoide form. The Y-zeolite exchanged with K+-ions shows quite another interaction with the disaccharides. The affinity due to the position of the glycosidic linkage at the fructosyl moiety was quite another: 3>1>6>5=2
Remarkable is the extraordinary high complexationof K+-ions by turanose compared with the extreme low affinity towards divalent cations. As the interaction of monosaccharides with K+-ions in solution was shown to be very low [8], a cooperative mechanism of K+ions inside the supercage and an interaction with the inner surface of the zeolite pore may be imaginated. It must be noted that Na+-ions do not exhibit such properties (Table 2). Probably, the K+ ion, which is isoelectronic with the Ca++-ion, can easier loose its hydration shell thus getting contact with the hydroxyl groups of the saccharides. The interaction of the disaccharides with the KY-zeolite can be compared with the affinities towards a strongly dealuminated Y-zeolite 1201 (Table 2). As the most surprising result the respective row of affinity due to the position of the glucosidic linkage at the fructosyl moiety is reversed when compared with the KY-zeolite: 2=5>6=1>3
One is tempted to postulate the preferential adsorption of the same isomers and a similar conformational state of the disaccharides in both types of zeolites. Considering the dealuminated zeolite, a high interaction of the sugar with the inner surface of the pores by the apolar CH-groups must be compared with the energy contribution of the remaining hydroxyl groups. According to current models of hydrophobic effects [28] this energy should be minimized by hydrogen bridges between these hydroxyl groups plus water molecules. The high selectivity (e.9. glucose: KE = 0.2 and sucrose: @ = 22) is obviously due to the ability of some molecules to change their conformation resulting in a high energy contribution by dispersive forces between the CH-moieties and the inner SiQ surface plus an optimal hydrophilic interaction located in the inner part of the zeolite pore (induced fitting). It seems that K+-ions present in a usual Y-zeolite interact with the same isomer and induce the same conformation of the guest-molecules.
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3.2
Analysls of the real Henry constants
The excess measures used so far cannot be directly interpreted as thermodynamic quantities. The zeolites are characterized by a defined pore volume. The true adsorption isotherm can be obtained by a pore-filling model [29]. Regarding low degrees of adsorption respective pores filled mainly with water, the true Henry constant is approximatelygiven by K=e+KE where e is the void volume of the pores completely filled with water. Usually this amount is measured by equilibration of the dried zeolite in an environment of constant humidity p]. However, in case of hydrophobic dealuminated zeolites this method fails [30]. To overcome this problem, the value of e was taken from the excess Henry constant for those saccharides which are assumed to be totally excluded from the pores:
0 = e + KEmin, With reference of the data in Table 2 an average value of KEmin= - 0.315 was assumed for all zeolites investigated. The free energy of adsorption is proportional to In K. Thus free energy relationships can be established between the adsorption on different types of zeolites. Figure 2 shows linear relations for the adsorption on the Ca++ containing Y- and X-zeolites and the Yzeolite exchanged with Sr++. The slope (2.2) is significantly greater than 1 indicating that the weakening effect of glucose bound on the fructosyl residue is increased for the interaction with Sr++-ion or within the less hydrophobic environment of the X-zeolite. Glucose cannot fit this straight line because of the absence of complexation. The exceptional state of turanose indicates that this disacharide is most similar to glucose; the complexation ability of the fructosyl residue seems to be masked. Figure 3 demonstrates the total different states of the saccharides in the presence of Ca++- or K+-ions. Figure 4 shows that decreasing complexation ability of fructose correlates with increasing hydrophobicity. The small slope (-0.3) reflects the significant higher selectivity caused by hydrophobic interactions. Again turanose is shown to be very similar to glucose. Figure 5 demonstrates the inverse affinity of fructose and all fructose plus glucose containing disaccharides (without maltulose). The slope is about - 1.O and shows that both K values are correlated by a pure reciprocal relationship. As expected, only glucose is located out of this line.
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iru
d
leu r
u
suc 0 imalt 0 glu 0 glu
-4
tur
0
-5
suc I
I
-3.5
-3.0
-2.5
1
1
1
-2.0
In
-1.5
-1.0
1
-0.5
0.0
KCaY
Figure 2. Linear free energy relationship of the adsorption on the 80CaNaY-with that on the 75CaNaX- (D)and the 65SrNaY-zeolite (0). In KCaX= -0.27 + 2.2 In kav , r = 0.95 In KSrY= 0.45 + 1.7 In Kcay , r = 0.94
0 tur -0.5 - 1 .o
imalt 0
0 glu
-1.5 In
fru 0
tre 0
KKY
-2.0
-2.5 SUC
-3.0
-3.5
1 -3.5
I
-3.0
I
-2.5
0
1
-2.0
In
1
-1.5
0 leu
1
-1.0
1
-0.5
0.0
KCaY
Figure 3. Comparison of the relative free energies of adsorption on the 80KNaY- and the 8OCaNaY-zeolite.
I370
,
0.0
In
-0.5
-
-1.0
-
-1.5
-
-2.0
-
-2.5
-
-3.0
-
KCaY
0 fru
0 leu suc
I
-3.5
0 tur
0 glu
-1
I
I
I
I
0
1
2
3
4
Figure 4. Linear free energy relationship of the adsorption on a dealuminated Nay-zeolite (Si/AI = 1 10) and that on the 80CaNaY-zeolite (SVAL2.4). In Kcay = -0.48 - 0.33 In Ksi , r = 0.75 0.0 tur
\a,
fru 0
-0.5 -
i
In ,K
-3.0 -3.5
O glu
1 f
-1
0 SUC 0 leu I
I
I
I
0
1
2
3
4
In K,, Figure 5. Linear free energy relationship of the adsorption on a dealuminated Nay-zeolite (Si/AI=llO) and that on the 80KNa-zeolite. In Kw = 0.24 - 0.88 In Ksi , r = 0.91
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