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Structural studies on galactomannans and their complexes
Food Hydrocolloids vol.! nO .5/6 pp.581 -582, 1987
Structural studies on galactomannans and their complexes W.T.Winter, B.K.Song and H.Bouckris Department of Chemistry and Polymer Research Institute, Polytechnic University, 333 Jay Street, Brooklyn. NY lJ201. USA
Subtle variations in the physical properties of galactomannans from different plant sources together with the persistent question of the influence of galactose distribution and abundance upon structure and properties has prompted an extensive re-examination of their molecular organization by diffraction methods (1-5; H.Bouckris and W.Winter, in preparation), molecular modeling and, recently, by solid state NMR (R.Marchessault, M.G. Taylor and W.T.Winter, in preparation). Table I summarizes the crystallogaphic lattice constants for a variety of galactomannans. The constancy of the c (fibre) axis is easily understood in terms of conformational restrictions on the mannan backbone imposed by the (3-(1-4) linkage. Refinement of several of the structures listed in Table I using the measured X-ray intensities has confirmed our hypothesis that the invariance in the a dimension reflects the orderly association of mannan chains stabilized by a network of hydrogen bonds (see Figure 1) involving mannan 0 (2) and, when not galactosylated , mannan 0 (6) as donors and mannan 0(5) or 0 (2) as acceptors. This sheet, since it does not involve the galactose units, is insensitive to the degree of galactose substitution. The third dimension, b, is much larger , reflecting the occurrence of the galactose units as 'spacers' between mannan sheets. This dimension is also most sensitive to hydration with reversible changes of 3 - 5 A occurring solely in response to humidification or drying . The local absence of a galactose moiety arising from their irregular substitution along the main chain leads to a void which is filled with loosely bound or ' free' water. Thus, gums with lower DS values lose their crystallinity more readily upon drying . Finally, as shown in Figure I, a binding site for cross-linking agents such as borate or zirconium ions exists between the sheets and should lead to three-dimensional network formation by galactose-borate (or Zr)-galactose interactions. NMR data is also consistent with this model of cross-linking in galactomannan gels (6,7).
Tab le I. Galactomannan lattice constants Source
% RH
G/M
a ( A)
b (A)
c (A)
Fenugreek (1) Fenugreek (1) Lucerne (I ) Lucerne (I) Guar (2, H.Bouckris et al. , in preparation) Tara (3) Locust bean (2. H.Bouckris et al. , in preparation) Mannan I (4,5)
71 0 50 0 81 88 88 0
0.93 0.93 0.92 0.92 0.64 0.39 0.32 0.00
9.12 8.94 8.98 9 .20
33.35 29.48 33.32 3 1.09 32.83 30.62 30.6 1 7.21
10.35 10.27 10.34 10.34 10.35 10.40 10.24 10.27
© lRL Press Limited, Oxford, England
9.13 8.9 1 9.04 8.92
581
W.T.Winter, B.K.Song and H.Bollckris
--------- b----------_
H
><1I
Fig. 1. Projectiondown the c-axis of a representative galactomannan showinggalactose- galactose association between sheets; a possible borate (or zirconium) binding site (B) and unbound water (H) filling a void created by the absence of a galactosyl residue. M designates the mannan chains. G, and Gj designate the galactosyl residues which assume two different linkage conformations in these structures.
Acknowledgements We gratefully acknowledge partial support of thiswork by the donors of the Petroleum Research Fund of the American Chemical Society and a grant from Hercules, Inc. References 1. 2. 3. 4. 5. 6. 7.
Song,B.K., Winter,W.T. and Taravel,F.R. (1987) Macromolecules, in press. Marchessault,R.H., Buleon,A., Deslandes.Y. and GOIO,T.) (1979) J. Colloid Interface Sci., 71, 375. Chien,Y.Y. and Winler,W.T. (1985) Macromolecules, 18, 1357. Chanzy,H., Perez.S,, Miller,D.P., Paradossi.G. and Winter,W.T. (1987) Macromolecules, 20, 2407. Nieduszynski,I.A. and Marchessault,R.H.M. (1972) Can. J. Chem., 50, 230-236. Noble.O. and TaravelFvk. (1987) Carbohydr. Res., 166,1. Gey,C., Noble,a., Perez.S. and Taravel,F.R. (1987) Carbohydr. Res., in press.
582
Structural studies on galactomannans and their complexes W.T.Winter, B.K.Song and H.Bouckris Department of Chemistry and Polymer Research Institute, Polytechnic University, 333 Jay Street, Brooklyn. NY lJ201. USA
Subtle variations in the physical properties of galactomannans from different plant sources together with the persistent question of the influence of galactose distribution and abundance upon structure and properties has prompted an extensive re-examination of their molecular organization by diffraction methods (1-5; H.Bouckris and W.Winter, in preparation), molecular modeling and, recently, by solid state NMR (R.Marchessault, M.G. Taylor and W.T.Winter, in preparation). Table I summarizes the crystallogaphic lattice constants for a variety of galactomannans. The constancy of the c (fibre) axis is easily understood in terms of conformational restrictions on the mannan backbone imposed by the (3-(1-4) linkage. Refinement of several of the structures listed in Table I using the measured X-ray intensities has confirmed our hypothesis that the invariance in the a dimension reflects the orderly association of mannan chains stabilized by a network of hydrogen bonds (see Figure 1) involving mannan 0 (2) and, when not galactosylated , mannan 0 (6) as donors and mannan 0(5) or 0 (2) as acceptors. This sheet, since it does not involve the galactose units, is insensitive to the degree of galactose substitution. The third dimension, b, is much larger , reflecting the occurrence of the galactose units as 'spacers' between mannan sheets. This dimension is also most sensitive to hydration with reversible changes of 3 - 5 A occurring solely in response to humidification or drying . The local absence of a galactose moiety arising from their irregular substitution along the main chain leads to a void which is filled with loosely bound or ' free' water. Thus, gums with lower DS values lose their crystallinity more readily upon drying . Finally, as shown in Figure I, a binding site for cross-linking agents such as borate or zirconium ions exists between the sheets and should lead to three-dimensional network formation by galactose-borate (or Zr)-galactose interactions. NMR data is also consistent with this model of cross-linking in galactomannan gels (6,7).
Tab le I. Galactomannan lattice constants Source
% RH
G/M
a ( A)
b (A)
c (A)
Fenugreek (1) Fenugreek (1) Lucerne (I ) Lucerne (I) Guar (2, H.Bouckris et al. , in preparation) Tara (3) Locust bean (2. H.Bouckris et al. , in preparation) Mannan I (4,5)
71 0 50 0 81 88 88 0
0.93 0.93 0.92 0.92 0.64 0.39 0.32 0.00
9.12 8.94 8.98 9 .20
33.35 29.48 33.32 3 1.09 32.83 30.62 30.6 1 7.21
10.35 10.27 10.34 10.34 10.35 10.40 10.24 10.27
© lRL Press Limited, Oxford, England
9.13 8.9 1 9.04 8.92
581
W.T.Winter, B.K.Song and H.Bollckris
--------- b----------_
H
><1I
Fig. 1. Projectiondown the c-axis of a representative galactomannan showinggalactose- galactose association between sheets; a possible borate (or zirconium) binding site (B) and unbound water (H) filling a void created by the absence of a galactosyl residue. M designates the mannan chains. G, and Gj designate the galactosyl residues which assume two different linkage conformations in these structures.
Acknowledgements We gratefully acknowledge partial support of thiswork by the donors of the Petroleum Research Fund of the American Chemical Society and a grant from Hercules, Inc. References 1. 2. 3. 4. 5. 6. 7.
Song,B.K., Winter,W.T. and Taravel,F.R. (1987) Macromolecules, in press. Marchessault,R.H., Buleon,A., Deslandes.Y. and GOIO,T.) (1979) J. Colloid Interface Sci., 71, 375. Chien,Y.Y. and Winler,W.T. (1985) Macromolecules, 18, 1357. Chanzy,H., Perez.S,, Miller,D.P., Paradossi.G. and Winter,W.T. (1987) Macromolecules, 20, 2407. Nieduszynski,I.A. and Marchessault,R.H.M. (1972) Can. J. Chem., 50, 230-236. Noble.O. and TaravelFvk. (1987) Carbohydr. Res., 166,1. Gey,C., Noble,a., Perez.S. and Taravel,F.R. (1987) Carbohydr. Res., in press.
582