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FAIR-CT96-1624
The Strength of Wood Fibres: Association Between Hemicellulose and Cellulose at the Molecular Level |
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Contract No: | FAIR-CT96-1624 |
| Date Prepared: | June 2000 | |
| Source: | Final Report Abstract and Executive Summary |
The strength of heterogeneous fibres such as wood is largely determined by the strength of association between hemicellulose and cellulose. Understanding the association at the molecular level permits a rational evolution of pulping processes, and opens new avenues to the development of designer-made fibres based on agricultural products. In this multi-disciplinary project the association between hemicellulose and cellulose fibres was studied at several levels of molecular detail, both experimentally and theoretically.
Hemicelluloses such as xylan, glucomannan and xyloglucan bind tightly to cellulose in a very specific pattern. The co-crystallisation reduces the number of accessible conformations, which will lower the conformational entropy. The driving force behind efficient binding is hydrogen bonds. These interactions are highly specific, setting high demands on structural complementary. For flexible polymers such as carbohydrates structural changes upon association of the polymer to the substrate can be expected. For small carbohydrate complexes these structural changes are observed. For these small systems desolvational entropy contributes significantly to the affinity, which is probably even larger for polymeric systems. All these issues (specificity, structural changes, entropic effects, and desolvation) were addressed in varying molecular detail to obtain a comprehensive understanding of hemicellulose-cellulose association.
The methodological approach was based on state-of-the-art physico-chemical methods. Energetic aspects of the association between hemicellulose and cellulose was studied by binding assays of radioactive labelled oligomers. Isotherms valid for all densities validated by these experiments were derived from models with molecular detail, which related the structural information obtained in X-ray diffraction experiments to the binding experiments. Cellulose- hemicellulose interaction measurements were carried out by AFM. 13C solid state NMR of the complex provided structural and dynamical details, such as short-range order, structural effects due to the association. Structural observations were used to formulate a single atomistic model of the hemicellulose-cellulose complex with the aid of theoretical approaches such as Molecular Dynamics simulations and docking calculations. The structure of the hemicelluloses in solution, essential to this objective, was determined by solution state NMR.
Tetramers were found to be the shortest oligosaccharides to bind on cellulose surface. All data agreed that cello-oligomers are bound more tightly on cellulose surface than hemicellulose- oligomers (xylo-, manno- and xylogluco-oligomers). However, there is still some contradictory in some of the results, especially are manno-oligomers or xylo-oligomers binding more tightly with cellulose. It is clear that both xylan and mannan associate with cellulose. It seems that the binding is mainly unspecific as only very few specific binding sites were detected on cellulose surfaces. The specific binding is essentially entropy-driven and occurs without significant heat capacity decrease. It can be considered a hydrophilic entropic binding governed by the partial disorganisation of the water molecules before the association process. The specific binding sites may be on the specific defects (kinks, holes, corners) on the crystalline cellulose surface. The unspecific binding of hemicelluloses is very important for the pulp and paper properties as the amount of non-specifically bound oligosaccharides with higher dissociation constant was orders of magnitude higher than the amount of specifically bound oligosaccharides. Unspecific binding must therefore not to be overlooked.
The general picture obtained from EEB docking runs was that cello-, manno- and xylo- oligosaccharides all interact differently with the cellulose surface. Cello-oligosaccharides tended to extend a cellulose plane, and remain aligned with the cellulose layer. Manno-oligosaccharides appeared to lay flat on the cellulose surface and interacted with two cellulose chains. Xylo- oligosaccharidcs did not have a favoured orientation, due to the absence of the hydroxymethyl group that is responsible for the binding of the other oligosaccharides. Furthermore, xylo-oligomers did not conform to the two-fold helical structure that is needed for alignment with the surface, but they took conformations that were intermediate between a two-fold and a three-fold helical structure.
During pulping the structure of xylan and mannan is modified and thus interactions between hemicelluloses and cellulose are different of those in the native wood. Results clearly showed the orientation together with structural changes of modified xylan (not xylo-oligomers) along cellulose fibre axis demonstrating clear association of these polymers in pulp and paper. Association of mannan and cellulose seems to be different as mannan orients perpendicular to the cellulose chains. This difference in orientation of xylan and mannan may have a significant role in strength properties of paper sheets. The results obtained with AFM measurements suggest that adsorbed xylan (hemicellulose) may make cellulose more flexible, which may further increase the strength of the paper.
Even though it is not possible to conclude from the results at this point which hemicellulose would give the best paper products, it is clear that the results obtained will be very useful in the future for manufacturing of wood products. Different type of products can be obtained by decorating cellulose fibrils or fibres with different hemicelluloses or mixtures of them. The results are not only valid for pulp and paper industry but are also very valuable when preparing wood based composites in which strength is very important. The amount of hemicellulose- within pulp and paper can be controlled by processing conditions. However, the wood species (hardwood or softwood) used determine the type of hemicellulose present. In the future the woods can already be tailored by genetic engineering for specific applications. For example hemicellulose content, ratio of xylan and mannan, and hemicellulose structures can be manipulated to obtain better products and environmentally sound pulping methods. More research on structure-function properties of different hemicelluloses must, however, be conducted to be able to predict most optimal modifications.
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