Folding of Xylan onto Cellulose Fibrils in Plant Cell Walls Revealed by Solid-state NMR
Professor Paul Dupree, Department of Biochemistry, University of Cambridge, UK
Exploitation of plant lignocellulosic biomass is hampered by our ignorance of the molecular basis for its properties such as strength and digestibility. Xylan, the most prevalent non-cellulosic polysaccharide, binds to cellulose microfibrils. The nature of this interaction remains unclear, despite its importance. Here it is shown that the majority of xylan, which forms a 3-fold helical screw in solution, flattens into a 2-fold helical screw ribbon to bind intimately to cellulose microfibrils in the cell wall. 13C solid-state magic angle spinning NMR spectroscopy, supported by in silico predictions of chemical shifts, shows both 2- and 3-fold screw xylan conformations are present in fresh Arabidopsis stems. The 2-fold screw xylan is spatially close to cellulose, and has similar rigidity to the cellulose microfibrils, but reverts to the 3-fold screw conformation in the cellulose-deficient irx3 mutant. The discovery that induced polysaccharide conformation underlies cell wall assembly provides new principles to understand biomass properties. The cell wall structure suggests strategies for improved extraction of nanocellulose from wood.
Structure of Native Cellulose Microfibrils, the Starting Point for Nanocellulose Production
Dr Michael C Jarvis, School of Chemistry, Glasgow University and IBioIC, Strathclyde University, Glasgow
There is an emerging consensus that higher plants synthesise cellulose microfibrils that initially comprise 18 chains. However the mean number of chains per microfibril in situ is usually greater than 18, sometimes much greater. Microfibrils from woody tissues of conifers, grasses and dicotyledonous plants, and from organs like cotton hairs, all differ in detailed structure and mean diameter. Diameters increase further when aggregated microfibrils are isolated. Because surface chains differ, the tensile properties of the cellulose may be augmented by increasing microfibril diameter.
Association of microfibrils with anionic polysaccharides in primary cell walls and mucilages leads to in vivo mechanisms of disaggregation that may be relevant to the preparation of nanofibrillar cellulose products. For the preparation of nanocrystalline celluloses, the key issue is the nature and axial spacing of disordered domains at which axial scission can be initiated. These disordered domains do not, as has often been suggested, take the form of large blocks occupying much of the length of the microfibril. They are more likely to be located at chain ends or at places where the microfibril has been mechanically damaged, but their structure and the reasons for their sensitivity to acid hydrolysis need better characterisation.
Residual Wood Polymers Facilitate Compounding of Microfibrillated Cellulose with Non-polar Media
Professor Wolfgang Gindl-Altmutter, Institute of Wood Technology and Renewable Materials, BOKU-Vienna, Austria
Microfibrillated cellulose is a fascinating material with an obvious potential for composite reinforcement due to its excellent mechanics together with high specific surface area. However, in order to utilise this potential, commercially viable solutions to important technological challenges have to be found. Notably, the distinct hydrophilicity of microfibrillated cellulose prevents efficient drying without loss in specific surface area, necessitating storage and processing in wet condition. This greatly hinders compounding with important technical polymers immiscible with water. Differently from cellulose, the chemistry of the major wood polymers lignin and hemicellulose is much more diverse in terms of functional groups. Specifically, the aromatic moieties present in lignin and acetyl groups in hemicellulose provide distinctly less polar surface-chemical functionality compared to hydroxyl groups, which dominate the surface chemical character of cellulose. Experimental evidence demonstrates that the surface-chemical functionalities of lignin and hemicellulose can be utilised to the benefit dispersion of microfibrillated cellulose in non-polar media and promote interfacial adhesion between fibrils and polymer matrices. It should be noted that lignin and hemicellulose do not provide hydrophobicity to a comparable extent as achievable by chemical modification of cellulose, but may be a cost-efficient alternative to modification in a number of specific cases.
Molecular Interactions at Play in Nanocellulose Assembly
Dr Yoshiharu Nishiyama, CERMAV-CNRS, France
Cellulose is often described as strong, stiff and difficult to process, in contrast to commodity plastics. Hydrogen bonding and hydrophobic interactions are often mentioned in the literature as the origin of these particular behaviors, but the description remains qualitative. How can we quantify in a simple way the contribution of different types of molecular interactions in the context of material properties and processing? The crystal structure analysis, molecular modeling including quantum mechanical approaches, thermodynamics data of analogue molecules give clues to the dissection of the different types of energy contribution. Hydroxyl groups of cellulose form hydrogen bond comparable to other simple alcohols. Trends of heat of vaporization of alkyl-alcohols and alkanes suggests a stabilization by such hydroxyl-group hydrogen bonding to be of the order of 24 kJ/mol, while London dispersion force contributes to about 0.41 kJ/mol/dalton. Simple arithmetic gives striking agreement with experimental enthalpy of sublimation of small sugars, where the major cohesive energy comes from hydrogen bonds. For cellulose, due to the reduced number of hydroxyl groups, London dispersion force would be the major component in intermolecular cohesion. The role of multipolar interactions and hydrophobic interactions will be also discussed.