Modelling of xyloglucan, pectins and pectic side chains binding onto cellulose microfibrils

A study of the properties of hemicelluloses adsorbed onto microfibrillar cellulose isolated from apple parenchyma

Binding assays of commercially available hemicelluloses and pectins, and microfibrillar cellulose isolated form apple parenchyma were prepared. Initial studies showed that among all of the non-cellulosic polysaccharides examined, only the hemicelluloses (xyloglucan, xylan, glucomannan, ß-d-glucan) showed the ability to adsorb to microfibrillar cellulose. Among several adsorption models tested, the best fit was obtained for the Redlich-Peterson isotherm. Moreover, the linear vs. the branched structure and the size of the hemicelluloses have an influence over the extent of the adsorption to cellulose. The Fourier Transform Infrared and Raman spectra showed that a rather weak interaction took place between the hemicelluloses and cellulose. Also, the differential scanning calorimetry and the light scattering method results showed that after adsorption, cellulose has less mobility. Moreover, the mechanical properties of cellulose films changed after the addition of the chosen hemicelluloses and the films became less elastic but more resistant to a breaking force.

Building an extensible cell wall

This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.

Probing adhesion between nanoscale cellulose fibres using AFM lateral force spectroscopy: The effect of hemicelluloses on hydrogen bonding

Inter-fibre adhesion is a key contributing factor to the mechanical response and functionality of cellulose-based biomaterials. 'Dip-and-Drag' lateral force atomic force microscopy technique is used here to evaluate the influence of arabinoxylan and xyloglucan on interactions between nanoscale cellulose fibres within a hydrated network of bacterial cellulose. A cohesive zone model of the detachment event between two nano-fibres is used to interpret the experimental data and evaluate inter-fibre adhesion energy. The presence of xyloglucan or arabinoxylan is found to increase the adhesive energy by a factor of 4.3 and 1.3, respectively, which is consistent with these two hemicellulose polysaccharides having different specificity of hydrogen bonding with cellulose. Importantly, xyloglucan's ability to strengthen adhesion between cellulose nano-fibres supports emergent models of the primary plant cell walls (Park & Cosgrove, 2012b), which suggest that xyloglucan chains confined within cellulose-cellulose junctions play a key role in cell wall's mechanical response.

Kinetic aspects of the adsorption of xyloglucan onto cellulose nanocrystals

In this work, the adsorption of a neutral flexible polysaccharide, xyloglucan (XG), onto thin cellulose nanocrystal (CNC) surfaces has been investigated to get more insight into the CNC-XG association. Gold-coated quartz crystals were spin-coated with one layer of CNC, and XG adsorption was monitored in situ using a quartz crystal microbalance with dissipation (QCM-D). The adsorption of XG under flow at different concentrations did not result in the same surface concentration, which evidenced a kinetically controlled process. In an attempt to describe the binding of XG to CNCs, adsorption data were fitted to a kinetic model comprising a contribution from XG adsorption onto uncovered CNC surfaces and a contribution from XG adsorption after rearrangement. Kinetic studies evidenced the presence of two adsorption regimes as a function of XG concentration. For low XG concentrations, the kinetic constant for chain rearrangement is comparable to the kinetic constant for adsorption. This fact implies a rearrangement and alignment of XG molecules on CNCs. Differently, for higher XG concentrations, the kinetic constant related to the conformational rearrangement decreases, indicating that XG molecules have no time to laterally rearrange before new XG molecules adsorb.

Role of Polysaccharides on Mechanical and Adhesion Properties of Flax Fibres in Flax/PLA Biocomposite

The effect of alkali and enzymatic treatments on flax fibre morphology, mechanical, and adhesion properties was investigated. The multilength scale analysis allows for the correlation of the fibre's morphological changes induced by the treatments with mechanical properties to better explain the adherence properties between flax and PLA. The atomic force microscopy (AFM) images revealed the removal of primary layers, upon treatments, down to cellulose microfibrils present in the secondary layers. The variation in mechanical properties was found to be dependent, apart from the crystalline content, on interaction between cellulose microfibrils and encrusting polysaccharides, pectins and hemicelluloses, in the secondary layers. Finally, microbond tests between the modified fibres and PLA emphasize the important role of the outer fibre's surface on the overall composite properties. It was observed here that gentle treatments of the fibres, down to the oriented microfibrils, are favourable to a better adherence with a PLA drop. This paper highlights the important role of amorphous polymers, hemicellulose and pectin, in the optimisation of the adhesion and mechanical properties of flax fibres in the biocomposite.

Barley xyloglucan xyloglucosyl transferases bind xyloglucan-derived oligosaccharides in their acceptor-binding regions in multiple conformational states

Three barley xyloglucan endotransglycosylases (HvXETs), known as xyloglucan xyloglucosyl transferases (EC 2.4.1.207), were subjected to kinetic and computational docking studies. The k(cat) x K(m)(-1) values with the reduced [3H]-labelled XXXG, XXLG/XLXG and XLLG acceptor substrates were 0.02 x 10(-2), 0.1 x 10(-2) and 3.2 x 10(-2) s(-1) microM(-1), while the K(m) constants were 10.6, 8.6 and 5.3 mM, obtained for HvXET3, HvXET4 and HvXET6, respectively. Docking of XLLG in acceptor-binding regions revealed that at least two conformational states were likely to participate in all isoforms. The assessments of kinetic and computational data indicated that the disposition of aromatic residues at the entrance to the active sites and the flexibility of proximal COOH-terminal loops could orient acceptors more or less favourably during binding, thus leading to tighter or weaker K(m) constants. The data suggested that binding of acceptors in HvXETs is guided by contributions from the conserved residues in the active sites and by the of neighbouring loops.