FAIR-CT96-5006 Construction of plant cell wall analogues based on Acetobacter cellulose polymer interactions and mechanical properties

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Biopolymers/Gums : FAIR Area 1.1 - Biomass and Bioenergy Chain : FAIR Marie Curie Research Training Grants

Objectives:

In order to get a better understanding of the links between pectin structure, cell wall architecture and mechanical properties, the approach of re-constructing in vitro composite networks from isolated cell wall polymers has been used.

Activities and results:

Composites have been produced, based on cellulose synthesised by the bacterium Acetobacter xylinus. Cell wall deposition phenomena were mimicked by making pectins available at the point of cellulose synthesis. The fermentation yield was sufficient to produce enough material to carry out analytical and mechanical tests. The role of ionic and covalent cross-links were studied. In contrast with glucan-binding polymers, pectic substances did not demonstrate a driving force for molecular association with cellulose. A pre-formed pectic network was required for the formation of a composite of the two polymers. The best incorporation was obtained with pectin of intermediate gel strength in the presence of calcium, or a very low number of covalent cross-links. Though no covalent bonds were present between the two kind of polymers, extensive interactions were observed. Solid state NMR indicated a large proportion of microstructural pectins immobilised by the cellulose network. On the other hand, the presence of pectins modified greatly the cellulose fibril interactions, leading to a drastic increase of the composite uni-axial extensibility. This effect was not directly due to pectin presence in composites, but rather was due to pectin effect on cellulose network formation. This suggests that the composition of the matrix in which cellulose is deposited during cell wall synthesis could have a previously unsuspected importance in determining the architecture and subsequently the mechanical properties of the cell wall.

A biaxial extension testing device has been constructed to mimic the effect of turgor pressure on a cell wall.

Conclusions: The preliminary results obtained on Acetobacter cellulose composites indicated that whilst cellulose remains the main load bearing component the soluble polysaccharides (pectins, xyloglucans) play a very different mechanical role than under uni- axial extension. It demonstrated that cell wall analogues respond in characteristic ways to different applied stresses. This has lead to the development of models linking molecular interactions through microstructural organisation to mechanical properties.

Keywords: Pectin, cell wall polymers

Contacts

Scientific Supervisor

• Unilever UK Central Resources Ltd Plant Science Unit / UNITED KINGDOM