Model engineered cellulose-hemicellulose-pectin nanomaterials for understanding plant cell wall assembly

There is a growing interest in the application of lignocelluloses in recent years. Apart from their traditional utilization for construction, paper production and combustion heating, lignocelluloses are being examined for next generation renewable biofuels and raw materials for new sustainable, engineered composites. The structure of lignocelluloses thus becomes an important subject essential for improved utilization and new application development. The plant cell wall is considered to be an ideal natural material exhibiting excellent mechanical properties. Wood such as Douglas-fir exhibits a Young's modulus of 13 GPa and ultimate strength (compression) of 50 MPa. A consequence of the molecular and nanoscale composition and organization responsible for this excellent behavior is a high degree of recalcitrance, which inhibits the efficient use of these materials for biofuel production. To better understand structure-property relationships, many recent studies have focused on modeling plant cell walls in vitro or in silico. A variety of plant cell wall models have been proposed, often representing differing conceptualizations of the wall architecture. To develop a better understanding of how various cell wall polymers interact and organize, model materials and systems are important to isolate specific behaviors key in cell wall assembly. Simpler model systems with fewer components and well defined constituents may also provide a much better experimental venue for connection to computational simulations, which may be difficult to apply to natural systems and their associated complexity and diversity.;To establish a more suitable system for studying cellulose-hemicellulose/pectin interactions and assembly, the selection of appropriate model cellulose materials was first explored. Specifically, model cellulose substrates representing the crystalline or highly ordered component as well as the amorphous or disordered component of natural cellulose were prepared using processes optimized not to alter the natural cellulose surface chemistry.;Cellulose nanowhiskers (CNWs) were chosen to simulate the crystalline portion of natural cellulose. CNWs were produced using both sulfuric acid and hydrochloric acid treatments. .;A model amorphous cellulose substrate was produced using CNWs as a starting material. It is produced by swelling, dissolution and regenerating CNW in high concentration of phosphoric acid (phosphoric acid swollen cellulose nanowhiskers, PASCNWs). This substrate was chosen to study cell wall polysaccharide assembly since its molecular weight remained the same as the starting material and no chemical modification of the cellulose was observed.;To understand the impact of different hemicellulose and pectin on cellulose assembly, four typical cell wall polysaccharides including xyloglucan, xylan, arabinogalactan and apple pectin (the majority of the polysaccharides is a homogalacturonan) were chosen. A novel cellulose producing system was used to mimic the cellulose synthesis component of cell wall formation. Specifically, the bacterium (Gluconacetobacter xylinus) strain ATCC 700178, which produces a spherical form of cellulose under agitated incubation, was grown in the presence of 0.5% (w/v) xyloglucan, xylan, arabinogalactan and pectin.;To further understand the role of xyloglucan and pectin in cellulose assembly, cellulose was also grown in cultures containing blends of both xyloglucan and pectin with different ratios. Results show that xyloglucan had the dominant impact on the assembly of cellulose, suggesting that xyloglucan and pectin may interact with cellulose at different points in the assembly process, or in different regions. .;To understand the adsorption of cell wall polysaccharides to cellulose, highly ordered CNWs (representing the crystalline part of cellulose) and disordered PASCNWs (representing the amorphous part of cellulose) were used to understand the adsorption behaviors. Four cellulose substrates including CNWs from G. xylinus (cellulose Ialpha rich) or cotton (cellulose Ibeta] dominant and disordered PASCNWs derived from these two types CNWs with xyloglucan, xylan, arabinagalactan and pectin were used as model substrates.;Finally, to explore any correlation between the adsorption examined above and mechanical performance, model cell wall nanocomposites derived from G. xylinus cellulose or its CNWs were prepared. Xyloglucan and/or pectin were either added to static G. xylinus cultivations to obtain bacterial cellulose (BC) composite films, or blended to CNWs to obtain CNW composite films. The mechanical properties of the air-dried films were examined under extension conditions. The Young's modulus, strain and stress at break for the films were recorded. (Abstract shortened by UMI.).