Model engineered cellulose-hemicelluose-pectin nanomaterials for understanding plant cell wall assembly
Open Access
- Author:
- Gu, Jin
- Graduate Program:
- Plant Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 26, 2012
- Committee Members:
- Jeffrey M Catchmark, Dissertation Advisor/Co-Advisor
Jeffrey M Catchmark, Committee Chair/Co-Chair
Nicole Robitaille Brown, Committee Member
Seong H Kim, Committee Member
Teh Hui Kao, Committee Member - Keywords:
- cellulose
hemicellulose
pectin
bacterial cellulose
cellulose nanowhiskers - Abstract:
- 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. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, combustion gas analysis and N2 adsorption were used to quantify the degree of desulfation of H2SO4-prepared CNWs with or without desulfation. The results show that commercial cotton cellulose as received contained sulfur. Hydrochloric acid treatment did not result in any cellulose chemical modification. The sulfur content of H2SO4-prepared CNWs was higher than that exhibited by the original cellulose due to the esterification process. Hydrolysis using H2SO4 introduced sulfate groups onto the cellulose surface. Two desulfation methods, acid-catalyzed and solvolytic desulfation, were used in an attempt to remove the sulfate groups from H2SO4-prepared CNWs. However, they only resulted in partial desulfation of the cellulose nanowhiskers. The CNW surface esterification was estimated using a model of the cellulose structure in conjunction with both surface area measurements using N2 adsorption analysis, and a calculated surface area based on the measured dimensions of the CNWs produced. Calculations suggest that more than one third of the surface hydroxyls were substituted by sulfate groups. Based on this analysis, HCl-prepared CNWs were chosen since they retained the most natural cellulose surface and should be used as an improved substrate to study cellulose-cell wall polysaccharide interactions. 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. Previous work has shown that the structure of the cellulose spheres consists of dense layers of cellulose which extend spherically from nearly the center of the sphere to the outer edge, making this a unique system for assessing the impact of hemicellulose/pectin addition on cellulose formation on scales which extend from elementary fibril formation on the molecular level (0.5-5 nm), to layer formation on the micro scale (1-25 m), to overall sphere formation on the micro to macro level (100 m to 5 mm). Cellulose samples with xyloglucan and pectin had different macro structures compared to the other culture conditions. The micro structures showed that these two samples formed dense cellulose layers and had fewer cellulose fiber connections between layers. Cellulose samples with xylan and xyloglucan were found to contain more Iβ cellulose as found in higher plants, and exhibited decreased crystallinity and crystalline sizes according to X-ray diffraction patterns. IR spectroscopy confirmed the changes of crystal allomorph. 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. This part of the study highlights that different hemicelluloses/pectins could affect cellulose assembly process in different steps of the synthesis process. 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 Iα rich) or cotton (cellulose Iβ dominant) and disordered PASCNWs derived from these two types CNWs with xyloglucan, xylan, arabinagalactan and pectin were used as model substrates. The binding behavior was characterized by adsorption isotherm and Langmuir models. The maximum adsorption and the binding constant of xyloglucan, xylan and pectin to any CNWs were generally higher than to PASCNWs derived from the same source, suggesting that these polymers have a stronger affinity for an ordered cellulose surface. The binding affinity of xyloglucan, xylan and pectin to G. xylinus cellulose was always higher than to cotton cellulose, showing that binding interactions depended on the biological origin of cellulose and associated differences in its structure. The interaction energy was quantified when hemicellulose/pectin adsorbed to CNWs with isothermal titration calorimetry measurements. The exothermic interaction nature of cellulose-xyloglucan was confirmed. However, little interaction energy was observed when other hemicellulose was injected into cellulose suspensions. Further study revealed that the surface area, porosity and crystalline plane could all have impact on cellulose-hemicellulose/pectin interactions. 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. Results showed that the mechanical properties of CNW composites were similar to the pure CNW at low non-cellulosic polysaccharides concentration. With relatively high (60%) xyloglucan/pectin concentration, the modulus of the CNW composites was slightly reduced. However, if the xyloglucan and/or pectin were added to the bacterial cellulose culture, the BC composites became much less stiff and more brittle. The tensile tests together with morphological and crystal structure analysis indicate that surface coating or tethering of xyloglucan or pectin alone does not have significant impact on the mechanical properties of model cell wall materials. Xyloglucan and pectin could change the mechanical performance of model cell wall assembly during the cellulose synthesis process by either changing the cellulose bundling or cellulose network formation.