Elucidating Emergent Properties of Lignocellulosic Biopolymer Assemblies Through Macromolecular Modeling

Lignocellulosic materials are a class an incredibly complex, highly diverse, and multifunctional polymer nanocomposites. Even within a cell wall, the biopolymer structure and composition vary locally to provide different properties and functionalities to each tissue type. For example, the secondary cell walls found in xylem tissue contain large bundles elementary cellulose fibrils, decorated with hemicellulose and lignin, to impart mechanical integrity. In contrast, the compound middle lamella found between adjoining cells contains little or no cellulose but is rich in lignin and pectin to provide adhesive properties. Aside from differing biopolymer compositions, modifications to monomer composition (such as the ratio of syringyl to guaiacyl units in lignin) and backbone decoration (such as acetylation of xylan) are also used by plants to impart specialized functionality to lignocellulosic assemblies. Furthermore, process-induced modification, such as lignin extraction and enzymatic hydrolysis, impart additional changes that alter the properties of the material. In this presentation, I will overview molecular modeling methods used by our group to identify structure/property/function relationships in lignocellulosic biopolymer assemblies. First, I will describe the construction of macromolecular models of secondary cell walls containing cellulose, hemicellulose, and lignin using best available experimental data to assign their spatial proximity and nanoscale arrangement. Variations of these models are employed in simulations of mechanical deformation to investigate the impacts of polymer composition and macromolecular architecture. We find that alterations is in lignin monomer ratio impacts mechanical moduli and propensity to form defects in cellulose fibrils during mechanical deformation. Next, I will present our recent molecular models for the compound middle lamella tissue, which has been identified as the primary location of failure during chemo-mechanical deconstruction. Variations in biopolymer composition and moisture content produce large changes in the mechanical properties of this tissue type, which have practical implications for biorefining and bio-based material design strategies.