Biophysical Origins of Biomass Recalcitrance
Molecular Dynamics Simulation of Cellulose and Lignin
The plant cell wall is made of cellulose microfibrils which are embedded in a matrix of polysaccharides (hemicelluloses and pectins) and lignin. The main function of the cell wall is to provide structural rigidity and protection to the cell.
Plant cell wall structure has come under renewed interest recently in the context of the production of bioethanol from enzymatic hydrolysis of lignocellulosic biomass. Cellulosic ethanol production is a two stage process involving first the hydrolysis of cellulose by cellulases to smaller oligosaccharides and then fermentation of the sugars to ethanol. The hydrolysis step is the bottleneck of the process because of the natural resistance, or "recalcitrance", of plant cell walls to degradation.
Figure: Atomic-detailed model of lignin (brown) coating a cellulose microfibril (green) inside a water box.
There are two main physical factors contributing to biomass recalcitrance. Firstly, cellulose is found in crystalline fibrils the compact structure of which impedes enzymatic access. In comparison, amorphous cellulose is readily digested by enzymes. Secondly, matrix polysaccharides (hemicellulose and pectins) and lignin coat the cellulose fibril and act as a physical barrier preventing enzymes from reaching the cellulose. Confirmation of the contribution of lignin to biomass recalcitrance is provided by the finding that removing lignin from biomass increases the cellulose-hydrolysis yield from 20% to 98%.
Computer simulation is a powerful tool for complementing experiment in obtaining an understanding of the molecular-level structure and dynamics of lignocellulose. In this context, our group is working towards a computational model of lignocellulose.
The projects currently undertaken in the Center for Molecular Biophysics:
