Cosolvent pretreatment in cellulosic biofuel production: effect of tetrahydrofuran-water on lignin structure and dynamics

The deconstruction of cellulose is an essential step in the production of ethanol from lignocellulosic biomass. However, the presence of lignin hinders this process. Recently, a novel cosolvent based biomass pretreatment method called CELF (Cosolvent Enhanced Lignocellulosic Fractionation) which employs tetrahydrofuran (THF) in a single phase mixture with water, was found to be highly effective at solubilizing and extracting lignin from lignocellulosic biomass and achieving high yields of fermentable sugars. Here, using all-atom molecular-dynamics simulation, we find that THF preferentially solvates lignin, and in doing so, shifts the equilibrium configurational distribution of the biopolymer from a crumpled globule to coil, independent of temperature. Whereas pure water is a bad solvent for lignin, the THF : water cosolvent acts as a theta solvent, in which solvent : lignin and lignin : lignin interactions are approximately equivalent in strength. Under these conditions, polymers do not aggregate, thus providing a mechanism for the observed lignin solubilization that facilitates unfettered access of celluloytic enzymes to cellulose.