(498a) A Free Energy Landscape Study Of The Reverse Hydrolysis Reactions Of Xylose In Acidic Aqueous Solutions

Biofuels converted from lignocellulosic biomass provide an alternative to the traditional fossil fuels. Pretreatment is one of the crucial steps during biomass conversion to hydrolyze hemicelluloses and relocate lignin. During dilute acid pretreatment, hemicelluloses such as xylan are hydrolyzed into monomer xylose. Under acidic environment and relative high monomer concentration, reverse hydrolysis of xylose can occur thus lowering the sugar yield and the overall biomass conversion efficacy. Here we present a free energy study of the acid-catalyzed reverse hydrolysis of xylose into xylobiose by using the metadynamics sampling technique in the frame of Car-Parrinello molecular dynamics. This history-dependent method allows an efficient sampling of the configurational space along the reaction coordinates and an accurate estimation of the free energy landscape. The reaction pathways and transition states of forming the glycosidic bonds were located on the landscape. The activation energies and free energy changes were also obtained and show a good agreement with experiment. Our estimations of ΔG and understanding of the reaction mechanisms can be used to develop accurate kinetic models for this reverse hydrolysis process.