(574g) Free Energy of Desorption of Cello-Oligosaccharides From a Cellulose Crystal Surface

Enzymatic hydrolysis is an important step in the process of manufacture of ethanol from cellulosic biomass.  Cello-oligosaccharide molecules are produced as intermediates during the action of enzymes on the cellulose surface.  These cello-oligosaccharide molecules get re-adsorbed on the cellulose crystal surface, thus hindering further action of the enzyme on cellulose.  A detailed understanding of the energetics of the process of removal of these intermediates from the crystal surface will be important for developing strategies for optimizing cellulosic ethanol production. 

In this work we have used molecular dynamics simulations to determine the free energy change of the process of desorption of short, medium and long cello-oligosaccharides from cellulose crystal surface in the presence of water.  The choice of the path connecting the adsorbed and the desorbed states of the molecule i.e. the reaction coordinate will play an important role in governing the calculated value of the free energy change in a highly multidimensional system such as the one being studied here.  Indeed, several choices are available for the reaction coordinate that is to be used in this free energy calculation; a simple example will be the normal distance between the center of mass of the desorbing cello-oligosaccharide molecule and the top layer of the cellulose crystal.  We will present a comparison of the results for free energy profiles that are obtained using different choices of the reaction coordinate.  Furthermore, results will also be presented for the free energy calculations carried out on different crystal surfaces.