Monday, April 30, 2007

Is there a structural basis for xylan biphasic kinetics during dilute acid pretreatment?

Stephanie E. Porter1, Shi-You Ding2, Bryon S. Donohoe2, Todd B. Vinzant1, Claudia Ishizawa1, Michael E. Himmel1, and David K. Johnson2. (1) Chemical and Biosciences Center, National Renewable Energy Lab, 1617 Cole Blvd., Golden, CO 80401, (2) Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

The kinetics of xylan hydrolysis during treatment with dilute sulfuric acid can best be described by two first-order reactions, an initial rapid reaction followed by a much slower reaction. Our goal was to determine if this apparent biphasic hydrolysis was due, either fully or partially, to structural characteristics in plant cell walls.  We found that isolated corn stem rinds did exhibit biphasic hydrolysis kinetics, suggesting the biphasic release was not simply due to the mixture of tissue types in milled whole stover.  Based on further pretreatment kinetics, our initial results showed an increase in the fraction of fast hydrolyzing xylan as temperature and acid concentration were increased.  Pretreated corn stover rinds were analyzed by several microscopic methods that spanned the 100 to 0.01 mm range.  The samples were also probed with a xylan-specific monoclonal antibody and imaged in a confocal laser scanning microscope (CLM).  Based on the CLM images, it appears that migration of xylan within the cell wall structure may play a key role in the kinetics of xylan hydrolysis.  At temperatures of 160oC or above, xylan migration in corn stem rinds is rapid so that fast xylan hydrolysis dominates.  At temperatures at or below 140oC xylan, migration is slower and the kinetics are a more equivalent mix of slow and fast kinetics.  We have developed a hypothesis for biphasic kinetics that involves migration of xylan, in close association with lignin, to the exposed surfaces of the cell walls. This abstract is subject to government rights.