Tuesday, May 3, 2011
Previous research has revealed that lignin droplets formed as a result of dilute acid pretreatment contain xylan. However, it is not clear whether that xylan is covalently bound to the lignin forming a lignin-carbohydrate complex (LCC) or simply adsorbed; and to what extent xylan was associated with the lignin droplets. The goal of this study was to evaluate and chemically characterize the presence of putative LCCs in lignin droplets and determine how much xylan they may contain. A method for selectively isolating the lignin droplets from the surface of pretreated cell walls using organic solvent has been developed. Surface lignin droplet extracts were analyzed by high resolution NMR and at least 2 types of LCCs were found, phenyl glycoside and g-ester types. The amount of carbohydrate in the lignin droplets was relatively low (2 to 6%) and decreased with increasing pretreatment severity along with a concurrent drop in the molecular weight of the lignin. It appears that the frequency of the LCC linkages to lignin are as low as those found in native lignin (i.e., about one cross link per 80 to 100 lignin monomer units). LCCs found in lignin droplets are not likely the root cause of low xylose yields or contribute to the biphasic nature of xylan hydrolysis kinetics. Clear evidence of lignin de-polymerization was observed.