6-01: The effect of lignin chemistry and structure on carbohydrate conversion

Lignin is considered one of the most recalcitrant factors during the enzymatic hydrolysis of lignocellulosic biomass.  Possible mechanisms of lignin-induced inhibition are physical blockage of carbohydrates, non-productive adsorption of cellulases onto lignin, and chemical blockage of binding sites by lignin carbohydrate complexes.  However, the details have not been fully resolved due to the complex characteristics of lignin.  A commercial lignin was fractionated into different molecular weight groups and its functional groups were modified to evaluate the inhibitory role of lignin chemistry and structure during enzymatic hydrolysis.  Additionally, lignins were isolated from various biomasses (e.g. softwood vs. hardwood, unpretreated vs. pretreated, and alkaline vs. acid-treated) to study the effect of residual lignin chemistry following different pretreatments.  These lignins were precipitated onto bleached pulps to mimic the lignocellulosic biomass and spatial deposition on the cell wall surface was evaluated using confocal microscopy.  A commercial cellulase cocktail and mono-component enzymes were used in this study. Inverse correlations were found between total hydroxyl content (1.7 to 1.1, phenolic and aliphatic) and cellulase adsorption (9% to 70%), and degree of condensation measured by NMR (0.02 to 0.64) and digestibility of reconstructed substrates (83.2% to 58.2%) for all samples tested.  Furthermore, large molecular weight (>2,000 M_w) lignin showed a greater inhibitory effect on carbohydrate conversion at the same lignin content.  The data suggest that the degree of condensation, total hydroxyl content, and molecular weight of lignin are critical factors impacting efficient enzymatic hydrolysis.  The mechanisms of lignin-derived inhibition will be discussed based on these experimental observations.