Inhibitory Effect of Condensed and Non-Condensed Phenolic Moieties in Lignin on Enzymatic Hydrolysis

The residual lignin plays an inhibitory role in the enzymatic hydrolysis of cellulosic biomass. In this study, we examined the structure change of the isolated lignins from hot water hydrothermal pretreatment of aspen and their inhibitory effects on the enzymatic hydrolysis of Avicel. The functional groups of the isolated lignins were determined by quantitative ¹³C, 2D HSQC and ³¹P NMR. The increase of pretreatment temperature significantly increased the condensed and non-condensed syringyl and guaiacyl OH group contents in the isolated lignins, and decreased the aliphatic OH, p-hydroxybenzoate OH and carboxylic OH group contents. A compelling adverse association (r²=0.998) was observed between the condensed syringyl and guaiacyl phenolic OH group contents in lignins and their inhibitory effects on enzymatic hydrolysis. Langmuir adsorption isotherms showed that higher pretreatment temperature resulted in higher binding ability between isolated lignins and enzymes, which led to more non-productive binding. It is hypothesized that condensed syringyl and guaiacyl phenolic units are mainly responsible for the inhibitory effect of lignin on enzymatic hydrolysis, in which the condensed aromatic rings enhance the hydrophobic interactions and the phenolic OH group boost the hydrogen bonding. The combination of hydrophobic interactions and hydrogen bonding can further strengthen the non-productive binding.