Pretreatment of wheat straw with laccase mediator system degrades lignin and improves saccharification

Abstract

Agricultural by-products such as wheat straw are attractive feedstocks for the production of second generation bioethanol due to their high abundance [1]. Biofuel production from lignocellulosic material requires deconstruction of the cell-wall into individual polymers, and hydrolysis of the carbohydrates into monomeric sugars. In this context, one of the major factors causing biomass recalcitrance towards saccharification is correlated with the content and composition of lignin [2,3], whose presence hampers the enzymatic hydrolysis of carbohydrates. Hence, the purpose of this work is to study the ability of an enzymatic pretreatment based in the laccase-mediator system to remove/modify the lignin from wheat straw for improving saccharification. Pretreatment with Pycnoporus cinnabarinus laccase and 1-hydroxybenzotriazole (HBT) as mediator, followed by an alkaline peroxide extraction, removed up to 48% lignin from ground wheat straw. The lignin removal directly correlated with increases (~60%) in glucose yields after cellulase hydrolysis. The structural modifications occurring in the lignin polymer during the enzymatic pretreatment were studied ¿in situ¿ by two-dimensional nuclear magnetic resonance analysis (2D NMR) of the whole pretreated material at the gel-state [4]. The main lignin modifications included a reduction in the number of aliphatic side-chains involved in the major inter-unit linkages (ß-O-4' alkyl-aryl ethers and ß-5' phenylcoumarans) per aromatic lignin unit. Simultaneously, the removal of p-hydroxyphenyl, guaiacyl and syringyl lignin units and of p-coumaric and ferulic acids, as well as a moderate decrease of tricin units, was observed without a substantial change in the wood polysaccharide signals. Especially noteworthy was the formation of C¿-oxidized lignin units that indicates an oxidative depolymerisation mechanism.

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