pH-Induced Lignin Surface Modification to Reduce Nonspecific Cellulase Binding and Enhance Enzymatic Saccharification of Lignocelluloses

Authors

  • Prof. Hongming Lou,

    1. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou (PR China)
    2. USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin (USA)
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    • This work was conducted while H. Lou and T. Lan were visiting scientists at the USDA Forest Products Laboratory and while J. Y. Zhu was on official government time.

  • Prof. J. Y. Zhu,

    Corresponding author
    1. USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin (USA)
    2. Department of Biological Systems Engineering, University of Wisconsin-Madison, Madison, Wisconsin (USA)
    • USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin (USA)
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  • Tian Qing Lan,

    1. USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin (USA)
    2. School of Light Industry and Food Sciences, South China University of Technology, Guangzhou (PR China)
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    • This work was conducted while H. Lou and T. Lan were visiting scientists at the USDA Forest Products Laboratory and while J. Y. Zhu was on official government time.

  • Huanran Lai,

    1. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou (PR China)
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  • Prof. Xueqing Qiu

    1. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou (PR China)
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Abstract

We studied the mechanism of the significant enhancement in the enzymatic saccharification of lignocelluloses at an elevated pH of 5.5–6.0. Four lignin residues with different sulfonic acid contents were isolated from enzymatic hydrolysis of lodgepole pine pretreated by either dilute acid (DA) or sulfite pretreatment to overcome recalcitrance of lignocelluloses (SPORL). The adsorption isotherms of a commercial Trichoderma reesi cellulase cocktail (CTec2) produced by these lignin residues at 50 °C were measured in the pH range of 4.5–6.0. The zeta potentials of these lignin samples were also measured. We discovered that an elevated pH significantly increased the lignin surface charge (negative), which causes lignin to become more hydrophilic and reduces its coordination affinity to cellulase and, consequently, the nonspecific binding of cellulase. The decreased nonspecific cellulase binding to lignin is also attributed to enhanced electrostatic interactions at elevated pH through the increased negative charges of cellulase enzymes with low pI. The results validate the hypothesis that the increases in enzymatic saccharification efficiencies at elevated pH for different pretreated lignocelluloses are solely the result of decreased nonspecific cellulase binding to lignin. This study contradicts the well-established concept that the optimal pH is 4.8–5.0 for enzymatic hydrolysis using Trichoderma reesi cellulose, which is widely accepted and exclusively practiced in numerous laboratories throughout the world. Because an elevated pH can be easily implemented commercially without capital cost and with minimal operating cost, this study has both scientific importance and practical significance.

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