Synthesis and Characterization of New 5-Linked Pinoresinol Lignin Models

Authors

  • Dr. Fengxia Yue,

    1. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640 (P.R. China), Fax: (+1) 608-265-2904
    2. Dept. of Biochemistry, the Wisconsin Energy Institute and the DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI 53726 (USA)
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  • Dr. Fachuang Lu,

    Corresponding author
    1. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640 (P.R. China), Fax: (+1) 608-265-2904
    2. Dept. of Biochemistry, the Wisconsin Energy Institute and the DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI 53726 (USA)
    • State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640 (P.R. China), Fax: (+1) 608-265-2904===

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  • Prof. Dr. Runcang Sun,

    1. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640 (P.R. China), Fax: (+1) 608-265-2904
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  • Prof. Dr. John Ralph

    1. Dept. of Biochemistry, the Wisconsin Energy Institute and the DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI 53726 (USA)
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Abstract

Pinoresinol structures, featuring a β-β′-linkage between lignin monomer units, are important in softwood lignins and in dicots and monocots, particularly those that are downregulated in syringyl-specific genes. Although readily detected by NMR spectroscopy, pinoresinol structures largely escaped detection by β-ether-cleaving degradation analyses presumably due to the presence of the linkages at the 5 positions, in 5-5′- or 5-O-4′-structures. In this study, which is aimed at helping better understand 5-linked pinoresinol structures by providing the required data for NMR characterization, new lignin model compounds were synthesized through biomimetic peroxidase-mediated oxidative coupling reactions between pre-formed (free-phenolic) coniferyl alcohol 5-5′- or 5-O-4′-linked dimers and a coniferyl alcohol monomer. It was found that such dimers containing free-phenolic coniferyl alcohol moieties can cross-couple with the coniferyl alcohol producing pinoresinol-containing trimers (and higher oligomers) in addition to other homo- and cross-coupled products. Eight new lignin model compounds were obtained and characterized by NMR spectroscopy, and one tentatively identified cross-coupled β-O-4′-product was formed from a coniferyl alcohol 5-O-4′-linked dimer. It was demonstrated that the 5-5′- and 5-O-4′-linked pinoresinol structures could be readily differentiated by using heteronuclear multiple-bond correlation (HMBC) NMR spectroscopy. With appropriate modification (etherification or acetylation) to the newly obtained model compounds, it would be possible to identify the 5-5′- or 5-O-4′-linked pinoresinol structures in softwood lignins by 2D HMBC NMR spectroscopic methods. Identification of the cross-coupled dibenzodioxocin from a coniferyl alcohol 5-5′-linked moiety suggested that thioacidolysis or derivatization followed by reductive cleavage (DFRC) could be used to detect and identify whether the coniferyl alcohol itself undergoes 5-5′-cross-linking during lignification.

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