title: Synthesis and characterization of new 5-linked pinoresinol lignin models.
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.
authors.affiliation: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, PR China.
authors.forename: Fengxia
authors.initials: F
authors.lastname: Yue
authors.forename: Fachuang
authors.initials: F
authors.lastname: Lu
authors.forename: Runcang
authors.initials: R
authors.lastname: Sun
authors.forename: John
authors.initials: J
authors.lastname: Ralph
mesh_terms: D003432:Cross-Linking Reagents / Q000737:chemistry*; D005663:Furans / Q000138:chemical synthesis* / Q000737:chemistry*; D017705:Lignans / Q000138:chemical synthesis* / Q000737:chemistry*; D008031:Lignin / Q000138:chemical synthesis* / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D010636:Phenols / Q000138:chemical synthesis* / Q000737:chemistry*
chemical_list: D003432:Cross-Linking Reagents; D005663:Furans; D017705:Lignans; D010636:Phenols; D008031:Lignin; C010559:coniferyl alcohol; C103298:pinoresinol