Initial Stage of Guaiacyl-Syringyl Lignin Formation from Coniferyl and Sinapyl Alcohols

Peroxidase-catalyzed copolymerization of coniferyl alcohol (CA) and sinapyl alcohol (SA) was conducted using a flow microreactor to investigate the early stages of guaiacyl (G)-syringyl (S) lignin formation. As initial dimerization products, all possible β-β and β-5 dimers, and small amounts of G(β-O-4)G dimers, were obtained in a ratio of 94:6. This suggests that β-β and β-5 dimers serve as main nucleation sites for GS lignin formation. Five oligolignols were identified when CA/SA was added dropwise. These products indicate that the cross-coupling of CA with the S-end unit of oligolignols was favored. For the G(β-O-4)S(β-β)S(β-O-4)G tetramer, erythro-β-O-4 was dominant over threo-β-O-4 (92:8). In contrast, CA-only polymerization produced equal amounts of erythro and threo isomers. These results indicate that the formation of erythro or thero isomers is governed by whether the lignin ends are S or G units when the β-O-4 substructure forms through the coupling of growing lignin ends with monolignols.