Initial Stage of Syringyl Lignin Formation from Sinapyl Alcohol

Elucidating the detailed structure and formation mechanism of lignin, especially understudied syringyl (S) lignin, advances our knowledge of lignocellulosic biomass. To examine the early stages of S-lignin formation from sinapyl alcohol (SA), the FMR (flow microreactor) method and the Zutropf (gradual addition of SA) method with limited amounts of H₂O₂ were employed for the peroxidase-catalyzed dehydrogenative polymerization of SA. Only β-β dimers and not β-O-4 dimers were obtained as initial dimerization products. Six new oligoligognols up to pentamers with β-β and β-O-4 structures were identified. The erythro isomer was preferentially formed over the threo isomer in the β-O-4 structures, similar to that found in naturally occurring S-rich hardwood lignin. Although minor substructures, the α-oxidized β-β and β-O-4 structures and spirodienone (β-1) structure identified in this study demonstrate the characteristic features of S-rich lignin. Based on the identified products, the initial formation mechanism of S-lignin from SA was proposed.