Abstract

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.

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