Origin of Reduced Lignin Structures: Quinone Methides

Recent discoveries have shown the presence of reduced lignin structures in softwood lignin, such as dihydroconiferyl alcohol and secoisolariciresinol. During lignin polymerization, the oxidative radical coupling of monolignols generates quinone methides as intermediates. Several reduction agents were tested on a quinone methide model. The products were analysed by GC-MS and {1}H, {13}C, DEPT and HSQC NMR techniques were used to confirm the structure of the reduced quinone methide. An uncatalyzed reduction of this quinone methide model was achieved with β-NADH, a biologically significant reducing agent, demonstrating that the formation of reduced lignin structures is not necessarily biologically controlled.