Mechanism of oxidative C(α)-C(β) cleavage of a lignin model dimer by Phanerochaete chrysosporium ligninase. Stoichiometry and involvement of free radicals

The hemoprotein ligninase of Phanerochaete chrysosporium Burds. catalyzes the oxidative cleavage of lignin model dimers between C(α) and C(β) of their propyl side chains. The model dimers hitherto used give multiple products and complex stoichiometries upon enzymatic oxidation. Here we present experiments with a new model dimer, 1-(3,4-dimethoxyphenyl)-2-phenylethanediol (dimethoxyhydrobenzoin, DMHB) which is quantitatively cleaved by ligninase in air to give benzaldehyde and veratraldehyde according to the stoichiometry: 2DMHB + O₂ → 2pHCHO + 2Ph(OMe)₂CHO. Catalytic amounts of H₂O₂ are required for this aerobic reaction. Under anaerobic conditions, ligninase uses H₂O₂ as the oxidant for cleavage: DMHB + H₂O₂ → phCHO + Ph(OMe)₂CHO. Electron spin resonance experiments done in the presence of spin traps, 2-methyl-2-nitrosopropane or 5,5-dimethyl-1-pyrroline-N-oxide, show that C(α)-C(β) cleavage yields α-hydroxybenzyl radicals as intermediate products. Under anaerobic conditions, these radicals react further to give the final aldehyde products. In air, O₂ adds to the carbon-centered radicals, probably giving α-hydroxybenzylperoxyl radicals which fragment to yield superoxide, benzaldehyde, and veratraldehyde. These results lead us to propose a mechanism for C(α)-C(β) cleavage in which attack by ligninase and H₂O₂ on the methoxylated ring of DMHB yields a cation radical, which then cleaves to give either benzaldehyde and an α-hydroxy(dimethoxybenzyl) radical or veratraldehyde and an α-hydroxybenzyl radical. Similar mechanisms probably apply to the enzymatic C(α)-C(β) cleavage of natural lignin.