Decarboxylation and demethoxylation of naturally occurring phenols during coupling reactions and polymerization

Phenolic compounds originating from plant residue decomposition or microbial metabolism form humic-like polymers in the presence of various phenoloxidases or metal oxides. Enzyme-mediated reactions were reported to result in the decarboxylation or demethoxylation of substrate molecules; decarboxylation was also observed with metal oxides. To obtain more information on these phenomena, several humic precursors were incubated with various phenoloxidases (peroxidase, laccase, tyrosinase) or birnessite (delta -MnO2) and monitored for CO2 evolution and methanol production. Additionally, some reaction mixtures were analyzed for methane evolution. By using the test compounds labeled with C-14 in three different locations (carboxyl group, aromatic, or aliphatic chain), we demonstrated that (CO2)-C-14 evolution (ranging from 4.6 to 63.5% of the initial radioactivity) was mainly associated with the release of carboxyl groups. Minimal mineralization of C-14-labeled aromatic rings or aliphatic carbons occurred in ferulic or p-coumaric acids (0-5.6%). Demethoxylation ranged from 0.5 to 13.9% for 2,6-dimethoxyphenol and syringic acid, respectively. The methyl groups in 2-, 3-, and 4-methylphenol resisted release, as indicated by the lack of methane or methanol production. In previous studies, chlorophenols incubated with various phenoloxidases or birnessite were subject to dehalogenation. It appears that dehalogenation, decarboxylation, and demethoxylation of phenolic substrates are controlled by a common mechanism, in which various substituents are released if they are attached to carbon atoms involved in coupling. According to the experimental data, electron-withdrawing substituents, such as -COOH and -Cl, are more susceptible to release than electron-donating ones, such as -OCH3 and -CH3. The release of organic substituents during polymerization of humic precursors may add to CO2 production in soil.