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

Phenolic compounds originating from plant residue decomposition or microbia l metabolism form humic-like polymers in the presence of various phenoloxid ases or metal oxides. Enzyme-mediated reactions were reported to result in the decarboxylation or demethoxylation of substrate molecules; decarboxylat ion was also observed with metal oxides. To obtain more information on thes e phenomena, several humic precursors were incubated with various phenoloxi dases (peroxidase, laccase, tyrosinase) or birnessite (delta -MnO2) and mon itored for CO2 evolution and methanol production. Additionally, some reacti on mixtures were analyzed for methane evolution. By using the test compound s labeled with C-14 in three different locations (carboxyl group, aromatic, or aliphatic chain), we demonstrated that (CO2)-C-14 evolution (ranging fr om 4.6 to 63.5% of the initial radioactivity) was mainly associated with th e release of carboxyl groups. Minimal mineralization of C-14-labeled aromat ic 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 s yringic acid, respectively. The methyl groups in 2-, 3-, and 4-methylphenol resisted release, as indicated by the lack of methane or methanol producti on. In previous studies, chlorophenols incubated with various phenoloxidase s or birnessite were subject to dehalogenation. It appears that dehalogenat ion, decarboxylation, and demethoxylation of phenolic substrates are contro lled by a common mechanism, in which various substituents are released if t hey are attached to carbon atoms involved in coupling. According to the exp erimental data, electron-withdrawing substituents, such as -COOH and -Cl, a re more susceptible to release than electron-donating ones, such as -OCH3 a nd -CH3. The release of organic substituents during polymerization of humic precursors may add to CO2 production in soil.