Reaction rates of phenolic humus constituents and anilines during cross-coupling

Oxidative coupling processes operative in soil during the formation of stable humic materials and bound residues involving anilines and soil organic matter were studied. We examined the relationship between chemical structure and reactivity for the peroxidase-mediated coupling of potential phenolic humus constituents of plant and microbial origin. The results demonstrated that the acrylic group significantly increased the reactivity of the naturally-occurring phenolic compounds tested. These results suggested that lignin-derived phenols containing the acrylic group, e.g. ferulic acid, may be utilized preferentially during the peroxidase-mediated synthesis of humic materials in soils. We also investigated the peroxidase-mediated cross-coupling of mono-substituted anilines with various phenolic compounds. Reactivity of aniline, nitroaniline and chloroaniline was greatly enhanced in the presence of a highly-reactive electron donor such as ferulic acid. The enhanced reactivity suggested that the aniline compounds were reacting by a different mechanism than when they are present as sole electron donors. The most plausible mechanism is a secondary chemical reaction between anilines and intermediates or products produced during the enzymatic oxidation of phenolic electron donors.