Comparison of reaction products from the transformation of catechol catalyzed by birnessite or tyrosinase

Both tyrosinase, a Cu-containing polyphenoloxidase, and birnessite (delta-R-MnO2) are able to catalyze the transformation of phenolic compounds through oxidative polymerization, a process that leads to humification, but the reaction mechanisms are not fully understood. The objective of this study was to characterize or identify the reaction products generated from catechol (1,2-dihydroxybenzene) by birnessite or tyrosinase. Birnessite and tyrosinase catalyzed the transformation of catechol to oligomers, polycondensates, and fragments. The reaction products formed after catalysis by tyrosinase were brown colored, while those resulting from the birnessite-catechol system were green colored; the former had a higher absorbance between 200 and 620 mn than the latter. This indicated a higher degree of aromatic ring condensation in products of the tyrosinase-catechol system relative to those of the birnessite-catechol system. In addition, the products of birnessite catalysis contained polycondensates and fragments, including aliphatic components, with lower molecular weights than did the products derived from catalysis by tyrosinase. Fourier-transform infrared (FTIR) analysis and mass spectrometry indicated that the reaction products formed an organic coating on the birnessite granules.