Kinetics and mechanisms for sulfamethoxazole transformation in the phenolic acid-laccase (Trametes versicolor) system

Oxidation of phenolic acids (PCs) by laccase could produce various kinds of reactive oxygen species (ROS), which is expected to have substantial impact on the transformation of antibiotics like sulfamethoxazole (SMX) in soil and aquatic environments. In this study, the formation of semiquinones radical (SQ(center dot-)), superoxide anion radical (O2(center dot-)), hydrogen peroxide (H2O2), hydroxyl radical (center dot OH), and singlet oxygen (O-1(2)) in a laccase-gallic acid (GA) reaction system was confirmed. Meanwhile, GA would be transformed to its monomeric quinone and quinones of di- and tri-polymers. Transformation of SMX by laccase alone is negligible, while which was greatly enhanced in the presence of GA at the optimal pH of 5.5. The dissolved O-2 was the requisite for transformation of SMX due to its fundamental role in the formation of SQ(center dot-), the key species initializing the chain reactions for the generation of other ROS. The quenching experiments indicated O-2(center dot-) and O-1(2) were the main ROS responsible for SMX transformation. A total of thirteen products were proposed for the SMX transformation, with the pathways including the breaking of S-N bond, the cleavage of oxazole ring, electrophilic substitution, Michael addition, and condensation reactions. Moreover, the existence of electron-withdrawing substitution group on the benzene ring of PCs and less stability of SQ(center dot-) was believed to be favorable for the transformation of SMX. The results above expand our understanding on the role of oxidation of PCs by laccase in the SMX transformation in environments and are of significance in relation to use of laccase in dealing with SMX pollution.

Automated summary

This study confirms the production of various reactive oxygen species (ROS) through oxidation of phenolic acids (PCs) by laccase, which has a significant impact on the transformation of sulfamethoxazole (SMX) in soil and aquatic environments. The presence of gallic acid (GA) enhances the transformation of SMX by laccase, and the formation of ROS such as semiquinones radical (SQ(center dot-)), superoxide anion radical (O2(center dot-)), hydrogen peroxide (H2O2), hydroxyl radical (center dot OH), and singlet oxygen (O-1(2)) is confirmed. The dissolved O-2 plays a fundamental role in the formation of SQ(center dot-), which initiates chain reactions for the generation of other ROS. The transformation pathways of SMX include the breaking of S-N bond, cleavage of oxazole ring, electrophilic substitution, Michael addition, and condensation reactions. The presence of electron-withdrawing substitution groups on the benzene ring of PCs and the less stability of SQ(center dot-) are believed to favor the transformation of SMX. These results improve our understanding of the role of laccase-mediated oxidation of PCs in SMX transformation and have implications for the treatment of SMX pollution using laccase.