Manganese peroxidase mediated oxidation of sulfamethoxazole: Integrating the computational analysis to reveal the reaction kinetics, mechanistic insights, and oxidation pathway

Highlights: • Manganese peroxidase was used to induce the in vitro oxidation of sulfamethoxazole. • The reaction kinetics were investigated under different conditions. • The release of Mn³⁺ from protein was responsible for sulfamethoxazole oxidation. • Proton-coupled electron transfer process dominated the Mn³⁺ transformation. • Possible oxidation pathways were proposed with single-electron transfer mechanism. In this study, manganese peroxidase (MnP) was applied to induce the in vitro oxidation of sulfamethoxazole (SMX). The results indicated that 87.04% of the SMX was transformed and followed first-order kinetics (k_obs = 0.438 h⁻¹) within 6 h when 40 U L⁻¹ of MnP was added. The reaction kinetics were investigated under different conditions, including pH, MnP activity, and H₂O₂ concentration. The active species Mn³⁺ was responsible for the oxidation of SMX, and the Mn³⁺ production rate was monitored to reveal the interaction among MnP, Mn³⁺, and SMX. By integrating the characterizations analysis of the MnP/H₂O₂ system with the density functional theory (DFT) calculations, the proton-coupled electron transfer (PCET) process dominated the catalytic circle of MnP and the transformation of Mn³⁺. Additionally, possible oxidation pathways of SMX were proposed based on single-electron transfer mechanism, which primarily included the S–N bond cleavage, the C–S bond cleavage, and one electron loss without bond breakage. It was then transformed to hydrolysis, N–H oxidation, self-coupling, and carboxylic acid coupling products. This study provides insights into the atomic-level mechanism of MnP and the transformation pathways of sulfamethoxazole, which lays a significant foundation for the potential of MnP in wastewater treatment applications.