Investigation on humic substance and tetracycline interaction mechanism: biophysical and theoretical studies and assessing their effect on biological activity

Tetracycline (TC) is a widely used antibiotic, and evaluating its interaction with humic substances (HS) that act as a complexing agent in the environment is essential to understanding the availability of this contaminant in the environment. This study evaluated the interaction between HS and TC using different spectroscopic techniques, theoretical studies, and biological assays simulating environmental conditions. TC interacts with HS, preferably by electrostatic forces, with a binding constant of 9.2 × 10 3 M −1 (30 °C). This process induces conformational changes in the superstructure, preferably in the HS, like protein fraction. Besides, studies using the 8-anilino-1-naphthalene sulfonate (ANS) probe indicated that the antibiotic alters the hydrophobicity degree on HS’s surface. Synchronized fluorescence shows that the TC interaction occurs preferentially with the protein-like fraction of soil organic matter ( K SV = 26.28 ± 1.03 M −1 ). The TC epitope was evaluated by 1 H NMR and varied according to the pH (4.8 and 9.0) of the medium, as well as the main forces responsible for the stabilization of the HS-TC complex. The molecular docking studies showed that the formation of the HS-TC complex is carried out spontaneously (Δ G = −7.1 kcal mol −1 ) and is stabilized by hydrogen bonds and electrostatic interactions, as observed in the experimental spectroscopic results. Finally, biological assays indicated that HS influenced the antimicrobial activity of TC. Thus, this study contributed to understanding the dynamics and distribution of TC in the environment and HS’s potential in the remediation of antibiotics of this class in natural systems, as these can have adverse effects on ecosystems and human health. Graphical Abstract