Molecular Modeling of the Binding Structures in the Interlayer Adsorption of a Tetracycline Antibiotic by Smectite Clays

A controlling factor in the fate of antibiotics in the environment is their sequestration in soil particles including clay minerals. Of special interest is the interlayer adsorption by smectite clays, which has been shown to influence both the bioavailability and persistence of antibiotics in the soil environment. However, the interlayer structures of the bound antibiotics, essential to an accurate understanding of the adsorption mechanisms, are not well understood. Molecular simulations of oxytetracycline (OTC) with a model montmorillonite (MONT) clay were performed to gain insights into these structures for tetracycline antibiotics. Monte Carlo simulations were used for explorations of the clay layer spacing required for the adsorption of the antibiotic under different hydration states of the clay interlayer; these preliminary results were validated with previous X-ray diffraction patterns obtained following sorption experiments of OTC with MONT. Molecular dynamics relaxation simulations were performed subsequently in order to obtain geometry-optimized structures of the binding conformations of the intercalated antibiotic in the model MONT layers. This study contributes to a mechanistic understanding of the factors controlling the interlayer adsorption of the tetracycline antibiotics by the expandable smectite clay minerals. Figure 1. Optimized Monte Carlo simulation cell of OTC in the interlayer of MONT: perspective side view (top) and bottom view (bottom).