Tuning antibiotic adsorption via aromatic functionalization of chitosan monomers: a computational study

This work describes the application of molecular dynamics (MD) simulations and density-functional theory (DFT) calculations to unveil how the association propensity of the antibiotics tetracycline (TC), ciprofloxacin (CPX), levofloxacin (LVX), and cephalexin (CLX) in different protonation states varies with the increasing aromaticity and hydrophobicity of the adsorbent species. It is shown that the increasing aromaticity of the chitosan substituent enhances the adsorption of all antibiotics. Both TC and CLX appear to adsorb better under low and neutral pH conditions than in alkaline media. Conversely, the adsorption of CPX and LVX is usually higher and it is less affected by the pH conditions. Regarding the association mechanism, it is apparent from the MD simulations and the DFT calculations that van der Waals and Coulomb interactions, as well as π-π stacking, play a major role in the capture of the antibiotic by the adsorbent, while H-bonding has a minor contribution even though it occasionally aids the stabilization of the complex.