Influence of Water on the Adsorption Sites of Cycline Antibiotics onto Hydroxyapatite Surfaces

Hydroxyapatite (HA) functionalized nanoparticles are particularly important in bone and tooth regeneration. This study continues previous work aimed at understanding the interaction of cycline antibiotics with different HA surfaces, now incorporating the effect of hydration on adsorption energies as calculated using density functional theory. The strength of these interactions was analyzed through the quantum theory of atoms in molecules (QTAIM). The adsorption isotherm of nearly stoichiometric HA followed Langmuir-Freundlich kinetics. Minocycline exhibited higher adsorption energy than doxycycline and tetracycline. The nonhydrated adsorption behavior of minocycline is compared to earlier results for doxycycline and tetracycline, while this study also evaluates adsorption on hydrated HA surfaces for all three molecules. Minocycline displays more stable adsorption than the others, in agreement with experimental findings. Our results reveal that the influence of water cannot be neglected: although it does not prevent cycline-HA interactions, it can stabilize previously unstable configurations through hydrogen bonding between cyclines and water molecules. QTAIM analysis indicates that a combination of electrostatic interactions and medium-to-strong covalent character governs the adsorption mechanism.