Combined experimental and DFT studies of amino acid adsorption on biomimetic apatite: application to serine

Biomimetic apatite nanocrystals, as synthetic analogs of bone apatite, offer a more realistic bone mineral model than regular stoichiometric hydroxyapatite (HAP) for studying biomolecule-bone mineral interactions. This is due to the presence, on the constitutive nanocrystals of bone apatite and biomimetic counterparts, of a hydrated ionic surface layer that closely mimics the bone mineral surface. In the present work, a relevant model of the biomimetic apatites in solution, in particular focusing on their very peculiar interface structuration, is proposed using first-principles calculations. Moreover, the adsorption of a model amino acid relevant to biomineralization, namely serine, on synthetic biomimetic apatite is addressed under aqueous conditions using a combined experimental and first-principles computational approach. Experimentally, adsorption isotherms were established at a physiological pH and were compared to the literature data, and the release of calcium and phosphate ions in solution upon serine adsorption was monitored for the first time to better understand the underlying sorption mechanism. Adsorption data fitted well with the Freundlich model, and calculated parameters showed a low adsorbed amount as well as a low affinity for the surface of the apatite substrate. The results indicate that serine adsorbs onto the apatite surface without significantly disturbing the ionic environment. Second, density functional theory (DFT) simulations, using an explicit water solvent model, revealed several energetically favorable adsorption configurations. In these configurations, the serine molecule interacts with the non-apatitic hydrated surface layer through electrostatic interactions involving (COO^-/Ca²⁺) and (NH₃⁺/HPO₄^2-) ions. The water molecules at the water/biomimetic apatite interface also play a role in the serine adsorption. Computed interatomic distances and Bader charge analysis confirmed that the adsorption is predominantly governed by electrostatic forces and hydrogen bonding, with no charge transfer. This study points to the relevance of such an integrative experimental/computational approach when dealing with complex surface structures as in bone(-like) apatites, to enhance our understanding of the interaction between biomimetic apatite surfaces and biomolecules such as the amino acid serine.