Protein adsorption on nanohydroxyapatites: Spectroscopic investigations at molecular level

Proper thermal treatments allowed to modify the number of surface Ca ²⁺ able to coordinate water molecules on the surface of hydroxyapatite (HA) nanoparticles surrounded by an amorphous layer. Despite the consequent significant difference in the first hydration level between untreated and treated HA, the amount of adsorbed BSA, used as a model protein, remained essentially unchanged and the native structure of adsorbed protein was retained (as indicated by mid-IR ATR). Near-IR spectroscopy evidenced that adsorbed proteins should be in direct contact with surface Ca²⁺ through a displacement of H₂O molecules by charged acidic residues. In agreement with a previous study that evidenced the heterogeneity of surface Ca²⁺ ions in terms of Lewis acidity, it was then proposed that the adsorption of BSA on such nano-HA should be ruled by some feature of the local structure of surface Ca²⁺ sites, prevailing on the total number of cationic sites exposed and the related features of the first hydration layer.