Ab-initio modeling of protein/biomaterial interactions: influence of amino acid polar side chains on adsorption at hydroxyapatite surfaces

The adsorption from gas-phase of five different amino acids (AA), namely Gly, Ser, Lys, Gln and Glu, on three surface models of hexagonal hydroxyapatite (HA) has been studied at B3LYP level with Gaussian type basis set within a periodic approach. The AA adsorption was simulated on the (001) and (010) stoichiometric surfaces, the latter both in its pristine and water reacted form. Low/high AA coverage has been studied by doubling the HA unit cell size. The AA have been docked to the HA surfaces following the electrostatic complementarity between the electrostatic potentials of AA and the bare HA. Gly adsorbs as a zwitterion at the (001) surface, whereas at the (010) ones the proton of the COOH group is transferred to the surface resulting in an HA + /Gly - ion pair. For the other AA the common COOH-CH-NH 2 moiety behaves like in Gly, while the specific side-chain functionalities adsorb at the HA surfaces by maximizing electrostatic and H-bond interactions. The interactions between the side chains and the HA surface imparts a higher stability compared to the Gly case, with Glu being the strongest adsorbate due to its high Ca affinity and H-bond donor propensity. For AA of large size, the adsorption is more favorable in conditions of low coverage as repulsion between adjacent AA is avoided. For all considered AA, the strongest interaction is always established on the (010) faces rather than on the (001) one, therefore suggesting an easier growth along the c-direction of hydroxyapatite crystals from AA solutions.