Ab initio modelling of protein–biomaterial interactions: influence of amino acid polar side chains on adsorption at hydroxyapatite surfaces

The adsorption from the gas phase of five different amino acids (AAs), 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 AAs 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 AAs, 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 impart a higher stability compared with the Gly case, with Glu being the strongest adsorbate owing to its high Ca affinity and H-bond donor propensity. For AAs of large size, the adsorption is more favourable in conditions of low coverage as repulsion between adjacent AAs is avoided. For all considered AAs, 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 HA crystals from AA solutions.