First-principles study of structural and surface properties of (001) and (010) surfaces of hydroxylapatite and carbonated hydroxylapatite

Since it was first discovered that the main component of the mineral phase of bone, dentine and enamel is made from non-stoichiometric hydroxylapatite [Ca10(PO4)6(OH)2; OHAp], many successful efforts have been made to characterize its structure physico-chemically and to use it as a biomaterial for tissue repair and reconstruction. For the latter, it has been suggested that the biomimetic features of OHAp can be improved by vacancies and ionic substitutions, as typically found in natural bone tissues. In the present work, this line of thought has been followed, and the structural and electrostatic potential features of the (001) and (010) surfaces of OHAp and defective type A, type B and type AB carbonated hydroxylapatite (COHAp) have been studied using ab initio quantum mechanics at the DFT/B3LYP level. The results are in good agreement with previous experimental and preliminary theoretical work. They provide a deep analysis of the modulation of OHAp features caused by carbonate substitutions, and extend the current knowledge of the structural and surface properties of apatites. The (001) and (010) surfaces of hydroxylapatite are key to understanding and modulating the mineral-organic interactions in bone tissues. The present work provides the structural and electrostatic properties of both stoichiometric and carbonated hydroxylapatite surfaces obtained with an ab initio quantum mechanics approach.