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

Abstract

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.

Equation of state of hexagonal hydroxylapatite (P63) as obtained from density functional theory simulations

“…Recently, the structural, vibrational, surface properties and second‐order elastic constants of hydroxylapatite by ab initio quantum mechanical approach were investigated, obtaining results in good agreement with available experimental data. The aim of the present work is to further extend the research and calculate the equation of state of stoichiometric OHAp (space group P6 3 ) by using the well‐known quasi‐harmonic approximation (QHA), to fill the knowledge gap on the thermomechanical behavior of this mineral phase.…”

First principle investigation of the thermomechanical properties of type A carbonated apatite

“…The well‐known hybrid DFT Hamiltonian B3LYP was selected as it was very suitable for the investigation of structural, vibrational, thermodynamics, and mechanical properties of apatite minerals . A correction to take into account dispersive forces using the DFT‐D2 approach was included in its recently adjusted form for the hybrid B3LYP functional (B3LYP‐D* approach) …”

“…The all‐electron Gaussian‐type orbital basis sets adopted were the same used in previous works on OHAp and type A Cap . Calcium has been described with an 86‐511G(2d), with outer shell exponents α sp = 0.453 bohr −2 , α d1 = 3.1910 and 0.8683 bohr −2 , and α d2 = 0.2891 bohr −2 .…”

“…The effects of type A, type B, and type A‐B substitutions on the unit cell variations and on the spatial orientation of the carbonate ion within the OHAp lattice were extensively studied at both the experimental and theoretical levels, aiming at a better comprehension of the mineralization, demineralization, and remineralization properties of carbonated hydroxylapatite. In addition, several efforts were made to investigate the surface morphology and electrostatic potential of COHAp, influencing the adsorption processes at the mineral‐organic interface in a biological environment, which could be affected by the and other ionic substitutions …”

Interactions between Clinically Used Bisphosphonates and Bone Mineral: from Coordination Chemistry to Biomedical Applications and Beyond

“…%), organic fraction (in 90 % type I collagen) and water . Biological apatite contains various anionic and cationic inclusions, substitutions, and vacancies that make the material thermodynamically more stable . It is characterized by two nonequivalent binding motifs of Ca 2+ which differ by their coordination environments and can be described as protein‐bound and free sites …”