Ab-initio QM study of hydroxyapatite (001) and (010) surfaces in interaction with water

Hydroxyapatite [HA, Ca10(PO4)6(OH)2] is the principal constituent of the mineral phase of bone as well as of teeth enamel. Much recent work has investigated the role of calcium hydroxyapatite within the wide range of biomaterials, both experimentally and theoretically. Considering bioglasses, it was shown by Hench and coworkers that a thin crystalline layer of a material, very similar in structure to HA, is formed on the surface of a bioglass when it is immersed in body plasma solution (in vivo tests) or in SBF (Simulated Body Fluid, usually used for in vitro studies). Due to the fundamental function carried out by HA, the knowledge of its structure at the atomic level is considered of great importance. In the present work, ab-initio B3LYP periodic electronic methods using the CRYSTAL06 code have been applied to study the most common HA (001) and (010) surfaces. Actually, HA crystals are elongated in the [0001] direction in both bone matrix and tooth enamel, but some proteins bind preferentially to the (010) crystal faces. Surface models with different thickness have been characterised, both free and in interaction with water. The computed water binding energy is ~ 20/30 kcal/mol, in agreement with microcalorimetric data. The vibrational frequencies of adsorbed water molecule show a blue shift of the bending mode (average value ~ 80 cm-1), and a red shift (~ 100-1200 cm-1) of the OH stretching modes, as consequence of the hydrogen bond between water and the surface oxygens of the phosphates groups. These results are in remarkable agreement with the experimental IR data. For the future, we will aim at modelling amino acids adsorptions and at simulating the interaction between HA and silica based materials, with the purpose to better understand the formation of the thin HA layer at the bioglasses materials.