Hydroxyapatite [HA, Ca10(PO4)6(OH)2] is the main constituent of the mineral phase in mammalian bones and teeth enamel. Moreover, according to Hench’s mechanism, a thin layer of crystalline carbonate hydroxyapatite (HCA) is forming after immersion of bioglasses in SBF solutions (Simulated Body Fluid). Just because of this thin layer, bone reparation can be promoted and the bioactive glass implant result strongly attached. Chemical structure of HCA is very similar to HA’s one, therefore, studying hydroxyapatite microstructure allows to better understand the crucial step of bioglass bonding to living tissues. In the present work, in order to study the interfa- cial reactions between HA and bioglasses, ab-initio electronic structure methods have been applied to describe hydroxyapatite (001) and (100) surfaces, with the periodic ab-initio CRYSTAL code (Gaussian basis set and B3LYP functional). Actually, HA growth is elongated in the  direction in bone matrix and in tooth enamel, but it is Figure 1 known that some proteins (e.g., multiphosphorylated) bind preferentially to (100) crystal faces. Models with different thickness have been characterised, both free (Fig. 1) and in interaction with Figure 2 water molecules (Fig. 2). The computed water binding energy is ~ 20/30 kcal/mol, in agreement with micro-calorimetric data. The vibrational frequencies of adsorbed water have also been calculated and show, in comparison with those of free water, a blue shift of the bending mode (average value ~ 80 cm-1), whereas a red shift (~ 100-1200 cm-1) has been calculated for the OH stretching. This behaviour is a consequence of the hydrogen bond between water and oxygens of superficial phosphates groups (hydrogen bond length varies from 1.5 to 2 Å). 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, thus approaching to Hench’s reaction steps.
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