Structural and chemical properties of the hydroxyapatite surface. A computational ab initio and a microcalorimetric/IR spectroscopic characterization

Bioactive glasses, when implanted in the body or simply immersed in simulated body fluids (SBF), develop a biologically active hydroxyapatite (HA) layer which in turn does promote the bone-tissue formation. In fact hydroxyapatite, which exhibits strong similarities to the mineral phase of the mammalian bones and teeth, does play a key role during the bioactive glasses integration processes, in that it facilitates adhesion and subsequent proliferation of the osteocytes, so allowing the damaged bone tissues to be repaired. The first step of these processes is the adsorption of biomolecules at the active surface of HA. Therefore, studies aimed at quantitatively describing the structural and chemical properties of the HA surface are of greatest interest, in the attempt to elucidate at nano-level the interfacial processes involved in the biological fixation of inorganic materials to the living tissues. In the present study, ab initio methods and experimental techniques have been used to characterize the adsorption features of HA surfaces using H2O and CO as molecular probes. Periodic ab initio B3LYP calculations using CRYSTAL06 code have run to characterize the (001) and (010) bare surfaces for both hexagonal and monoclinic HA phases. On the geometrically relaxed surfaces the adsorption of H2O and CO has been simulated, from low to high coverage. Energies of adsorption and the vibrational features of H2O and CO have been computed as a function of coverage and compared with the corresponding microcalorimetric data and infrared spectra. The main conclusions from the simulations are that both H2O and CO adsorb on the exposed Ca2+ ions which are characterized by rather strong local electric fields, although not strong enough to dissociate H2O. The (001) surface is more active than (010) when considering CO as a probe. H2O adsorption studies on the (010) are, at the moment, still in progress. For the CO case, comparison between the computed and the measured CO stretching frequencies confirms that the exposed Ca2+ ions behave as a Lewis acidic sites, in that bathochromic shifts have been measured and computed. Microcalorimetric data for the CO adsorption are still in progress, whereas the RT adsorption of H2O on a nanosized HA specimen has already been investigated. In Figure 1 the heat of adsorption of H2O vapour is reported as a function of the adsorbed amount. The energy of interaction is quite high not only in the early stage of the process but also at high coverage (correspondent to the formation of a second shell of coordinated H2O). The interaction energies computed at B3LYP level are in fair agreement with the experimental heats, confirming that the interaction (which is also partially irreversible upon evacuation) is quite strong (see Figure 1). It is suggested that the strong, but still molecularly coordinated H2O on the cus Ca2+ cations at the HA surface has likely an implication in the adsorption of proteins at the hydrated layer interface. Indeed, if reactive Ca-OH groups were formed at the surface upon contact with H2O, denaturation of proteins would occur hampering cells adhesion. In conclusion, the joint use of experimental and computational approaches has been found extremely fruitful to elucidate the molecular events which are responsible of the adsorption processes occurring at the HA surfaces.