Hydroxyapatite surfaces in interaction with biomolecules: a periodic B3LYP approach

Hydroxyapatite [HA, Ca10(PO4)6(OH)2] is the main constituent of the mineral phase in mammalian bones and teeth enamel. For this reason, HA is widely applied as an orthopaedic and dental biomaterial, both per se and together with other classes of materials, in the form of coating for metal alloys, in composites with polymers and so on. Despite these various applications, fundamental knowledge of the interaction mechanisms of hydroxyapatite with biomolecules is still poorly understood. Many experimental techniques, such as solid state NMR, IR spectroscopy, microscopy and others, have investigated the interaction of HA different samples with macromolecules as biphosphonates, organic acids and amino acids to highlight the inorganic-organic interface reactions. Along the same line, recently, theoretical methods have been devoted to simulate the most relevant HA faces, both bare and in interaction with biomolecules, depending on the theoretical approach. While molecular mechanics and dynamics modelling aims at characterizing the dynamical properties of large quantities of molecules on the surfaces, quantum mechanics can provide more quantitative information at a molecular level. In this communication, B3LYP periodic simulations of the adsorption processes of different molecules on some of the most studied HA surfaces, carried out by means of the quantum-mechanical CRYSTAL06 code, will be presented. Gaussian basis set of double-zeta quality in combination with the accurate B3LYP functional have assured the best compromise between cost of the calculation and reliability of the obtained results. Starting from the optimized bulk structure of an hexagonal HA model, the following five surfaces have been considered: the stoichiometric (001), (010) and (101) ones and two non stoichiometric (010) cases, namely the calcium-rich and the phosphorous-rich. All the surface models have been fully optimized as real 2D slabs and characterized with respect to their stability with thickness and to their electronic and vibrational properties. The adsorption processes have been simulated first for small probe molecules, as water and carbon monoxide, then considering the simplest organic acid – formic acid – to gain insight into the acid dissolution mechanism of HA related to teeth enamel and to the formation of dental caries. Subsequently, adsorption of glycine and other amino acids (such as lysine and glutamic acid) has been simulated, as a first step towards the knowledge of HA/protein interactions, which are actually occurring when a biomaterial is contacted with bones or teeth living tissues. The reference methodology and the complete characterization of the hydration process as well as the preliminary acid dissolution steps of HA surfaces will be addressed. Both energetic and vibrational properties of the models examined will be discussed and, compared to experimental data, deriving form microcalorimetric and IR spectroscopic measurements, when present. As for amino acids adsorption simulations, which are fundamental for the knowledge of HA in vivo behaviour, the stability of different forms of interaction, neutral vs zwitterionic, as well as H2O role in the interaction will be presented.