Hydroxyapatite as a key biomaterial: quantum-mechanical simulation of its surfaces in interaction with biomolecules

Hydroxyapatite [HA, Ca 10 (PO 4 ) 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. HA also covers the surface of the Hench Bioglass® 45S5 when put in contact with biofluids. Quantum mechanical simulations have been carried out at B3LYP level within periodic boundary conditions and adopting Gaussian basis sets as encoded in the CRYSTAL09 code [1] to model their surfaces. The (001), (010) and non-stoichiometric surfaces of HA were studied in interaction with Gly [2-3] and Ser, Lys, Gln and Glu aminoacids to characterize the most stable adsorbed structures and interaction energies. For the case of glycine, the quantum mechanical results were essential to assign the experimental infrared features measured after chemical vapour deposition of Gly on well formed nano-hydroxyapatite crystals. This work has been extended to model the interaction of a 12mers-peptide with the HA surfaces as it is usually observed that proteins denature due to contact with inorganic solid substrates. However, recent findings highlight HA surfaces as ideal platforms to stabilize (and even to induce) well-defined protein folded conformations upon adsorption, a fact which enables the retention of the peptide biological functionality. The results of our simulation indeed confirm that the oligo-peptide will prefer to stay as an a-chain with respect to its globular native conformation under the condition of being mutated with at least one Lys and one Glu aminoacids.