Hydroxyapatite [HA, Ca10(PO4)6(OH)2] is the main constituent of the mineral phase in mammalian bones and teeth enamel. Moreover, carbonate hydroxyapatite (HCA) also plays a leading role in bone reparation, because a thin layer of crystalline HCA in contact with the physiological phase is formed at the surface of bioactive glasses used as artificial replacers of the natural bones. Fortunately, HCA structure is very close to that of HA, therefore, understanding HA allows to progress in the realm of bioglasses/living tissues bonding mechanism. In the present work, ab-initio B3LYP periodic electronic methods using the CRYSTAL code have been applied to study the most common HA (001) and (100) surfaces. Actually, HA crystals are elongated in the [0001] direction in both bone matrix and tooth enamel, but some proteins bind preferentially to the (100) 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, suffer 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.
Ab initio QM study of hydroxyapatite (001) and (100) surfaces
CORNO, MARTA;UGLIENGO, Piero;CIVALLERI, Bartolomeo;
2006-01-01
Abstract
Hydroxyapatite [HA, Ca10(PO4)6(OH)2] is the main constituent of the mineral phase in mammalian bones and teeth enamel. Moreover, carbonate hydroxyapatite (HCA) also plays a leading role in bone reparation, because a thin layer of crystalline HCA in contact with the physiological phase is formed at the surface of bioactive glasses used as artificial replacers of the natural bones. Fortunately, HCA structure is very close to that of HA, therefore, understanding HA allows to progress in the realm of bioglasses/living tissues bonding mechanism. In the present work, ab-initio B3LYP periodic electronic methods using the CRYSTAL code have been applied to study the most common HA (001) and (100) surfaces. Actually, HA crystals are elongated in the [0001] direction in both bone matrix and tooth enamel, but some proteins bind preferentially to the (100) 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, suffer 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.
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