Mechanism of Zn stabilization in hydroxyapatite and hydrated (0 0 1) surfaces ofhydroxyapatite

Abstract

A basic understanding of Zn incorporation on bulk and hydrated (0 0 1) surfaces ofhydroxyapatite (HA) is attained through electronic structure calculations which use acombined first principles density functional (DFT) and extended Hückel tight binding(EHTB) methodology. A Zn substituted hydroxyapatite relaxed structure is obtainedthrough a periodic cell DFT geometry optimization method. Electronic structure propertiesare calculated by using both cluster DFT and periodic cell EHTB methods. Bond ordercalculations show that Zn preference for the Ca2 vacancy, near the OH channel and withgreater structural flexibility, is associated with the formation of a four-fold (bulk) andnearly four-fold (surface) coordination, as in ZnO. When occupying the octahedral Ca1vacancy, Zn remains six-fold in the bulk, but coordination decreases to five-fold in thesurface. In the bulk and surface, Zn2 is found to be more covalent than Zn1, due to adecrease in bond lengths at the four-fold site, which approach the 1.99 Å ZnO value. Znis however considerably less bound in the biomaterial than in the oxide, wherecalculated bond orders are twice as large as in HA. Surface phosphate groups(PO4) andhydroxide ions behave as compact individual units as in the bulk; no evidence is found for the presenceof HPO4. Ca–O bond orders decrease at the surface, with a consequent increase in ionicity.Comparison between DFT and EHTB results show that the latter method gives a goodqualitative account of charge and bonding in these systems.