First Principles Investigation of Mineral Component of Bone:  CO₃ Substitutions in Hydroxyapatite

Abstract

First principles calculations are used to investigate carbonated hydroxyapatite, a naturally occurring mineral and also the inorganic component of animal bone. Two types of carbonate substitution are studied: A-type in which the carbonate ion substitutes for an OH and B-type where the substitution is for a phosphate. Both types have unbalanced charges and various forms of charge compensation are treated. The methods, which are based on density functional theory and first principles pseudopotentials, yield equilibrium atomic arrangements, changes in lattice parameters, and total energies for different types of substitution. When calculated energies of selected stable compounds are used, the formation energies of different carbonate substitutions with accompanying charge compensation defects can be compared. The results indicate that compact complexes are energetically favored, and a B-type material with charge compensation by a calcium vacancy together with a hydrogen atom which bonds to a neighboring phosphate is the most stable of all those considered.

Diffraction techniques and vibrational spectroscopy opportunities to characterise bones

“…Several investigations indicate that type B carbonate is the major species in biological apatites. This is in line with the research of Astala et al [22] where ❏ represents the vacancy. This representation is in fact far too simple.…”

Bisphosphonate binding affinity as assessed by inhibition of carbonated apatite dissolution in vitro

“…T i is given by Eq. (2) ( 2) where C eff is the number of moles of equivalent HAP dissolved per liter of added titrant (3) Titrant concentrations for sodium chloride (T NaCl ) and hydrochloric acid (T HCl ) were calculated from Eqs. (4) and (5)…”

“…[1][2][3] Thus HAP and its derivatives have been used extensively in in vitro studies to investigate the complex nature of skeletal mineral scaffolding. However, this ignores the role of minor components (e.g.…”

“…3), as suggested by the results of other studies. 3,6,7 Although the biological importance of carbonate in bone is not fully understood, it has been shown to act as a hydrogen ion buffer, 9 to increase crystal lattice strain, and to reduce the overall crystallite size. 10,11 Bisphosphonates (BPs) are nonhydrolyzable analogues of naturally occurring inorganic pyrophosphates; they display strong affinity for bone mineral, and are potent inhibitors of bone mineral resorption.…”

In Silico Study of Hydroxyapatite and Bioglass®: How Computational Science Sheds Light on Biomaterials

“…HA is the main component of the inorganic phase of bones and teeth, but, as many studies have demonstrated, the mineral present in those tissues is neither crystalline nor without defects (Fleet & Liu, 2003;Fleet & Liu, 2004;Fleet & Liu, 2007;Rabone & de Leeuw, 2005;Astala & Stott, 2005;Astala et al, 2006;de Leeuw et al, 2007 ). Indeed, it incorporates many other elements, ions or compounds, such as Mg or other alkaline earth metals instead of Ca, and, above all, the carbonate anion.…”