Bone mineral: new insights into its chemical composition

Stanislas Von Euw,Christophe Drouet,Nadine Nassif,Guillaume Laurent,Yan Wang,Florence Babonneau,Thierry Azaïs

doi:10.1038/s41598-019-44620-6

Abstract

Some compositional and structural features of mature bone mineral particles remain unclear. They have been described as calcium-deficient and hydroxyl-deficient carbonated hydroxyapatite particles in which a fraction of the PO43− lattice sites are occupied by HPO42− ions. The time has come to revise this description since it has now been proven that the surface of mature bone mineral particles is not in the form of hydroxyapatite but rather in the form of hydrated amorphous calcium phosphate. Using a combination of dedicated solid-state nuclear magnetic resonance techniques, the hydrogen-bearing species present in bone mineral and especially the HPO42− ions were closely scrutinized. We show that these HPO42− ions are concentrated at the surface of bone mineral particles in the so-called amorphous surface layer whose thickness was estimated here to be about 0.8 nm for a 4-nm thick particle. We also show that their molar proportion is much higher than previously estimated since they stand for about half of the overall amount of inorganic phosphate ions that compose bone mineral. As such, the mineral-mineral and mineral-biomolecule interfaces in bone tissue must be driven by metastable hydrated amorphous environments rich in HPO42− ions rather than by stable crystalline environments of hydroxyapatite structure.

Highlights

Bone is a natural composite material whose main components are mineral and organic matrices[1,2]
The direct solid-state Nuclear Magnetic Resonance detection of the protons localized in bone mineral from an intact bone tissue sample is not possible
According to these results that were obtained from intact bone tissue samples, we demonstrated that the H2O and HPO42−-containing non-apatitic environments evidenced here do not originate from octacalcium phosphate (OCP) but can be safely attributed to the amorphous surface layer that coats bone mineral particles

Summary

Introduction

Bone is a natural composite material whose main components are mineral and organic matrices[1,2]. It is generally accepted that “the deficit in negative charge caused by the replacement of PO43− by either CO32− or HPO42− can be compensated by the loss of positive charge, as through removal of Ca2+ from the lattice”[7] Beside their presence within the hydroxyapatite’s crystal lattice, HPO42− ions were proposed to be present in the so-called amorphous surface layer that coats both biological[8,9,10,11] (bone, dentin) and biomimetic[11,12,13,14,15,16] nanocrystalline hydroxyapatite particles. A single pulse 31P ssNMR spectrum of bone mineral recorded under quantitative conditions was recorded to quantify these HPO42− ions with respects to the overall amount of inorganic phosphate ions that compose bone mineral

Methods

Results

Conclusion