Contribution to understand the biomineralization of bones.

Abstract

The goal is to propose a material scientific hypothesis for the atomic arrangement of calcium phosphates during the mineralization of bones. It was reached by the analysis of bones of healthy and osteoporotic rats using analytical transmission electron microscopic methods. Electron diffraction patterns show hydroxyapatite (HAP) as dominant phase within the mineralized areas. In the electron energy loss spectrum, a double peak of the phosphorous L-edge seems to be a characteristic feature of the phosphorous binding in biological HAP. The hypothesis bases on periodic features on the collagen surface which agree with distances between oxygen atoms in the (200) plane of octacalcium phosphate (OCP). Bridge pillars for the HAP network consist of OCP coupled with a half unit cell on collagen by oxygen-hydrogen bridges. Possibly, the metastable OCP bridges are only a transient step, while the mineralization is starting. OCP and HAP couple by similar distances of calcium atoms in an interface close to the (100) planes of the OCP and the HAP network. To reach the perfect overlap of the equidistant Ca atoms, the HAP network has to be rotated by 22.5° around the a-axis, 11.5° around the c-axis of HAP, and 10.1° around an axis perpendicular to a and c. A supercell based on this idea is able to explain the dominance of HAP in the electron diffraction patterns, the arrangement of the (002) lattice planes perpendicular to the collagen fiber axis, and sections of high-resolution TEM images.