In-vitro Synthesis of Mg-enriched Hydroxylapatites Towards Understanding of Human Enamel

Objectives: Recent findings in the Joester Group especially by Karen Derocher, have improved our picture about the localication of trace elements (<5at%) in the enamel crystallites of Human teeth. It was found that at Mg-rich (1-1.5 at%) region exists inside of the hydroxylapatite nanocrystals. So far, synthetic means towards magnesium enriched hydroxylapatites were and led always to other phases e.g. such as whitlockite.

Methods: A state-of-art method of synthesizing Mg-rich hydroxylapatites has been applied. The obtained crystals were characterized by single crystal xray-diffraction structurely and by atom probe tomogrogphy, ICP/MS and EDS regarding their elemental composition.
Results: We set up an in-vitro system to synthesize Mg-rich (0.1-1wt%, rel. to 0.5-5 At% in hydroxylapatite) hydroxylapatites under hydrothermal conditions under very acidic conditions (pH 3-4) with a surface-active agent, supported by mcirowave heating. Under these conditions at higher Mg-loadings (>0.12 wt%) most of the magnesium ends up in whitlockite. Nevertheless, the minor hydroxylapatite phase could be distinguished by morphology to that we were able to obtain single crystals. The crystal structure revealed the preferred positon of the Mg in the hydroxylapatite crystal lattice. APT, ICP/MS and EDS were obtained to clarify the elemental composition of the single crystals.
Conclusions: A preliminary structure evaluation shows a lower occupation at the Ca2 site in the hydroxylapatite lattice indicating presumably the site where the Mg is located. This site is coordinated by five phosphate-oxygens and the hydroxyl-oxygen. Composition of these crystals based on EDS and structural refinement is Ca_4.7Mg_0.3(PO₄)₃OH (relating to 1.3 At% Mg), thereby very close to the atomic fraction found in the human enamel by APT by Karen Derocher