Influence of magnesium ions and amino acids on the nucleation and growth of hydroxyapatite

Abstract

It has long been known that magnesium (Mg) ions are effective inhibitors of hydroxyapatite (HAP) crystal nucleation and growth. During the HAP nucleation, it is now generally recognized that the involvement of assembled amorphous calcium phosphate (ACP) nuclei and prenuclei may be of greater importance in the overall crystallization mechanism. In the calcium-phosphate (Ca–P) biomineralization process significant amounts of Mg ions are present in animal skeletons where they are thought to stabilize ACP mineral phases. However, there is still some uncertainty concerning the precise roles of Mg in the nucleation and growth of apatite minerals. Herein, a slow, well-controlled, Ca–P crystallization study under a precisely defined thermodynamic driving force shows that Mg ions are able to prevent the nucleation of HAP in supersaturated Ca–P solutions by stabilizing gel-like ACP phases, prolonging the induction and subsequent transformation times, under near-physiological conditions (pH 7.400, ionic strength (I) = 0.150 mol L⁻¹, 37.0 °C). Relatively large HAP crystals growing from the ACP surfaces were characterized by HRTEM and SEM. Surprisingly, EDS spectra revealed that no Mg ions were present in either the gel-like ACP or final crystalline phase. Following the addition of amino acids to the supersaturated solutions both in the presence and absence of Mg ions, the nucleation of HAP was promoted by shortening the induction and transformation times, and at the same time, the sizes of ACP and HAP crystals decreased as compared with the controls. Our results suggest that the formation of magnesium-phosphate (Mg–P) ion pairs may reduce the thermodynamic driving force for nucleation and phase transformation to HAP.