Consideration of the functional significance of dicarboxylic amino acids in calcium apatite precipitation

Abstract

The inhibitory activities of amino acids on the crystal growth of calcium apatite were examined using synthetic hydroxyapatite as seeds in dilute supersaturated solutions. Phosphoserine exhibited the strongest inhibition among the adsorbates tested, while dicarboxylic amino acids also provided partial inhibition in the concentration range up to 10mM. In particular, the inhibitory activity of aspartic acid (Asp) was stronger than those of glutamic acid and succinic acid by comparison on the basis of the total molecular concentration in the solution. Parallel experiments were also conducted to determine the adsorption parameters of Asp onto the seed crystals and to assess the effect of the Ca-Asp complex on the equilibrium free concentration of Asp and on the kinetics of precipitation. The results clearly indicate that even concerning a weak adsorbate such as Asp, the observed crystal growth inhibition can be interpreted in terms of the fractional coverage of specific adsorption sites and the formation of ion pairs with calcium, thereby decreasing the driving force (degree of supersaturation) for crystal growth.