Biomineralization process of calcium phosphate: Modulation of the poly-amino acid with different hydroxyl/carboxyl ratios

Organic species are thought to exert an important effect on the formation of the mineral materials by their electrostatic attraction with the cations of minerals. In this work, a series of polyaspartic acids with different hydroxylation degrees (PAsp-x%OH) have been used as crystal growth modifiers to direct the synthesis of calcium phosphate. The change of x in the PAsp-x%OH can precisely adjust its electrostatic interaction with calcium ions and, thereby, modulate the formation process and property of calcium phosphate, such as morphology, crystallinity, organic component content and calcium-to-phosphate ratio. Two competitive reactions are suggested in this system, that is, the combination of calcium ions to PAsp-x%OH and the precipitation of calcium hydrogen phosphate dihydrate. The trends of these two reactions are determined by variation of x in the PAsp-x%OH: lower value of x tends to involve the former, while the higher tends to latter. It has been found that the mineralization process involving PAsp-15%OH displayed a special point to counterbalance the two competitive reactions, leading to the longest induction time. These findings indicate that how an organic species controls the morphology and the formation dynamics of inorganic crystals in biomineralization by the slight modification of its molecular structure.