Research On Biomimetic Mineralization Of Calcium Phosphates Regulated By Various Organic Biomolecules

Biomineralization is an important process in tissue formation. The tissue formed underthe regulation of organic substrate possesses a hierarchical structure, the properties of whichare much better than synthetic materials. Hence, it is of great importance to explore theregulation mechanism of organic substrate on the formation of inorganic minerals.In this study, the biomimetic mineralization of calcium phosphate was regulated throughfive types of biomolecules （citrate, Asp, Glu, Lys and BSA）. The effect of each biomoleculeon the type, morphology and size of the formed calcium phosphate was investigated viachanging the amount of the biomolecule and the concentrations of calcium and phosphate.The composition and structure of the calcium phosphate was characterized by XRD, FTIR,SEM, DSC and TG. The action of each biomolecule was compared and the mechanism ofregulation was explored.The results indicated that compared with Lys and BSA, the other three biomoleculeshaving carboxyl groups would prohibit the nucleation. This prohibition mainly depended onthe concentration ratio of biomolecule and calcium. For citrate, the solution was transparentand no precipitate was formed when Cit/Ca²⁺≥1.6. This meant that3citrate molecules couldcombine2calcium ions, which left no vacant site for calcium to attract phosphate. For Asp,the solution was clear when Asp/Ca²⁺≥0.752, suggesting that1.5Asp molecules couldcombine2calcium ions. And the solution became clear when Glu/Ca²⁺≥0.68, so1.3Glumolecules could combine2calcium ions. Therefore, the order of the prohibition ability wasGlu> Asp> citrate.The study showed that biomolecules could regulate the type of the formed calciumphosphate. In the case of citrate, the type depended on the concentration ratio of citrate andcalcium. When Cit/Ca²⁺≤0.2and0.2＜Cit/Ca²⁺＜1.6, DCPD and HA was formed,respectively. For acidic amino acid, the solution was acid and in favor for the formation ofDCPD. So Asp and Glu primarily led to the formation of DCPD. For Lys and BSA, the typedepended on the amount of the biomolecule and the concentrations of Ca²⁺and PO43-. Themain products were DCPD and HA. While in acid condition, the metastable DCPD wouldreadily turn into HA under the regulation of Lys. The results also revealed the significant difference in morphologies of the formedcalcium phosphates regulated by different biomolecules. This meant that there was aninteraction between biomolecule and calcium. This interaction could change the relativegrowth rate of each crystal face and thus regulate the morphology and size of calciumphosphate. When the concentration of calcium was0.15M and Ca/P was1.67, differentDCPDs were formed under different biomolecules. Citrate could obtain a needle-like DCPD,and the length increased（from3μm to6μm） with the concentration of citrate （from0.0083Mto0.0332M）. For Asp, the morphology would turn from irregular sheet into diamond whilethe concentration of Asp increased from0.00376M to0.094M. These diamonds packedtogether and formed a stereoscopic structure. While the amount of BSA increased from0.2gto1.5g, a stereoscopic structure would turn into regular sheet. The DCPD in Lys wasirregular and had a rough surface, whereas the DCPD in control （without any biomolecule）was sheet-like and had a length-diameter ratio. As to Glu, the morphology of DCPD dependedon the concentration ratio of Glu and calcium. When Glu/Ca²⁺＜0.136, aggregate of sheetswas formed. A stereoscopic structure was obtained while the Glu/Ca²⁺was around0.136.When the concentration increased further, the stereoscopic structure turned into sheets again.When the formed calcium phosphate was HA, aggregate of small sheets was generated in thecontrol while aggregate of fine particles in the biomolecules.