Theoretical analysis of protein effects on calcium phosphate precipitation in simulated body fluid

Abstract

The effects of protein on the nucleation and growth rate of calcium phosphate precipitation, including hydroxyapatite (HA) and octacalcium phosphate (OCP) precipitation, in simulated body fluid were analyzed based on the classical nucleation and diffusion-controlled growth theory. To obtain quantitative results for analysis, a model system of bovine serum albumin and lysozyme added to revised simulated body fluid was studied. The parameters, such as the concentration of protein binding to calcium and phosphorous ions and the viscosity of the revised simulated body fluid with protein additions, were experimentally measured to assist quantitative analysis. The analysis focused on the effects of protein on calcium phosphate nucleation including changes in the ion concentration, interfacial energies and pH values of the simulated body fluid system. The protein effects on calcium phosphate growth were also analyzed based on a diffusion-controlled growth model. The analysis results indicate that proteins present in simulated body fluid act to increase the free energy of nucleation and reduce the nucleation rates by reducing the effective level of supersaturation in simulated body fluid. The proteins also affect calcium phosphate nucleation by changing the interfacial energies in simulated body fluid. However, the analysis indicates that the effects of protein on the interfacial energies could be to either promote or retard the nucleation. The protein effects on the nucleation induced by altering the pH value of the simulated body fluid can be neglected. The analysis also indicates that protein addition in revised simulated body fluid does not change the basic thermodynamic nature of HA and OCP formation; that is, HA precipitation is more thermodynamically favourable than OCP while OCP precipitation is more kinetically favourable than HA. Proteins presenting in simulated body fluid always exhibit an inhibitory effect on the growth rate of calcium phosphate precipitates because proteins increase the diffusion resistance of calcium and phosphate ions by increasing the revised simulated body fluid viscosity.