(708h) A Polarizable Force Field of Inorganic Phosphates and Hydroxyapatite Based on the Classical Drude Oscillator

Calcium phosphate (CaP) is a ubiquitous mineral with a major contribution to the formation of human bone and tooth enamel. The biomedical applications of calcium phosphate include reconstructive surgeries and porous scaffolds for tissue engineering. Despite its fundamental significance, the early-stage nucleation mechanism of CaP particles from a saturated solution remains speculative because the critical nanoscale details pertaining to the nanometer-sized “pre-nucleation” clusters remain inaccessible to characterization techniques. In this contribution, we develop a polarizable force field of phosphate ions, in various protonation states, and hydroxyapatite (HAP) using the classical Drude oscillator model. The force filed reproduces the key properties of ions in solution as well as the properties of the HAP crystal. The quantum mechanical electrostatic potential maps, atomic partial charges, molecular polarizabilities, and molecular dipoles are well reproduced. The experimental properties of the solvated ions and HAP crystals including solvation free energy of ions, lattice constants and density of crystal, cleavage energies, and mechanical properties of HAP are reproduced in agreement with experiments. The force filed is further applied to study nucleation process of calcium phosphate particles. The results show instantaneous growth of branched polymeric assemblies prior to the formation of spherical particles.