Small molecule-mediated control of hydroxyapatite growth: free energy calculations benchmarked to density functional theory.

The unique, plate-like morphology of hydroxyapatite (HAP) nanocrystals in bone lends to the hierarchical structure and functions of bone. Proteins enriched in phosphoserine (Ser-OPO3) and glutamic acid (Glu) residues have been proposed to regulate crystal morphology; however, the atomic-level mechanisms remain unclear. Previous molecular dynamics studies addressing biomineralization have used force fields with limited benchmarking, especially at the water/mineral interface, and often limited sampling for the binding free energy profile. Here, we use the umbrella sampling/weighted histogram analysis method to obtain the adsorption free energy of Ser-OPO3 and Glu on HAP (100) and (001) surfaces to understand organic-mediated crystal growth. The calculated organic-water-mineral interfacial energies are carefully benchmarked to density functional theory calculations, with explicit inclusion of solvating water molecules around the adsorbate plus the Poisson-Boltzmann continuum model for long-range solvation effects. Both amino acids adsorb more strongly on the HAP (100) face than the (001) face. Growth rate along the [100] direction should then be slower than in the [001] direction, resulting in plate-like crystal morphology with greater surface area for the (100) than the (001) face, consistent with bone HAP crystal morphology. Thus, even small molecules are capable of regulating bone crystal growth by preferential adsorption in specific directions. Furthermore, Ser-OPO3 is a more effective growth modifier by adsorbing more strongly than Glu on the (100) face, providing one possible explanation for the energetically expensive process of phosphorylation of some proteins involved in bone biomineralization. The current results have broader implications for designing routes for biomimetic crystal synthesis.