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.

Theoretical study of bone sialoprotein in bone biomineralization.

Bone sialoprotein (BSP) is an acidic, non-collagenous protein specific to bone proposed previously to promote hydroxyapatite (HAP) nucleation and modulate HAP nanocrystal growth. Specifically, two phosphorylated acidic amino acid sequences in BSP, highly conserved across several vertebrates, are the proposed active sites. We selected one of these sites, i.e. (Sp)(2)E(8), where Sp represents a phosphoserine as a model peptide to study the role of BSP. We used molecular dynamics simulations to determine whether an α-helix or a random coil peptide conformation promotes templated HAP nucleation. A bioinformatics method helps infer preferential crystal growth directions by predicting the likely peptide conformations adsorbed on the (001), (100), and (110) crystal faces of HAP. Results suggest that, independent of conformation, no stable nucleating template is formed and, thus, the ion distributions in the vicinity of the peptide that eventually lead to a stable nucleus start out with disordered arrangements of ions. When adsorbed on all three faces, the Sp residues bind strongly regardless of the peptide conformation, and the Glu residues show different propensities to form helical conformations. The lack of geometrical templating between the peptide residues and all HAP surface sites indicates that adsorption and subsequent crystal growth modulation may be structurally nonspecific.