Facet-Selective Templating of Hydroxyapatite by Bioinspired Polycarboxylate Peptides: Distinguishing (001) Hydroxyapatite Stabilization from Octacalcium Phosphate Promotion

The selective stabilization of hydroxyapatite (HAP) crystal facets by acidic biomolecules plays a central role in biomineralization, yet the mechanistic distinction between facet-specific HAP stabilization and promotion of metastable calcium phosphate phases remains unclear. Here, we systematically examine HAP crystallization in the presence of polycarboxylate ligands─EDTA, citrate, polyglutamate (E6 and E13), and the dentin-derived (DSS)4 repeat─to determine how carboxylate spacing and ligand architecture influence calcium phosphate phase selection and morphology. Scanning electron microscopy, powder X-ray diffraction, and multinuclear solid-state NMR reveal that EDTA, citrate, and polyglutamate favor the formation of sheet-like HAP enriched in (001) surface exposure, whereas (DSS)4 promotes ribbon-like nanocrystals dominated by octacalcium phosphate (OCP). Hydrothermal treatment converts OCP-containing products to phase-pure HAP, enabling direct comparison of ligand-dependent defect structures. 1H MAS NMR identifies hydroxylated surface calcium sites across all samples and detects hydrogen phosphate and crystalline water environments diagnostic of OCP in (DSS)4-templated materials, while 31P CP-MAS spectra confirm complete OCP-to-HAP transformation after hydrothermal conversion. These results support a geometric recognition model in which short-spaced carboxylates preferentially stabilize the HAP (001) surface, whereas extended anionic spacing favors OCP-related lattice arrangements. This work establishes design principles for peptide-mediated control of calcium phosphate crystal growth and facet expression.