Temperature-induced mobility in octacalcium phosphate impacts crystal symmetry: water dynamics studied by NMR crystallography

• Published in: Faraday discussions Vol. 255; pp. 451 - 482
• Main Authors: Nelson, Adam, Papawassiliou, Wassilios, Paul, Subhradip, Hediger, Sabine, Hung, Ivan, Gan, Zhehong, Venkatesh, Amrit, Franks, W. Trent Trent, Smith, Mark E, Gajan, David, De Paëpe, Gaël, Bonhomme, Christian, Laurencin, Danielle, Gervais, Christel
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 08.01.2025
• Subjects: Ambient temperature; Biocompatibility; Calcium isotopes; Calcium phosphates; Chemical Sciences; Crystallography; Material chemistry; Materials substitution; Molecular dynamics; NMR; Nuclear magnetic resonance; Room temperature; Symmetry; Water chemistry; NMR; DNP
• ISSN: 1359-6640; 1364-5498; 1364-5498
• DOI: 10.1039/d4fd00108g

Octacalcium phosphate (OCP, Ca 8 (PO 4 ) 4 (HPO 4 ) 2 ·5H 2 O) is a notable calcium phosphate due to its biocompatibility, making it a widely studied material for bone substitution. It is known to be a precursor of bone mineral, but its role in biomineralisation remains unclear. While the structure of OCP has been the subject of thorough investigations (including using Rietveld refinements of X-ray diffraction data, and NMR crystallography studies), important questions regarding the symmetry and H-bonding network in the material remain. In this study, it is shown that OCP undergoes a lowering of symmetry below 200 K, evidenced by 1 H, 17 O, 31 P and 43 Ca solid-state NMR experiments. Using ab initio molecular-dynamics (MD) simulations and gauge including projected augmented wave (GIPAW) DFT calculations of NMR parameters, the presence of rapid motions of the water molecules in the crystal cell at room temperature is proved. This information leads to an improved description of the OCP structure at both low and ambient temperatures, and helps explain long-standing issues of symmetry. Remaining challenges related to the understanding of the structure of OCP are then discussed. Octacalcium phosphate (OCP) exhibits water mobility, evidenced via variable-temperature NMR. Molecular-dynamics simulations then offer a new description of OCP symmetry.