Composites composed of microcrystalline calcium-deficient hydroxyapatite (HAp) and collagen were formed at 38 degrees C via an acid-base reaction between calcium phosphate precursors in the presence of a collagen matrix. Formation of composites having HAp:collagen weight ratios of 4.5:1, 11:1, and 22:1, along with that of pure mineral were investigated. Isothermal calorimetry and X-ray diffraction indicated complete reaction within 5 h resulting in hardened monoliths. The rate of HAp formation increased with an increase in the proportion of collagen present. Electron microscopy indicated that the acceleratory effect of collagen was associated with the provision of nucleation sites for HAp crystallization. Analysis of the solution chemistry also showed that collagen affected the calcium and phosphate concentrations and the pH. While collagen was shown to effect the kinetics of HAp formation, the rate limiting step, as shown by X-ray diffraction and solution chemistry, was the dissolution of the acidic calcium phosphate precursor, CaHPO4. Preliminary mechanical data indicated that the Young's modulus, yield strength, and work to fracture were at the lower end of the range of those values reported for bone. The porosities observed in these composites suggest that they might be osteoinductive while their compositions should allow their eventual resorption. Thus, microstructure, kinetics, and mechanical data suggest that these composites might be suitable as bone substitutes which form in vivo.

Macromolecular crystallographic refinement relies on sometimes dubious stereochemical restraints and rudimentary energy functionals to ensure the correct geometry of the model of the macromolecule and any covalently bound ligand(s). The ligand stereochemical restraint file (CIF) requires a priori understanding of the ligand geometry within the active site, and creation of the CIF is often an error-prone process owing to the great variety of potential ligand chemistry and structure. Stereochemical restraints have been replaced with more robust functionals through the integration of the linear-scaling, semiempirical quantum-mechanics (SE-QM) program DivCon with the PHENIX X-ray refinement engine. The PHENIX/DivCon package has been thoroughly validated on a population of 50 protein-ligand Protein Data Bank (PDB) structures with a range of resolutions and chemistry. The PDB structures used for the validation were originally refined utilizing various refinement packages and were published within the past five years. PHENIX/DivCon does not utilize CIF(s), link restraints and other parameters for refinement and hence it does not make as many a priori assumptions about the model. Across the entire population, the method results in reasonable ligand geometries and low ligand strains, even when the original refinement exhibited difficulties, indicating that PHENIX/DivCon is applicable to both single-structure and high-throughput crystallography.

A mechanism for the formation of hydroxyapatite (HAp) surface layers compositionally different from the bulk compositions is described. Surface layer formation is a consequence of HAp being a compound of variable composition: Ca 10 (PO 4 ) 6 (OH) 2 to ∼Ca 9 HPO 4 (PO 4 ) 5 OH, existing over Ca/P ratio from 1.67 for stoichiometric to ∼1.5 for fully calcium-deficient HAp. One consequence of this compositional variability is that only one HAp composition (Ca/P ratio) dissolves congruently (the solution and solid have the same Ca/P ratio). HAp compositions having Ca/P ratios different from the congruently dissolving composition will form surface layers. Equilibration of stoichiometric HAp with water will result in the formation of a surface layer having a Ca/P ratio of less than 1.67. In general, for solid HAp compositions having Ca/P ratios greater than that of the congruently dissolving composition, a nonstoichiometric surface layer will form and the Ca/P ratio in solution will exceed that of the surface layer. For solid compositions having lower Ca/P ratios than the congruently dissolving composition, the opposite will be true.

The mechanical properties of monoliths of calcium-deficient and carbonated hydroxyapatite formed by dissolution-precipitation reactions at 38 °C have been determined. Particulate solid reactants were mixed at liquid-to-solid weight ratios of 0.11 and 0.2 and pressed into various configurations on which mechanical tests were carried out. Testing was performed on wet specimens which had been allowed to react for 8 h by which time phase-pure hydroxyapatite had formed. Calcium-deficient hydroxyapatite produced at a liquid-to-solids ratio of 0.11 exhibited a tensile strength as high as 18 MPa, an average compressive strength of 174 MPa and a Young's modulus of 6 GPa. These values were lower when a larger proportion of water (liquid-to-solid 0.2) was used in sample preparation. However, the compressive strengths of calcium-deficient hydroxyapatite prepared at 38 °C are comparable to the compressive strengths of sintered hydroxyapatite containing an equivalent total porosity. Carbonated hydroxyapatite showed mechanical properties inferior to those exhibited by calcium-deficient material. These differences appear to be related to the microstructural variations between these compositions.

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