Hydroxyapatite nanocrystals modified with acidic amino acids

HA-aspartic acid (HA-ASP) and HA-glutamic acid (HA-GLU) composite nanocrystals were synthesized in aqueous medium. The relative amino acid content, determined through HPLC analysis, increases with the amino acid concentration in solution up to a maximum of about 7.8 wt.-% for HA-ASP and 4.3 wt.-% for HA-GLU composites. The dimensions of the composite nanocrystals decrease on increasing the amino acid content. The line profile analysis of the X-ray diffraction reflections was performed using: (i) the Scherrer method, (ii) the Warren-Averbach approach, and (iii) the Rietveld refinement. The results indicate that the broadening of the diffraction peaks is partly due to the decrease of crystallite size on increasing amino acid content, and even more to the simultaneous increase of microstrain. The different extent of structural disorder induced by the two amino acids, in agreement with their different incorporation, suggests a greater affinity of aspartic acid for hydroxyapatite structure.