A systematic study of all substitutions in biological apatite has been investigated and discussed using NMR and DFT calculations. The lowest energy is found for system containing grouped associations of four carbonate groups, substituting four consecutive phosphates, organized in zigzag fashion which confirms the tendency to carbonate clustering. The multiple-B substitutions, mono B-substitutions and A type substitution were also compared. With this set of models, 1 H, 13 C and 31 P chemical shifts observed experimentally in the synthetic CHAp sample were fairly well reproduced.
Abstract
Biological apatites (main constituent of natural bones) correspond to non-stoichiometric hydroxyapatite HAp, presenting a large variety of ions as substituents (CO3 2−, F−, SiO4 4−, Mg2+, Na+…). The precise location and configuration of ionic substitutes in the HAp matrix are generally difficult to identify and characterize. This contribution details the structural characterization based on NMR data of a particular case of hydroxyapatite substitution by carbonates. For this purpose, all substitution mechanisms proposed to our knowledge in the literature are modeled by DFT and the corresponding calculated NMR parameters allowed to propose or confirm some interpretations of a certain number of experimental observations to rationalize the dependencies of the 13C chemical shift and energy on these structural parameters. The presented results open the way for a fast interpretation of 13C NMR experiments on defective HAp materials and will allow to predict the most stable arrangement of CO3 2− for a given family of defects.
