Molecular Orbital Study of Apatite Nucleation at Silica Bioceramic Surfaces

Abstract

AbstractWe have used Hartree-Fock molecular orbital calculations to model the interactions of Ca2+, H2PO41-, HPO42- and H2O with bioceramic reactive surface sites represented by [Si3O6H6] to determine the reaction sequence for apatite precipitation at bioceramic surfaces. By comparing predicted reaction energies, viberational frequencies, abd NMR shifts for 29Si, and 31P with experimental values we were able to identify stable intermediates during reaction progress.At the pH of blood, our calculations predict that (i) the 3-ring is the active surface site, (ii) formation of [SiO-Ca-OPO3H] bonds is energetically preferred over direct Si-O-P bonds, and (iii) an acidic precursor with bidentate Ca>OPO3H bonds, [Si3O6H5CaHPO4(H2O)3]1-, nucleates rapidly (minutes to 1 hour) at the bioceramic surface. Predicted precursor complex, [Si3O6H5CaHPO4(H2O)3]1-, and young bone share unique IR bands at 631 and 1125–1145 cm-1. Going beyond the scope of our calculations, a literature survey suggests that subsequent slow (1–2 weeks) aggregation of oligomers by H-bonds between surface Si-OH and HPO4 groups results in apatite formation. Analogously, carboxyl and phosphoryl groups on sialoprotein and hydroxyl-terminations on collagen may provide the nucleation and H-bonding sites for natural bone formation in the absence of the bioceramic.