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Modeling apatite nucleation in the human body and in the geochemical environment

Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison 53706; sahai{at}geology.wisc.edu

This paper serves to show that under similar physicochemical conditions, universal chemical reaction pathways can be involved in the interactions of fluids, biomolecules and minerals, whether in the human body or in the geochemical environment. The concept is illustrated by comparing heterogeneous apatite nucleation at bone sialoprotein (BSP) surfaces, bioceramic implant surfaces and on marine, diatomaceous sediments where phosphorite deposits are formed. The present approach combines crystallographic considerations, experimental NMR data and ab initio molecular orbital calculations of NMR parameters to elucidate reactive site geometry, mineral nucleation and inhibition reaction pathways. This technique may be applicable in future investigations of biomineralization mechanisms.

The peptide sequence, S(P)EE, is proposed to constitute an active site on BSP, where the three acidic groups are arranged at 60° from each other. The equilateral triangle provides a stereochemical match for Ca²⁺ on the (001) face of apatite. The analogous active site on silicate bioceramics is the cyclic silicate trimer or three-ring containing surface silanol groups. One possible reaction pathway for calcium phosphate (CaP) nucleation consistent with experimental ³¹P NMR data involves the following steps: Ca²⁺ sorption at the active site and HPO₄^2– attachment resulting in the critical nucleus, followed by nucleus growth and phase transformation to apatite. The present ab initio calculations cannot provide information on which CaP phase is nucleated, so we assume that the CaP nucleus is amorphous. Magnesium inhibits nucleation by adsorbing faster than calcium, as an outer-sphere surface complex, at the active site.