Solid state NMR - An indispensable tool in organic-inorganic biocomposite characterization; refining the structure of octacalcium phosphate composites with the linear metabolic di-acids succinate and adipate

Abstract

Octacalcium phosphate (OCP; Ca₈(HPO₄)₂(PO₄)₄. 5H₂O) is a plausible precursor phase of biological hydroxyapatite, which composites with a number of biologically relevant organic metabolites. Widely used material science physicochemical structure determination techniques successfully characterize the mineral component of these composites but leave details of the structure, and interactions with mineral, of the organic component almost completely obscure. The metabolic linear di-acids succinate (SUC) and adipate (ADI) differentially expand the hydrated (100) layer of OCP. ¹³C¹³C correlation (proton driven spin diffusion, PDSD) experiments on OCP composited with (U-¹³C₄)-SUC, and (U¹³C₆)-ADI, show that the two di-acids per unit cell adopt non-centrosymmetric but mutually identical structures. ¹³C{³¹P}, rotational echo double resonance (REDOR) shows that one end of each linear di-acid is displaced further from the surface of the apatitic OCP layer relative to the other end. Overall the results indicate two di-acids per unit cell disposed perpendicularly across the OCP hydrated layer with one carboxylate of each di-acid substituting a hydrated surface OCP phosphate group. This study re-affirms the unique advantages of ssNMR in elucidating structural details of organic-inorganic biocomposites, and thereby mechanisms underlying the roles of small metabolites in influencing biomineralization mechanisms and outcomes.

Keywords: Adipate; Biocomposite; Biomineralization; Hydroxyapatite precursor; OCP; Octacalcium phosphate; PDSD; REDOR; Succinate.