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.
Highlights
Bone is a complex hydrated organic-inorganic composite material
Empirical formulae were calculated as described in Materials and Methods and were in excellent agreement with the data of Markovic et al [18] (OCP-SUC Ca8(PO4)4(HPO4)1.080.91 ⋅ 5–6H2O; Octacalcium phosphate (OCP)-ADI Ca8(PO4)4(HPO4)1.271.1 ⋅ 5–6H2O Elemental compositions confirm that OCP-SUC and OCP-ADI both have two acid molecules per unit cell i.e. per sixteen calcium atoms
OCP-SUC and OCP-ADI should have very similar structures, with adipates substituting the P5 phosphate group [7] and oriented analogously to the succinates in OCP-SUC. 31P spectra of different batches of both composites are very similar to each other, and to 31P data already published for OCP [20,22] and OCP-SUC [10] (Fig. 1) including 2D SQ-DQ 31P – 31P (Fig. S2) and 1H–31P HETCOR (Fig. S3) correlation experiments
Summary
Bone is a complex hydrated organic-inorganic composite material. The networks of organic matrix, mineral, and hydrating water, are impossible to isolate without disruption of their respective structures. It is well established that citrate is frequently a significant component of mammalian bone mineral [2,3,4], and other small metabolites such as lactate can be, and frequently are, incorporated [5]. It is conceivable that different bone samples may have different metabolite incorporations in the mineral, which could contribute to variations in material and chemical properties such as hardness, solubility, and re-absorption. Well chosen model calcium phosphate – small molecule composites, which can be synthesized with well defined homogeneous compositions in sufficient quantities for detailed structural analysis, can help elucidate how the incorporation of organic metabolites may affect biomineral structure and physical properties
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