NMR crystallography, which encompasses first-principles calculations, solid-state Nuclear Magnetic Resonance (NMR) spectroscopy and diffraction techniques, is a comparatively new approach to the study of crystalline materials. Previously we have used NMR crystallography to re-examine the structure of a biologically-important calcium phosphate phase, octacalcium phosphate (OCP), which allowed us to reassign the experimental 31P solid-state NMR spectrum and to identify the presence of an extended hydrogen-bonding network that we argued was critical to the structural stability of the material.1
Continuing with this approach, we now propose a structure for the octacalcium phosphate-citrate inclusion complex (OCP-CIT). This material is formed when citrate molecules are trapped within the water channels of OCP, and while citrate is known to comprise ca. 1 wt% of bone,2 the mode of its inclusion and the role that it plays have not been unambiguously determined. We have found that the citrate anion in OCP-CIT bridges the apatitic layers in a disordered fashion, and therefore propose this organic-inorganic composite material as a model for the incorporation of citrate into bone mineral, where it would act to prevent the formation of large, well-ordered mineral crystals, and may actively control the degree of disorder in bone mineral. In order to assess the relevance of OCP-CIT to bone mineral, we present for the first time 17O NMR data on bone, and compare with 17O NMR data obtained for octacalcium phosphate-citrate and other biologically-relevant phases of calcium phosphate.