posters 5th Asia-Pacific NMR Symposium 2013

Citrate Provides Essential Disorder for Bone Mineral (#115)

Erika Davies 1 , Karin H Muller 2 , Chris J Pickard 3 , David G Reid 4 , Jeremy N Skepper 2 , Melinda J Duer 4
  1. UNSW Mark Wainwright Analytical Centre, Kensington, NSW, Australia
  2. Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, United Kingdom
  3. Department of Physics & Astronomy, University College London, London, United Kingdom
  4. Department of Chemistry, University of Cambridge, Cambridge, United Kingdom

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.

  1. Davies, E.; Duer, M. J.; Ashbrook, S. E.; Griffin, J. M. J. Am. Chem. Soc. 2012, 134, 12508-12515.
  2. Hu, Y.-Y.; Rawal, A.; Schmidt-Rohr, K. P. Natl. Acad. Sci. U.S.A. 2010, 107, 22425-22429.