Citrate bridges between mineral platelets in bone.
We provide evidence that citrate anions bridge between mineral platelets in bone and hypothesize that their presence acts to maintain separate platelets with disordered regions between them rather than gradual transformations into larger, more ordered blocks of mineral. To assess this hypothesis, we take as a model for a citrate bridging between layers of calcium phosphate mineral a double salt octacalcium phosphate citrate (OCP-citrate). We use a combination of multinuclear solid-state NMR spectroscopy, powder X-ray diffraction, and first principles electronic structure calculations to propose a quantitative structure for this material, in which citrate anions reside in a hydrated layer, bridging between apatitic layers. To assess the relevance of such a structure in native bone mineral, we present for the first time, to our knowledge, (17)O NMR data on bone and compare them with (17)O NMR data for OCP-citrate and other calcium phosphate minerals relevant to bone. The proposed structural model that we deduce from this work for bone mineral is a layered structure with thin apatitic platelets sandwiched between OCP-citrate-like hydrated layers. Such a structure can explain a number of known structural features of bone mineral: the thin, plate-like morphology of mature bone mineral crystals, the presence of significant quantities of strongly bound water molecules, and the relatively high concentration of hydrogen phosphate as well as the maintenance of a disordered region between mineral platelets.
Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom.
Erika
E
Davies
Karin H
KH
Müller
Wai Ching
WC
Wong
Chris J
CJ
Pickard
David G
DG
Reid
Jeremy N
JN
Skepper
Melinda J
MJ
Duer
D000818:Animals; D001842:Bone and Bones / Q000378:metabolism*; D002130:Calcium Phosphates / Q000378:metabolism; D019343:Citric Acid / Q000378:metabolism*; D006736:Horses; D009682:Magnetic Resonance Spectroscopy; D008903:Minerals / Q000378:metabolism*; D033362:Powder Diffraction; D011817:Rabbits
D002130:Calcium Phosphates; D008903:Minerals; C022045:octacalcium phosphate; D019343:Citric Acid
NMR crystallography; biomineralization