Recent directions in the solid-state NMR study of synthetic and natural calcium phosphates

Abstract

Materials containing a calcium phosphate component have been the subject of much interest to NMR spectroscopists, especially in view of understanding the structure and properties of mineralized tissues like bone and teeth, and of developing synthetic biomaterials for bone regeneration. Here, we present a selection of recent developments in their structural characterization using advanced solid state NMR experiments, highlighting the level of insight which can now be accessed. 1 H, 31 P, 43 Ca, 17 O  Ultra-high magnetic fields  Dynamic Nuclear Polarization  Micro-imaging under MAS  Computational modeling  Isotopic labeling Monetite CaHPO 4 Octacalcium-phosphate (OCP) Ca 8 (HPO 4) 2 (PO 4) 4 .5H 2 O t-Ca 2 (P 2 O 7).2H 2 O m-Ca 2 (P 2 O 7).4H 2 O Apatitic-like layer Hydrated layer Apatitic-like layer Examples of Calcium-orthophosphates Examples of Calcium-pyrophosphates

Solid-state NMR spectroscopy

“…Whereas most studies exploit 1 H, 13 C and 31 P NMR spectroscopy, 43 Ca and 17 O NMR experiments are becoming increasingly used 241 . Recent work performing ex vivo microimaging of an intact mouse tooth under MAS was able to selectively identify and locate the mineral and organic components with high spatial resolution (~100 μm) 275 .…”

A 43Ca nuclear magnetic resonance perspective on octacalcium phosphate and its hybrid derivatives

“…Numerous 1 H, 31 P, and 13 C NMR studies related to natural and biomimetic calcium phosphate phases have been published. These include 13 C- 31 P rotational echo double resonance (REDOR) [25][26][27][28][29] to study the interfaces between the mineral part of bone (crystalline hydroxyapatiterelated and non-apatitic, amorphous components) and the organic part (collagen and other organic molecules). Within the topic of the organic-inorganic interface in the bone, octacalcium phosphate (OCP), Ca 8 (HPO 4 ) 2 (PO 4 ) 4 .5H 2 O, occupies a very special position.…”

Pathological calcifications in the human joint

“…The hydrated surface layer responsible for the strong surface reactivity of nanocrystalline apatites (ageing/maturation, ionic exchange, adsorption) is the most interesting structural feature but also the most complex to characterize. The latter can often be achieved with a combination of methods including chemical titrations and spectroscopic techniques such as vibrational spectroscopies (FTIR and Raman) and solid state nuclear magnetic resonance (NMR) [119,120]. The model of apatite nanocrystals based on an apatitic core and a more or less structured surface hydrated layer including non-apatitic domains is accepted but the precise description of the organization within this hydrated layer is still discussed.…”