Tuesday, June 03, 2008

Chem. Mater., 20 (11), 3549–3550, 2008.

The Organic−Mineral Interface in Teeth Is Like That in Bone and Dominated by Polysaccharides: Universal Mediators of Normal Calcium Phosphate Biomineralization in Vertebrates?

David G. Reid, Melinda J. Duer,* Rachel C. Murray, and Erica R. Wise

Teeth of mammals and many other vertebrates are made up of the three distinct substances, enamel, dentin, and cement.1 Cement is in many respects modified bone; it occurs as a thin film attached to the outside of dentin and holds the collagen fibers of the periodontal ligament. Enamel provides the hard surface of teeth and is about 97% apatitic mineral. Dentin, the major constituent, is a less mineralized material, extremely tough and impact resistant. The mineral is apatite-like and is bound into an organic matrix in which collagen is prominent. As in bone, the relationship between these two phases must be fundamental for toughness and hardness. We have shown that the solid-state NMR (SSNMR) technique called rotational echo double resonance (REDOR)2 is a powerful tool for studying such interfaces in calcium phosphate biominerals.3 This is because practically all the phosphorus is confined to the mineral and all the carbon to the organic phases, so the 13C{31P} REDOR experiment, which restores the through-space dipole−dipole coupling between nuclei of the two elements, is highly selective for the biomolecules at the boundary layer. Using this approach, we have shown that the molecules in bone most strongly associated with the interface are sugars, probably acidic glycosaminoglycans (GAGs), and not collagenous or other proteins as is widely assumed.4 Their prominence at the boundary argues an important role in controlling the formation and properties of the composite material, perhaps by directing calcium phosphate solidification and preventing inappropriate overcrystallization. Here, we report that the organic−mineral interface in teeth is very similar to that in bone and also dominated by polysaccharides.

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