Citrate Key in Bone's Nanostructure

Photo: Image of bone structure

This diagram shows the effect of
citrate concentration on the size
of hydroxyapatite crystals fabri-
cated with self-assembling block
copolymer templates. ;
© U.S. Dept. of Energy's Ames

Using nuclear magnetic resonance (NMR) spectroscopy, Ames Laboratory scientist and Iowa State University chemistry professor Klaus Schmidt-Rohr and his colleagues studied bone, an organic-inorganic nanocomposite whose stiffness is provided by thin nanocrystals of carbonated apatite, a calcium phosphate, imbedded in an organic matrix of mostly collagen, a fibrous protein.

"The organic, collagen matrix is what makes bones tough," Schmidt-Rohr said, "while the inorganic apatite nanocrystals provide the stiffness. And the small thickness – about 3 nanometers – of these nanocrystals appears to provide favorable mechanical properties, primarily in prevention of crack propagation."

"We can see all the peaks clearly," he says of a spectral graph which shows the points at which specific components in bone samples resonate; these specific signatures are the key to NMR technology, "even those at the organic-inorganic interface, where the organic material's signal strength is relatively weak."

"We had gotten some crystalline collagen samples to study," he said, "and it turned out that the supplier, Sigma-Aldrich, had used citrate to dissolve the collagen. And the citrate signature in the collagen samples matched the signature we were seeing in bone."

The case for citrate was made most convincingly when graduate research assistant Yanyan Hu was able to extract citrate from cow bone and replace it with carbon 13 (C13) -enriched citrate, resulting in a 30-fold enhancement of the NMR signals of the bone sample. The peaks matched exactly, confirming the presence of citrate on the surface where the apatite nanocrystals had formed.

"Based on the old literature, we looked at the citrate levels in a variety of types of bone and found that herring spine had the highest citrate concentration – about 13 percent by weight," Schmidt-Rohr said. "So it should hold that the citrate signal for herring spine should be three times higher than for cow bone, and indeed it was."; Source: DOE/Ames Laboratory