After studying bone structure over a five-year period, it was actually serendipitous that Schmidt-Rohr came across a signature that appeared to match what he was seeing.
"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."
According to Schmidt-Rohr, the role of citrate in bone had been studied up until about 1975, but since that time, no mention was made in any of the newer literature on bone. So in essence, his research team had to rediscover it.
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.
Schmidt-Rohr further hypothesized that, since citrate is too large to be incorporated into the apatite crystal lattice, it must be bound to the nanocrystals' surface where it stabilizes the nanocrystals' size by preventing their further growth. The findings were published in the Dec. 28, 2010 issue of the Proceedings of the National Academy of Sciences.
"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."
In further studies, the group found that higher concentration of citrate, the thinner the apatite nanocrystals in bone. This was further confirmed on bone-mimetic nanocomposites in a collaborat
|Contact: Kerry Gibson|