Natural-abundance 43Ca solid-state NMR spectroscopy of bone

Jiadi Xu, Peizhi Zhu, Zhehong Gan, Nadder Sahar, Mary Tecklenburg, Michael D. Morris, David H. Kohn, Ayyalusamy Ramamoorthy

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


Structural information about the coordination environment of calcium present in bone is highly valuable in understanding the role of calcium in bone formation, biomineralization, and bone diseases like osteoporosis. While a high-resolution structural study on bone has been considered to be extremely challenging, NMR studies on model compounds and bone minerals have provided valuable insight into the structure of bone. Particularly, the recent demonstration of 43Ca solid-state NMR experiments on model compounds is an important advance in this field. However, application of 43Ca NMR is hampered due to the low natural-abundance and poor sensitivity of 43Ca. In this study, we report the first demonstration of natural-abundance 43Ca magic angle spinning (MAS) NMR experiments on bone, using powdered bovine cortical bone samples. 43Ca NMR spectra of bovine cortical bone are analyzed by comparing to the natural-abundance 43Ca NMR spectra of model compounds including hydroxyapatite and carbonated apatite. While 43Ca NMR spectra of hydroxyapatite and carbonated apatite are very similar, they significantly differ from those of cortical bone. Raman spectroscopy shows that the calcium environment in bone is more similar to carbonated apatite than hydroxyapatite. A close analysis of 43Ca NMR spectra reveals that the chemical shift frequencies of cortical bone and 10% carbonated apatite are similar but the quadrupole coupling constant of cortical bone is larger than that measured for model compounds. In addition, our results suggest that an increase in the carbonate concentration decreases the observed 43Ca chemical shift frequency. A comparison of experimentally obtained 43Ca MAS spectra with simulations reveal a 3:4 mol ratio of Ca-I/Ca-II sites in carbonated apatite and a 2.3:3 mol ratio for hydroxyapatite. 2D triple-quantum 43Ca MAS experiments performed on a mixture of carbonated apatite and the bone protein osteocalcin reveal the presence of protein-bound and free calcium sites, which is in agreement with a model developed from X-ray crystal structure of the protein.

Original languageEnglish
Pages (from-to)11504-11509
Number of pages6
JournalJournal of the American Chemical Society
Issue number33
StatePublished - Aug 25 2010


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