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

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.