We will apply solid-state magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) to measure changes in the chemical structure of bone tissue under mechanical load. MAS-NMR will allow us to examine ultrastructure contributions to bone quality that have, to date, been unobtainable using traditional imaging techniques or other biophysical techniques. The enabling advance is a special NMR cell that allows compressive loading and displacement measurements. High-resolution structural information will be correlated to strain of bone tissue from mature murine tibiae as the specimens are deformed under uniaxial compression.
In Aim 1 we will assess distortion of the bone mineral lattice as changes in the mineral ion spacings and water mobility using a several different 1D and 2D solid-state NMR pulse sequences. We will also study load- induced changes in a series of synthesized carbonated apatites, which are model compounds for bone mineral to guide interpretation of our bone mineral results and to enable measurements with isotopically enriched nuclei, especially 43Ca. In these simplified model compounds we can accurately measure inter-ion distances, movement of mineral impurities, and reorganization of local symmetry and resolve uncertainties in the measurements of the more complex biomineral.
In Aim 2 we will investigate the role of water in stabilizing both the mineral and matrix components via hydrogen bonding and enthalpic stabilization. Changes in collagen secondary structure will be measured through 13C resonances. We will use a unique protocol for controlled disordering of the collagen and mineral by partial and complete displacement of native matrix water with deuterium oxide, which forms weaker hydrogen bonds. NMR will probe water bridges and glycine-proline bonds, as well as mineral changes, with correlations to measured strain and applied load.
In Aim 3 we will examine the loading response of mineral deformation, collagen conformation changes and hydrogen bonding in native and damaged bone tissue from mice of various ages in order to understand how age-related changes in the mineral and matrix compromise the mechanical competence of bone tissue. Controlled partial replacement of hydrogen ion with deuterium ion will be used to provide a range of collagen conformation changes.
Fragility fractures are a major health threat in aging populations and understanding the factors that contribute to bone failure is vitally important to the development of new interventional approaches. Through the development of methods for measuring NMR spectra of bone under compressive load, this project, for the first time, brings to this problem the power of solid-state nuclear magnetic resonance spectroscopy to understand the changes in bone structure when subjected to mechanical loading.
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