Unraveling Water-Mediated 31 P Relaxation in Bone Mineral.

Bone is a dynamic tissue composed of organic proteins (mainly type I collagen), inorganic components (hydroxyapatite), lipids, and water that undergoes a continuous rebuilding process over the lifespan of human beings. Bone mineral is mainly composed of a crystalline apatitic core surrounded by an amorphous surface layer. The supramolecular arrangement of different constituents gives rise to its unique mechanical properties, which become altered in various bone-related disease conditions. Many of the interactions among the different components are poorly understood. Recently, solid-state nuclear magnetic resonance (ssNMR) has become a popular spectroscopic tool for studying bone. In this article, we present a study probing the interaction of water molecules with amorphous and crystalline parts of the bone mineral through 31 P ssNMR relaxation parameters ( T 1 and T 2 ) and dynamics (correlation time). The method was developed to selectively measure the 31 P NMR relaxation parameters and dynamics of the crystalline apatitic core and the amorphous surface layer of the bone mineral. The measured 31 P correlation times (in the range of 10 -6 -10 -7 s) indicated the different dynamic behaviors of both the mineral components. Additionally, we observed that dehydration affected the apatitic core region more significantly, while H-D exchange showed changes in the amorphous surface layer to a greater extent. Overall, the present work provides a significant understanding of the relaxation and dynamics of bone mineral components inside the bone matrix.