Unraveling Water-Mediated 31P Relaxation in Bone Mineral.

Abstract

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 31P ssNMR relaxation parameters (T1 and T2) and dynamics (correlation time). The method was developed to selectively measure the 31P NMR relaxation parameters and dynamics of the crystalline apatitic core and the amorphous surface layer of the bone mineral. The measured 31P 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.