Monitoring Dehydration-Driven Motional Dynamics in Cortical Bone by Employing Solid-State NMR Methodologies

This study investigates the impact of dehydration on the molecular structure and spin dynamics of both the organic protein matrix and the inorganic hydroxyapatite components of bone. A comprehensive comparison between natural and dehydrated bovine cortical bones is conducted to elucidate the role of water in maintaining bone structure and dynamics. The organic protein components and inorganic hydroxyapatite are monitored separately using a combination of solid-state NMR techniques: wPMLG-detected ¹H MAS, ¹³C CP-MAS, ¹³C 2D PASS CP-MASS, ¹H-¹³C HETCOR, and ¹³C relaxometry for the organic (mainly type I collagens) and ³¹P MAS, ³¹P 2D PASS, and ³¹P relaxometry for hydroxyapatite. Our findings highlight a significant and previously overlooked effect of dehydration on the inorganic hydroxyapatite components of bone. Dehydration induces more pronounced changes in the spinning CSA sideband pattern of ³¹P nuclei residing on the inorganic hydroxyapatite components rather than the ¹³C spinning CSA sideband pattern of the organic matrix. There is no substantial change in the ¹³C chemical shift anisotropy (CSA) parameter, which suggests that dehydration does not lead to the decomposition of the organic matrix. The spin-lattice relaxation time is getting elongated in the dehydrated bone compared to the intact bone, suggesting that the motional degrees of freedom of the collagenous matrix are being affected due to the removal of free water from the surface of collagen. The remarkable alteration of the motional dynamics of ³¹P nuclei is demonstrated by quantitative measurements of the ³¹P spin-lattice relaxation time. The T₁ values increase by approximately 80% for the amorphous surface layer and by 40% for the crystalline apatite core upon dehydration. These experimental findings suggest that ³¹P 2D PASS NMR and ³¹P spin-lattice relaxation measurements serve as highly sensitive indicators of structural and dynamical changes in the hydroxyapatite phase, with potential implications for the early detection of pathological conditions such as osteoporosis. Collectively, this work addresses critical gaps in understanding how dehydration influences the molecular structure and nuclear spin dynamics in organic protein components and inorganic hydroxyapatite components of bone. Additionally, this detailed information about the local electronic environment and nuclear spin dynamics of carbon and phosphorus nuclei will illuminate the path to designing biomimetic bone-like materials.