Abstract
While obtaining high-resolution structural details from bone is highly important to better understand its mechanical strength and the effects of aging and disease on bone ultrastructure, it has been a major challenge to do so with existing biophysical techniques. Though solid-state NMR spectroscopy has the potential to reveal the structural details of bone, it suffers from poor spectral resolution and sensitivity. Nonetheless, recent developments in magic angle spinning (MAS) NMR technology have made it possible to spin solid samples up to 110 kHz frequency. With such remarkable capabilities, 1H-detected NMR experiments that have traditionally been challenging on rigid solids can now be implemented. Here, we report the first application of multidimensional 1H-detected NMR measurements on bone under ultrafast MAS conditions to provide atomistic-level elucidation of the complex heterogeneous structure of bone. Our investigations demonstrate that two-dimensional 1H/1H chemical shift correlation spectra for bone are obtainable using fp-RFDR (finite-pulse radio-frequency-driven dipolar recoupling) pulse sequence under ultrafast MAS. Our results infer that water exhibits distinct 1H−1H dipolar coupling networks with the backbone and side-chain regions in collagen. These results show the promising potential of proton-detected ultrafast MAS NMR for monitoring structural and dynamic changes caused by mechanical loading and disease in bone.
Highlights
Over the past years, there has been growing interest in obtaining molecular-level structural and dynamic information on the various constituents of bone
We demonstrate the use of 2D 1H/1H fp-RFDR31,32 based chemical shift correlation experiments under 100 kHz magic angle spinning (MAS) to investigate the atomic-level interactions that exist among the various constituents of bone, and to further understand the role played by water in the stabilization and self-assembly of collagen triple-helical conformation as well as its impact on the nucleation and distribution of the bone mineral nanoparticles within the organic matrix
Bone water is found in different forms with various binding conditions: free water filling the bulk of the microscopic pores in the calcified matrix, water bound to organic matrix mainly in the collagen network and organic-mineral interface, and water associated with the mineral phase[43,44,45]
Summary
There has been growing interest in obtaining molecular-level structural and dynamic information on the various constituents of bone. Despite that very fast MAS frequencies can cause a reduction in the spin diffusion rate among protons in a solid framework, the recoupling of the 1H–1H dipolar interactions by a rotor-synchronized π -pulse train during the mixing time of the fp-RFDR pulse sequence renders the 1H–1H spin diffusion processes rapid and effective even in the ultrafast MAS regime[14,31,32,33,34] In this context, recent studies have demonstrated the feasibility of these experiments under ultrafast MAS on small organic solids as well as under slow MAS on model membrane-bound peptides and proteins[15,23,25,26,27,35,36,37,38,39], which are typically mobile solids with relatively weak proton-proton dipolar couplings
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