Changes to the Disordered Phase and Apatite Crystallite Morphology during Mineralization by an Acidic Mineral Binding Peptide from Osteonectin

Noncollagenous proteins regulate the formation of the mineral constituent in hard tissue. The mineral formed contains apatite crystals coated by a functional disordered calcium phosphate phase. Although the crystalline phase of bone mineral was extensively investigated, little is known about the disordered layer’s composition and structure, and less is known regarding the function of noncollagenous proteins in the context of this layer. In the current study, apatite was prepared with an acidic peptide (ON29) derived from the bone/dentin protein osteonectin. The mineral formed comprises needle-shaped hydroxyapatite crystals like in dentin and a stable disordered phase coating the apatitic crystals as shown using X-ray diffraction, transmission electron microscopy, and solid-state NMR techniques. The peptide, embedded between the mineral particles, reduces the overall phosphate content in the mineral formed as inferred from inductively coupled plasma and elemental analysis results. Magnetization transfers between disordered phase species and apatitic phase species are observed for the first time using 2D ¹H–³¹P heteronuclear correlation NMR measurements. The dynamics of phosphate magnetization transfers reveal that ON29 decreases significantly the amount of water molecules in the disordered phase and increases slightly their content at the ordered-disordered interface. The peptide decreases hydroxyl to disordered phosphate transfers within the surface layer but does not influence transfer within the bulk crystalline mineral. Overall, these results indicate that control of crystallite morphology and properties of the inorganic component in hard tissue by biomolecules is more involved than just direct interaction between protein functional groups and mineral crystal faces. Subtler mechanisms such as modulation of the disordered phase composition and structural changes at the ordered–disordered interface may be involved

Interfacial Mineral–Peptide Properties of a Mineral Binding Peptide from Osteonectin and Bone-like Apatite

Osteonectin is a regulator of bone mineralization. It interacts specifically with collagen and apatite through its N-terminal domain, inhibiting crystal growth. In this work, we investigated the interface formed between the mineral and an acidic peptide, ON29, derived from the protein’s apatite binding domain. The structural properties of the peptide bound to the mineral and the mineral–peptide interface are characterized using NMR and computational methods. A biomaterial complex is formed by precipitation of the mineral in the presence of the acidic peptide. The peptide gets embedded between mineral particles, which comprise a disordered hydrated coat covering apatite-like crystals. ³¹P SEDRA measurements show that the peptide does not affect the overall proximity between phosphate ions in the mineral. {¹⁵N}¹³C REDOR measurements reveal an α-turn in the center of the free peptide, which is unchanged when it is bound to the mineral. {³¹P}¹³C REDOR and ¹H–¹³C HETCOR measurements show that Glu/Asp carboxylates and Thr/Ala/Val side chains from ON29 are proximate to phosphate and hydroxyl groups in the mineral phases. Predictions of ON29’s fold on and off hydroxyapatite crystal faces using ROSETTA-surface are used to model the molecular conformation of the peptide and its apatite-binding interface. The models constructed without bias from experimental results are consistent with NMR measurements and map out extensively the residues forming an interface with apatitic crystals

Ammonia Treatment of 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂ Material: Insights from Solid-State NMR Analysis

Li-rich cathode materials of the formula <i>x</i>Li₂MnO₃·<i>y</i>LiNi_<i>a</i>Co_<i>b</i>Mn_<i>c</i>O₂ (<i>x</i> + <i>y</i> = 1, <i>a</i> + <i>b</i> + <i>c</i> = 1) boast very high discharge capacity, ca. 250 mAh/g. Yet, they suffer capacity decrease and average voltage fade during cycling in Li-ion batteries that prohibit their commercialization. Treatment of the materials with NH₃(g) at high temperatures produces improved electrodes with higher stability of capacity and average voltage. The present study follows the changes occurring in the materials upon treatment with ammonia gas, through ⁶Li and ⁷Li solid-state NMR investigations of the untreated and ammonia treated 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂ as well as its constituent phases, Li₂MnO₃ and LiNi_0.4Co_0.2Mn_0.4O₂. The NMR analysis demonstrates the biphasic nature of these materials. Furthermore, it shows that the Li₂MnO₃ component phase in the integrated material is the phase mostly being affected by the gas treatment. A thickening of a protective surface film in the integrated material, with the right exposure time to the reactive gas, is observed, which further precludes Ni leach out from the bulk and leads to improved electrode performance. Formation of minor electrochemically inactive oxide phases in the integrated material and similarly in the Li₂MnO₃ alone upon longer exposure to the gas suggests that the performance deterioration observed can be linked to the rearrangement of ions in the Li₂MnO₃ constituent phase in the integrated material