Structural Characterization of Mg-Stabilized Amorphous Calcium Carbonate by Mg-25 Solid-State NMR Spectroscopy

Biogenic amorphous calcium carbonates (ACCs) play a crucial role in the mineralization process of calcareous tissue. Most biogenic ACCs contain Mg ions, but the coordination environment of Mg, which may influence the kinetics of the phase transformation of an ACC, remains poorly understood. We demonstrate that Mg-25 solid-state NMR can be used to probe the coordination shells of Mg in synthetic ACCs. The variation in Mg-25 chemical shifts suggests that Mg–O bond lengths increase as Mg content increases. On the basis of the Van Vleck second moments obtained from the double-resonance NMR experiments, we infer that the average number of carbonates surrounding the central Mg ion is in the range of 4–4.5 and that there is at least one water molecule coordinated to each Mg ion for the synthetic Mg-ACC samples. We suggest that the stability of Mg-ACC is owing to the structural water bound to Mg ions, which increases considerably the activation energy associated with the dehydration of Mg-ACC

Preparation and Structural Characterization of Free-Standing Octacalcium-Phosphate-Rich Thin Films

Free-standing films of calcium phosphates exhibit many favorable properties for tissue engineering. In this work, a thin film of calcium phosphate is prepared in a liposome suspension using the method of ammonia gas diffusion. The thickness of the film is about 10 μm, and the lateral dimensions are on the length scale of millimeter. The results of powder X-ray diffraction and transmission electron microscopy show that the thin films contain the mineral phases of hydroxyapatite and octacalcium phosphate (OCP). Using solid-state NMR spectroscopy, in particular the technique of heteronuclear correlation spectroscopy with variable contact time, the major crystalline phase of the thin film has been confirmed to be OCP

Characterization of the Crystallization Pathway of Calcium Phosphate in Liposomes

Electron microscopy is currently the most powerful method to discern the mechanisms of solid-state transformation and dissolution-reprecipitation for the studies of biomineralization. In this work, we show that solid-state NMR spectroscopy can serve as a useful complementary technique to characterize the crystallization pathway of a mineral phase. On the basis of the so-called NMR spin-diffusion method, direct evidence is given to support that the formation of the apatite phase within liposomes occurs via the solid-state transformation of the disordered phase. In this thermodynamically downhill process, the final step is the depletion of the structural water in the disordered phase, whose structural order of the phosphorus species is comparable to that of apatite

Calcium-43 NMR Studies of Polymorphic Transition of Calcite to Aragonite

Phase transformation between calcite and aragonite is an important issue in biomineralization. To shed more light on the mechanism of this process at the molecular level, we employ solid-state ⁴³Ca NMR to study the phase transformation from calcite to aragonite as regulated by magnesium ions, with ⁴³Ca enrichment at a level of 6%. Using the gas diffusion approach, the phase of Mg-calcite is formed initially and the system subsequently transforms to aragonite as the reaction time proceeds. Our ⁴³Ca solid-state NMR data support the dissolution-recrystallization mechanism for the calcite to aragonite transition. We find that the ⁴³Ca NMR parameters of Mg-calcite are very similar to those of pure calcite. Under the high-resolution condition provided by magic-angle spinning at 4 kHz, we can monitor the variation of the ⁴³Ca NMR parameters of the aragonite signals for the samples obtained at different reaction times. Our data suggest that in the presence of a significant amount of Mg²⁺ ions, aragonite is the most stable polymorph of calcium carbonate. The initial precipitated crystallites of aragonite have spine-like morphology, for which the ⁴³Ca spin–lattice relaxation data indicate that the ions in the lattice have considerable motional dynamics. As the crystallinity of aragonite improves further, the ⁴³Ca <i>T</i>₁ parameter of the aragonite phase changes considerably and becomes very similar to that obtained for pure aragonite. For the first time, the difference in crystal morphologies and crystallinity of the aragonite phase has been traced down to the subtle difference in the motional dynamics at the molecular level

Capturing the Local Adsorption Structures of Carbon Dioxide in Polyamine-Impregnated Mesoporous Silica Adsorbents

Interactions between amines and carbon dioxide (CO₂) are essential to amine-functionalized solid adsorbents for carbon capture, and an in-depth knowledge of these interactions is crucial to adsorbent design and fabrication as well as adsorption/desorption processes. The local structures of CO₂ adsorbed on a tetraethylenepentamine-impregnated mesoporous silica SBA-15 were investigated by solid-state ¹³C­{¹⁴N} S-RESPDOR MAS NMR technique and theoretical DFT calculations. Two types of adsorption species, namely, secondary and tertiary carbamates as well as distant ammonium groups were identified together with their relative concentrations and relevant ¹⁴N quadrupolar parameters. Moreover, a dipolar coupling of 716 Hz was derived, corresponding to a ¹³C–¹⁴N internuclear distance of 1.45 Å. These experimental data are in excellent agreement with results obtained from DFT calculations, revealing that the distribution of surface primary and secondary amines readily dictates the CO₂ adsorption/desorption properties of the adsorbent

Unraveling the Structure of Magic-Size (CdSe)₁₃ Cluster Pairs

Cadmium selenide is a II–VI semiconductor model system known for its nanoparticle preparation, growth mechanism, luminescence properties, and quantum confinement studies. For the past 2 decades, various thermodynamically stable “magic-size nanoclusters (MSCs)” of CdSe have been observed, isolated, and theoretically calculated. Nevertheless, none of the proposed structures were experimentally confirmed due to the small crystal domains beyond the diffraction limit. With a combination of nondestructive SAXS, WAXS, XRD, XPS, EXAFS, and MAS NMR techniques, we were able to verify the phase transformation, shape, size dimension, local bonding, and chemical environments of (CdSe)₁₃ nanoclusters, which are indicative of a paired cluster model. These experimental results are consistent with the size, shape, bond lengths, dipole moment, and charge densities of the proposed “paired-tubular geometry” predicted by computational approaches. In this article, we revisit the formation pathway of the mysterious (CdSe)₁₃ nanoclusters and propose a paired cluster structure model for better understanding of II–VI semiconductor nanoclusters

Effect of Charged Amino Acid Side Chain Length on Lateral Cross-Strand Interactions between Carboxylate-Containing Residues and Lysine Analogues in a β‑Hairpin

β-Sheets are one of the fundamental three-dimensional building blocks for protein structures. Oppositely charged amino acids are frequently observed directly across one another in antiparallel sheet structures, suggesting the importance of cross-strand ion pairing interactions. Despite the apparent electrostatic nature of ion pairing interactions, the charged amino acids Asp, Glu, Arg, Lys have different numbers of hydrophobic methylenes linking the charged functionality to the backbone. Accordingly, the effect of charged amino acid side chain length on cross-strand ion pairing interactions at lateral non-hydrogen bonded positions was investigated in a β-hairpin motif. The negatively charged residues with a carboxylate (Asp, Glu, Aad in increasing length) were incorporated at position 4, and the positively charged residues with an ammonium (Dap, Dab, Orn, Lys in increasing length) were incorporated at position 9. The fraction folded population and folding free energy were derived from the chemical shift deviation data. Double mutant cycle analysis was used to determine the interaction energy for the potential lateral ion pairs. Only the Asp/Glu-Dap interactions with shorter side chains and the Aad-Orn/Lys interactions with longer side chains exhibited stabilizing energetics, mostly relying on electrostatics and hydrophobics, respectively. This suggested the need for length matching of the interacting residues to stabilize the β-hairpin motif. A survey of a nonredundant protein structure database revealed that the statistical sheet pair propensity followed the trend Asp-Lys < Glu-Lys, also implying the need for length matching of the oppositely charged residues

Effect of Charged Amino Acid Side Chain Length at Non-Hydrogen Bonded Strand Positions on β‑Hairpin Stability

β-Sheets have been implicated in various neurological disorders, and ∼20% of protein residues adopt a sheet conformation. Therefore, studies on the structural origin of sheet stability can provide fundamental knowledge with potential biomedical applications. Oppositely charged amino acids are frequently observed across one another in antiparallel β-sheets. Interestingly, the side chains of natural charged amino acids Asp, Glu, Arg, Lys have different numbers of hydrophobic methylenes linking the backbone to the hydrophilic charged functionalities. To explore the inherent effect of charged amino acid side chain length on antiparallel sheets, the stability of a designed hairpin motif containing charged amino acids with varying side chain lengths at non-hydrogen bonded positions was studied. Peptides with the guest position on the N-terminal strand and the C-terminal strand were investigated by NMR methods. The charged amino acids (Xaa) included negatively charged residues with a carboxylate group (Asp, Glu, Aad in increasing length), positively charged residues with an ammonium group (Dap, Dab, Orn, Lys in increasing length), and positively charged residues with a guanidinium group (Agp, Agb, Arg, Agh in increasing length). The fraction folded and folding free energy for each peptide were derived from the chemical shift deviation data. The stability of the peptides with the charged residues at the N-terminal guest position followed the trends: Asp > Glu > Aad, Dap < Dab < Orn ∼ Lys, and Agb < Arg < Agh < Agp. The stability of the peptides with the charged residues at the C-terminal guest position followed the trends: Asp < Glu < Aad, Dap ∼ Dab < Orn ∼ Lys, and Agb < Arg ∼ Agp < Agh. These trends were rationalized by thermodynamic sheet propensity and cross-strand interactions