A High-Resolution ⁴³Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

High resolution 43Ca solid-state NMR studies of hydroxyapatite (Ca10(PO4)6(OH)2) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using 1H-43Ca rotational echo double resonance and 1H-43Ca cross polarization magic angle spinning experiments

Importance of Water in Maintaining Softwood Secondary Cell Wall Nanostructure

Water is one of the principal constituents by mass of living plant cell walls. However, its role and interactions with secondary cell wall polysaccharides and the impact of dehydration and subsequent rehydration on the molecular architecture are still to be elucidated. This work combines multidimensional solid-state 13C magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) with molecular dynamics modeling to decipher the role of water in the molecular architecture of softwood secondary cell walls. The proximities between all main polymers, their molecular conformations, and interaction energies are compared in never-dried, oven-dried, and rehydrated states. Water is shown to play a critical role at the hemicellulose–cellulose interface. After significant molecular shrinkage caused by dehydration, the original molecular conformation is not fully recovered after rehydration. The changes include xylan becoming more closely and irreversibly associated with cellulose and some mannan becoming more mobile and changing conformation. These irreversible nanostructural changes provide a basis for explaining and improving the properties of wood-based materials

Probing the Molecular Architecture of <i>Arabidopsis thaliana</i> Secondary Cell Walls Using Two- and Three-Dimensional ¹³C Solid State Nuclear Magnetic Resonance Spectroscopy

The plant secondary cell wall is a thickened polysaccharide and phenolic structure, providing mechanical strength to cells, particularly in woody tissues. It is the main feedstock for the developing bioenergy and green chemistry industries. Despite the role that molecular architecture (the arrangement of biopolymers relative to each other, and their conformations) plays in dictating biomass properties, such as recalcitrance to breakdown, it is poorly understood. Here, unprocessed dry ¹³C-labeled stems from the model plant <i>Arabidopsis thaliana</i> were analyzed by a variety of ¹³C solid state magic angle spinning nuclear magnetic resonance methods, such as one-dimensional cross-polarization and direct polarization, two-dimensional refocused INADEQUATE, RFDR, PDSD, and three-dimensional DARR, demonstrating their viability for the study of native polymer arrangements in intact secondary cell walls. All carbon sites of the two main glucose environments in cellulose (previously assigned to microfibril surface and interior residues) are clearly resolved, as are carbon sites of the other major components of the secondary cell wall: xylan and lignin. The xylan carbon 4 chemical shift is markedly different from that reported previously for solution or primary cell wall xylan, indicating significant changes in the helical conformation in these dried stems. Furthermore, the shift span indicates that xylan adopts a wide range of conformations in this material, with very little in the 3₁ conformation typical of xylan in solution. Additionally, spatial connections of noncarbohydrate species were observed with both cellulose peaks conventionally assigned as “surface” and as “interior” cellulose environments, raising questions about the origin of these two cellulose signals

Theoretical Investigation of Oxygen-17 NMR Shielding and Electric Field Gradients in Glutamic Acid Polymorphs

We present an assignment of the experimental 17O NMR shielding parameters for l-glutamic acid·HCl (Lemaitre, V.; Pike, K. J.; Watts, A.; Anupold, T.; Samoson, A.; Smith, M. E.; Dupree, R. Chem. Phys. Lett. 2003, 371, 91) based on first-principles quantum mechanical calculations. We use density functional theory and the gauge-including projector-augmented wave method (Pickard, C. J.; Mauri, F. Phys. Rev. B 2001, 63, 245101), which treats the true periodic nature of the crystal structure. We perform further theoretical calculations on a range of glutamic acid polymorphs and draw general conclusions about the influence of hydrogen bonding on 17O NMR shielding parameters

Solid-State ¹⁷O NMR as a Probe for Structural Studies of Proteins in Biomembranes

We report the first example of 17O NMR spectra from a selectively labeled transmembrane peptide, 17O-[Ala12]-WALP23, as a lyophilized powder and incorporated in hydrated phospholipid vesicles. It is shown that at high magnetic field it is feasible to apply 17O NMR to the study of membrane-incorporated peptides. Furthermore, we were able to estimate distances within the selectively labeled WALP peptide, which represents a consensus transmembrane protein sequence. This work opens up new applications of 17O solid-state NMR on biological systems

New Limits for Solid-State ¹⁷O NMR Spectroscopy: Complete Resolution of Multiple Oxygen Sites in a Simple Biomolecule

A solid-state 17O NMR 1H-decoupled double angle rotation (DOR) study of monosodium l-glutamate monohydrate (l-MSG) is reported. It is shown that all eight inequivalent sites can be resolved with DOR line widths (∼65 Hz) ∼120 times narrower than those in the MAS spectrum. The lines are tentatively assigned on the basis of their behavior under proton decoupling and the isotropic chemical shift and the quadrupole interaction parameter for each extracted by a combination of DOR and 3Q MAS at variable magnetic fields. With a shift range of ∼45 ppm for these similar oxygen sites and spectral resolution under DOR comparable to that for spin-1/2 nuclei, solid-state 17O NMR should have tremendous potential in the study of biomolecules

Experimental and Theoretical ¹⁷O NMR Study of the Influence of Hydrogen-Bonding on CO and O−H Oxygens in Carboxylic Solids

A systematic solid-state 17O NMR study of a series of carboxylic compounds, maleic acid, chloromaleic acid, KH maleate, KH chloromaleate, K2 chloromaleate, and LiH phthalate·MeOH, is reported. Magic-angle spinning (MAS), triple-quantum (3Q) MAS, and double angle rotation (DOR) 17O NMR spectra were recorded at high magnetic fields (14.1 and 18.8 T). 17O MAS NMR for metal-free carboxylic acids and metal-containing carboxylic salts show featured spectra and demonstrate that this combined, where necessary, with DOR and 3QMAS, can yield site-specific information for samples containing multiple oxygen sites. In addition to 17O NMR spectroscopy, extensive quantum mechanical calculations were carried out to explore the influence of hydrogen bonding at these oxygen sites. B3LYP/6-311G++(d,p) calculations of 17O NMR parameters yielded good agreement with the experimental values. Linear correlations are observed between the calculated 17O NMR parameters and the hydrogen bond strengths, suggesting the possibility of estimating H-bonding information from 17O NMR data. The calculations also revealed intermolecular H-bond effects on the 17O NMR shielding tensors. It is found that the δ11 and δ22 components of the chemical shift tensor at O−H and CO, respectively, are aligned nearly parallel with the strong H-bond and shift away from this direction as the H-bond interaction weakens

Probing Heteronuclear ¹⁵N−¹⁷O and ¹³C−¹⁷O Connectivities and Proximities by Solid-State NMR Spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing 15N−17O dipolar and J couplings are presented for [2H(NH3),1-13C,15N,17O2]glycine·2HCl and [15N2,17O2]uracil. Two-dimensional 15N−17O correlation spectra are obtained using the R3-HMQC experiment; for glycine·2HCl, the intensity of the resolved peaks for the CO and C−O2H 17O resonances corresponds to the relative magnitude of the respective 15N−17O dipolar couplings. 17O−15N REDOR curves are presented for glycine·2HCl; fits of the initial buildup (ΔS/S 15N−17O REAPDOR curves for the 15N resonances in glycine·2HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear 13C−17O and 15N−17O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin−echo experiment, SQ(τ) = SHET(τ)/SHOM(τ). For glycine·2HCl, 1JCO was determined as 24.7 ± 0.2 and 25.3 ± 0.3 Hz for the CO and C−O2H resonances, respectively, while for uracil, the average of the two NH···O hydrogen-bond-mediated J couplings was determined as 5.1 ± 0.6 Hz. In addition, two-bond intramolecular J couplings, 2JOO = 8.8 ± 0.9 Hz and 2JN1,N3 = 2.7 ± 0.1 Hz, were determined for glycine·2HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine·HCl [1JCO(CO) = 24.9 Hz, 1JCO(COH) = 27.5 Hz, 2JOO = 7.9 Hz] and uracil [2hJN1,O4 = 6.1 Hz, 2hJN3,O4 = 4.6 Hz, 2JN1,N3 = 2.7 Hz]

Amyloid Hydrogen Bonding Polymorphism Evaluated by ¹⁵N{¹⁷O}REAPDOR Solid-State NMR and Ultra-High Resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

A combined approach, using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and solid-state NMR (Nuclear Magnetic Resonance), shows a high degree of polymorphism exhibited by Aβ species in forming hydrogen-bonded networks. Two Alzheimer’s Aβ peptides, Ac-Aβ_16–22-NH₂ and Aβ_11–25, selectively labeled with ¹⁷O and ¹⁵N at specific amino acid residues were investigated. The total amount of peptides labeled with ¹⁷O as measured by FTICR-MS enabled the interpretation of dephasing observed in ¹⁵N­{¹⁷O}­REAPDOR solid-state NMR experiments. Specifically, about one-third of the Aβ peptides were found to be involved in the formation of a specific >C¹⁷O···H–¹⁵N hydrogen bond with their neighbor peptide molecules, and we hypothesize that the rest of the molecules undergo ± <i>n</i> off-registry shifts in their hydrogen bonding networks

Probing Heteronuclear ¹⁵N−¹⁷O and ¹³C−¹⁷O Connectivities and Proximities by Solid-State NMR Spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing 15N−17O dipolar and J couplings are presented for [2H(NH3),1-13C,15N,17O2]glycine·2HCl and [15N2,17O2]uracil. Two-dimensional 15N−17O correlation spectra are obtained using the R3-HMQC experiment; for glycine·2HCl, the intensity of the resolved peaks for the CO and C−O2H 17O resonances corresponds to the relative magnitude of the respective 15N−17O dipolar couplings. 17O−15N REDOR curves are presented for glycine·2HCl; fits of the initial buildup (ΔS/S 15N−17O REAPDOR curves for the 15N resonances in glycine·2HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear 13C−17O and 15N−17O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin−echo experiment, SQ(τ) = SHET(τ)/SHOM(τ). For glycine·2HCl, 1JCO was determined as 24.7 ± 0.2 and 25.3 ± 0.3 Hz for the CO and C−O2H resonances, respectively, while for uracil, the average of the two NH···O hydrogen-bond-mediated J couplings was determined as 5.1 ± 0.6 Hz. In addition, two-bond intramolecular J couplings, 2JOO = 8.8 ± 0.9 Hz and 2JN1,N3 = 2.7 ± 0.1 Hz, were determined for glycine·2HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine·HCl [1JCO(CO) = 24.9 Hz, 1JCO(COH) = 27.5 Hz, 2JOO = 7.9 Hz] and uracil [2hJN1,O4 = 6.1 Hz, 2hJN3,O4 = 4.6 Hz, 2JN1,N3 = 2.7 Hz]