Double-quantum Solid-state NMR Research Articles

Using Dynamic Bonds to Enhance the Mechanical Performance: From Microscopic Molecular Interactions to Macroscopic Properties

Polymeric materials combining good mechanical performances with self-healing ability and malleability have attracted dramatic attention, but it presently remains a challenge for the facile fabrication of such high-performance materials, not to mention the atomic-level characterization for understanding the molecular origin of the macroscopic properties. Herein, we proposed a facile strategy to fabricate a dual-cross-linked poly(n-butyl acrylate) polymer material, in which the self-complementary quadruple hydrogen bonding interactions between 2-ureido-4[1H]-pyrimidinone (UPy) dimers were utilized as the dynamic sacrificial cross-linkages, and thus to enhance the mechanical strength and toughness. The hydrogen bonding interactions between UPy dimers in such synthetic cross-linked polymer material were revealed in detail by selective saturation double-quantum (DQ) solid-state NMR spectroscopy under ultrafast magic-angle-spinning beyond 60 kHz. In the meantime, the self-healing capability and recyclability we...

Efficient 2D double-quantum solid-state NMR spectroscopy with large spectral widths.

2D double-quantum single-quantum correlation spectra with arbitrary spectral widths can be recorded with SR26 and related supercycled recoupling sequences when applying Supercycle-Timing-Compensation (STiC) phase shifts. This concept widely extends the applicability of supercycled sequences, most importantly for obtaining long-range distance constraints for structure determination with solid-state NMR.

Exploring Network Formation of Tough and Biocompatible Thiol‐yne Based Photopolymers

This work deals with the in-depth investigation of thiol-yne based network formation and its effect on thermomechanical properties and impact strength. The results show that the bifunctional alkyne monomer di(but-1-yne-4-yl)carbonate (DBC) provides significantly lower cytotoxicity than the comparable acrylate, 1,4-butanediol diacrylate (BDA). Real-time near infrared photorheology measurements reveal that gel formation is shifted to higher conversions for DBC/thiol resins leading to lower shrinkage stress and higher overall monomer conversion than BDA. Glass transition temperature (Tg ), shrinkage stress, as well as network density determined by double quantum solid state NMR, increase proportionally with the thiol functionality. Most importantly, highly cross-linked DBC/dipentaerythritol hexa(3-mercaptopropionate) networks (Tg ≈ 61 °C) provide a 5.3 times higher impact strength than BDA, which is explained by the unique network homogeneity of thiol-yne photopolymers.

β-Barrel Mobility Underlies Closure of the Voltage-Dependent Anion Channel

The voltage-dependent anion channel (VDAC) is themajor protein in the outer mitochondrial membrane, where it mediates transport of ATP and ADP. Changes in its permeability, induced by voltage or apoptosis-related proteins, have been implicated in apoptotic pathways. The three-dimensional structure of VDAC has recently been determined asa 19-stranded β-barrel with an in-lying N-terminal helix. However, its gating mechanism is still unclear. Using solid-state NMR spectroscopy, molecular dynamics simulations, and electrophysiology, we show that deletion of the rigid N-terminal helix sharply increases overall motion in VDAC's β-barrel, resulting in elliptic, semicollapsed barrel shapes. These states quantitatively reproduce conductance and selectivity of the closed VDAC conformation. Mutation of the N-terminal helix leads to a phenotype intermediate to the open and closed states. These data suggest that the N-terminal helix controls entry into elliptic β-barrel states which underlie VDAC closure. Our results also indicate that β-barrel channels are intrinsically flexible.

A large geometric distortion in the first photointermediate of rhodopsin, determined by double-quantum solid-state NMR

Double-quantum magic-angle-spinning NMR experiments were performed on 11,12-(13)C(2)-retinylidene-rhodopsin under illumination at low temperature, in order to characterize torsional angle changes at the C11-C12 photoisomerization site. The sample was illuminated in the NMR rotor at low temperature (~120 K) in order to trap the primary photointermediate, bathorhodopsin. The NMR data are consistent with a strong torsional twist of the HCCH moiety at the isomerization site. Although the HCCH torsional twist was determined to be at least 40°, it was not possible to quantify it more closely. The presence of a strong twist is in agreement with previous Raman observations. The energetic implications of this geometric distortion are discussed.

Probing Intermolecular Hydrogen Bonding in Sibenadet Hydrochloride Polymorphs by High-Resolution 1H Double-Quantum Solid-State NMR Spectroscopy

Molecular packing in two polymorphs of sibenadet hydrochloride (AR-C68397AA, Viozan™) is investigated using a combined experimental (1) H double-quantum (DQ) solid-state magic-angle spinning nuclear magnetic resonance and computational (gauge including projected augmented wave calculation of chemical shifts) approach. For Form I, NH-NH and NH-OH (1) H DQ peaks are observed corresponding to nearest distances of 2.62 and 2.87 Å, respectively, for the intermolecular hydrogen-bonding arrangement in the single-crystal X-ray diffraction structure. The same (1) H DQ peaks at the same (1) H chemical shifts are observed for Form II, for which there is no single-crystal diffraction structure, indicating the same intermolecular hydrogen-bonding arrangement of the benzothiazolone moieties as in Form I. (1) H DQ build-up (as a function of the DQ recoupling time) curves are presented for the resolved NH-NH and NH-OH DQ peaks for the two polymorphs. For Form I, the ratio of the maximum intensity for the NH-OH and NH-NH DQ peaks is in excellent agreement with the ratio of the summed squares of the H-H dipolar couplings, as determined using H-H distances from the crystal structure up to 4 Å. Small differences in the (1) H DQ build-up behaviour for the two polymorphs are attributed to differences in the longer-range NH-OH distances associated with different inter-layer arrangements.

Light Penetration and Photoisomerization in Rhodopsin studied by Numerical Simulations and Double-Quantum Solid-State NMR Spectroscopy

The penetration of light into optically thick samples containing the G-protein-coupled receptor rhodopsin is studied by numerical finite-element simulations and double-quantum solid-state NMR experiments. Illumination with white light leads to the generation of the active bathorhodopsin photostate in the outer layer of the sample but generates a large amount of the side product, isorhodopsin, in the sample interior. The overall yield of bathorhodopsin is improved by using monochromatic 420 nm illumination and by mixing the sample with transparent glass beads. The implications of these findings on the interpretation of previously published rhodopsin NMR data are discussed.

Structure of Tetrakis(melaminium) Bis(dihydrogenphosphate) Monohydrogenphosphate Trihydrate from X-ray Powder Diffraction and Solid-State NMR Spectroscopy

The crystal structure of the melamine phosphate salt tetrakis(melaminium) bis(dihydrogenphosphate) monohydrogenphosphate trihydrate with as much as ten independent moieties in the unit cell was determined by a direct-space global optimization technique from X-ray powder diffraction data using additional geometry constraints obtained by 31P double-quantum solid-state NMR spectroscopy. The structure analysis of the compound and its comparison with other melamine phosphates reveals the packing and bonding characteristics that are important for melamine-phosphate salts with a melamine-to-phosphor ratio larger than one, which are promising environmental-friendly flame retardants.

31P double-quantum solid-state NMR study of phosphoroorganic compounds with (O)P–O–P-(O), (S)P–O–P(S) and (S)P–S–P(O) unit

In this work we have tested applicability of the commonly used double quantum recoupling sequence POST-C7 to study of 31P– 31P geometrical constraints for phosphoroorganic model compounds with different chemical shift anisotropy (CSA) and distinct molecular dynamics in the crystal lattice. Our results clearly show that even with large CSA, POST-C7 gives good efficiency of 31P double-quantum excitations. Moreover, large amplitude molecular motion only slightly disturb 31P build-up curve. χ 2 error analysis is used for verification of values and orientations of chemical shift tensors (CST) parameters employed for simulation of POST-C7 buildup curves.

Accurate Measurements of13C−13CJ-Couplings in the Rhodopsin Chromophore by Double-Quantum Solid-State NMR Spectroscopy

A new double-quantum solid-state NMR pulse sequence is presented and used to measure one-bond 13C-13C J-couplings in a set of 13C2-labeled rhodopsin isotopomers. The measured J-couplings reveal a perturbation of the electronic structure at the terminus of the conjugated chain but show no evidence for protein-induced electronic perturbation near the C11-C12 isomerization site. This work establishes NMR methodology for measuring accurate 1JCC values in noncrystalline macromolecules and shows that the measured J-couplings may reveal local electronic perturbations of mechanistic significance.

Protein-induced bonding perturbation of the rhodopsin chromophore detected by double-quantum solid-state NMR.

We have obtained carbon-carbon bond length data for the functional retinylidene chromophore of rhodopsin, with a spatial resolution of 3 pm. The very high resolution was obtained by performing double-quantum solid-state NMR on a set of noncrystalline isotopically labelled bovine rhodopsin samples. We detected localized perturbations of the carbon-carbon bond lengths of the retinylidene chromophore. The observations are consistent with a model in which the positive charge of the protonated Schiff base penetrates into the polyene chain and partially concentrates around the C13 position. This coincides with the proximity of a water molecule located between the glutamate-181 and serine-186 residues of the second extracellular loop, which is folded back into the transmembrane region. These measurements support the hypothesis that the polar residues of the second extracellular loop and the associated water molecule assist the rapid selective photoisomerization of the retinylidene chromophore by stabilizing a partial positive charge in the center of the polyene chain.

Structural Studies of Biomaterials Using Double-Quantum Solid-State NMR Spectroscopy

Proteins directly control the nucleation and growth of biominerals, but the details of molecular recognition at the protein-biomineral interface remain poorly understood. The elucidation of recognition mechanisms at this interface may provide design principles for advanced materials development in medical and ceramic composites technologies. Here, we describe both the theory and practice of double-quantum solid-state NMR (ssNMR) structure-determination techniques, as they are used to determine the secondary structures of surface-adsorbed peptides and proteins. In particular, we have used ssNMR dipolar techniques to provide the first high-resolution structural and dynamic characterization of a hydrated biomineralization protein, salivary statherin, adsorbed to its biologically relevant hydroxyapatite (HAP) surface. Here, we also review NMR data on peptides designed to adsorb from aqueous solutions onto highly porous hydrophobic surfaces with specific helical secondary structures. The adsorption or covalent attachment of biological macromolecules onto polymer materials to improve their biocompatibility has been pursued using a variety of approaches, but key to understanding their efficacy is the verification of the structure and dynamics of the immobilized biomolecules using double-quantum ssNMR spectroscopy.

Conformation and Dynamics of Atactic Poly(acrylonitrile). 3. Characterization of Local Structure by Two-Dimensional 2H−13C Solid-State NMR

The detailed conformational structure of a disordered crystalline polymer, atactic poly(acrylonitrile) (aPAN), has been characterized by two-dimensional (2D) NMR experiments. A solid-state 2H−13C heteronuclear multiple-quantum correlation (HMQC) NMR experiment has been applied to an aPAN sample containing both α-hydrogen-deuterated and 13C⋮N-carbon-labeled repeat units. The 2D spectrum yields information in particular on the two successive torsion angles in racemo trans−trans dyads. Their average values are found as (180 ± 5°, 180 ± 5°), with distributions of standard deviation σ = 10 ± 5° for both torsion angles. The torsion angles and their distributions in racemo trans−trans dyads thus obtained are closer to the ideal trans conformation than are those in meso trans−trans dyads determined by 2D double-quantum solid-state NMR spectroscopy on a 13C⋮N-carbon-labeled aPAN sample. The differences are considered to originate from larger steric hindrance and larger electric dipole interaction between the C⋮N g...

Conformation of the glycosidic linkage in a disaccharide investigated by double-quantum solid-state NMR.

Double-quantum heteronuclear local field NMR is performed on a sample of a 13C2-labeled disaccharide, in which the two 13C spins are located on opposite sides of the glycosidic linkage. The evolution of the double-quantum coherences is found to be consistent with the solid-state conformation of the molecule, as previously determined by X-ray diffraction. The dependence of the double-quantum evolution on the glycosidic torsional angles is examined by using a graphical molecular manipulation program interfaced to a numerical spin simulation module.

Determination of a molecular torsional angle in the metarhodopsin-I photointermediate of rhodopsin by double-quantum solid-state NMR.

We present a solid-state NMR study of metarhodopsin-1, the pre-discharge intermediate of the photochemical signal transduction cascade of rhodopsin, which is the 41 kDa integral membrane protein that triggers phototransduction in vertebrate rod cells. The H-C10-C11-H torsional angles of the retinylidene chromophore in bovine rhodopsin and metarhodopsin-I were determined simultaneously in the photo-activated membrane-bound state, using double-quantum heteronuclear local field spectroscopy. The torsional angles were estimated to be [phi] = 160+/-10 degrees for rhodopsin and phi = 180+/-25 degrees for metarhodopsin-I. The result is consistent with current models of the photo-induced conformational transitions in the chromophore, in which the 11-Z retinal ground state is twisted, while the later photointermediates have a planar all-E conformation.

Conformation of Poly(ethylene oxide) Intercalated in Clay and MoS2 Studied by Two-Dimensional Double-Quantum NMR

The conformation of poly(ethylene oxide), PEO, intercalated in layered clays and MoS2 has been characterized by double-quantum solid-state NMR. Using PEO with 13% 13C−13C labeled units, the conformational statistics of the OC−CO bonds have been determined. The OC−CO bonds of the polymer in narrow interlayer gaps of ∼0.8, ∼0.85, and ∼1.0 nm thickness consistently exhibit a high gauche content of approximately 90%. This rules out models with highly trans-containing chains as found in certain complexes of PEO with HgCl2 or p-nitrophenol.

Trans content in atactic polystyrene estimated by double-quantum solid-state NMR

The conformational statistics of atactic polystyrene have been investigated by solid-state NMR, on a sample labeled with 25% 13 CH 2 groups. Double-quantum pulse sequences are used to select statistically formed 13 C– 13 C spin pairs. An experiment correlating the 13 C– 13 C internuclear vector with the 13 C chemical-shift anisotropy determines the conformational statistics of a single backbone torsion angle. Double-quantum experiments correlating the 13 CH 2 chemical-shift anisotropies of two adjacent segments provide information on two consecutive torsion angles. The analysis of the spectra yields a trans content of 68% (±10%). This represents the first direct experimental estimate of the trans/ gauche ratio in atactic polystyrene. The trans content found here agrees well with rotational-isomeric-state models but is inconsistent with results from more elaborate atomistic models that take packing effects into account.

Selective residual dipolar couplings in cross-linked elastomers by [formula omitted] double-quantum NMR spectroscopy

1 H double-quantum (DQ) solid-state NMR spectroscopy under fast magic-angle spinning (MAS) is introduced as a new spectroscopic tool for the investigation of the structure and local chain dynamics of elastomers. Dipolar connectivities between the protons of the various functional groups can be directly established from the highly resolved DQ solid-state NMR spectra as is shown for a series of cross-linked poly(styrene-co-butadiene). More quantitatively, residual dipolar couplings within and between the functional groups are evaluated selectively from the build-up curves of the double-quantum signals in the limit of the spin-pair approximation. In particular, the CH–CH and the CH 2–CH couplings of butadiene, which both act predominantly along the chain-segment direction, have been measured relative to the CH 2 coupling. The total build-up intensity is correlated with the cross-link density.

Complete Dipolar Decoupling of13C and Its Use in Two-Dimensional Double-Quantum Solid-State NMR for Determining Polymer Conformations

A multiple-pulse technique for complete dipolar decoupling of directly bonded13C-labeled sites is described. It achieves significant spectral simplifications in a recently introduced two-dimensional double-quantum solid-state NMR experiment for determining torsion angles. Both homonuclear and heteronuclear dipolar couplings are removed by combining a13C multiple-pulse sequence with continuous-wave irradiation on the protons. The13C sequence has a fundamental 10-pulse cycle which is a significantly modified magic-sandwich-echo sequence. The crucial heteronuclear decoupling is achieved by breaking the 360° “inner” pulses in the magic sandwich into 90° pulses and spacing them by1H 360° pulse lengths. Spectral artifacts typical of multiple-pulse sequences are eliminated by phase shifts between cycles. In contrast to many other multiple-pulse decoupling sequences, the long window in the cycle is the dwell time and can be longer than the inverse dipolar coupling, which makes the sequence practical for direct detection even with long pulse ring-down times. A modification of the sequence to scale the chemical shift and increase the effective spectral width is also presented. The 1D and double-quantum 2D experiments are demonstrated on polyethylene with 4%13C–13C spin pairs. The potential of this approach for distinguishing segmental conformations is illustrated by spectral simulations of the two-dimensional ridge patterns that correlate double-quantum and single-quantum chemical-shift anisotropies.

Direct Determination of a Peptide Torsional Angle ψ by Double-Quantum Solid-State NMR

Direct Determination of a Peptide Torsional Angle ψ by Double-Quantum Solid-State NMR