Fast Magic Angle Sample Spinning Research Articles

Proton-detected 3D 1H anisotropic/14N/1H isotropic chemical shifts correlation NMR under fast magic angle spinning on solid samples without isotopic enrichment

The chemical shift anisotropy (CSA) interaction of a nucleus is an important indicator of the local electronic environment particularly for the contributions arising from hydrogen (H)-bonding, electrostatic and π-π interactions. CSAs of protons bonded to nitrogen atoms are of significant interest due to their common role as H-bonding partners in many chemical, pharmaceutical and biological systems. Although very fast (∼100 kHz) magic angle sample spinning (MAS) experiments have enabled the measurement of proton CSAs directly from solids, due to a narrow chemical shift (CS) distribution, overlapping NH proton resonances are common and necessitate the introduction of an additional frequency dimension to the regular 2D 1H CSA/1H CS correlation method to achieve sufficient resolution. While this can be accomplished by using the isotropic shift frequency of 14N or 15N nuclei, the use of the naturally-abundant 14N nucleus avoids 15N isotopic labeling and therefore would be useful for a variety of solids. To this end, we propose a proton-detected 3D 1H CSA/14N/1H CS correlation method under fast MAS (90 kHz) to determine the CSA tensors of NH protons in samples without isotopic enrichment. Our experimental results demonstrate that the proposed 3D NMR experiment is capable of resolving the overlapping 1H resonances of amide (NH) groups through the 14N isotropic shift frequency dimension and enables the accurate measurement of site-specific 1H CSAs directly from powder samples under fast MAS conditions. In addition to the 3D 1H CSA/14N/1H CS experiment, an approach employing 14N-edited 2D 1H CSA/1H CS experiment is also demonstrated as an additional means to address spectral overlap of NH resonances with aliphatic and other proton resonances in solids.

Weak and Transient Protein Interactions Determined by Solid-State NMR.

Despite their roles in controlling many cellular processes, weak and transient interactions between large structured macromolecules and disordered protein segments cannot currently be characterized at atomic resolution by X-ray crystallography or solution NMR. Solid-state NMR does not suffer from the molecular size limitations affecting solution NMR, and it can be applied to molecules in different aggregation states, including non-crystalline precipitates and sediments. A solid-state NMR approach based on high magnetic fields, fast magic-angle sample spinning, and deuteration provides chemical-shift and relaxation mapping that enabled the characterization of the structure and dynamics of the transient association between two regions in an 80 kDa protein assembly. This led to direct verification of a mechanism of regulation of E. coli DNA metabolism.

Two-dimensional proton-detected (35)Cl/(1)H correlation solid-state NMR experiment under fast magic angle sample spinning: application to pharmaceutical compounds.

The determination of structure of hydrochloride salts of active pharmaceutical ingredients (HCl APIs) utilizing (35)Cl solid-state NMR studies has been of considerable interest in the recent past. Until now these studies relied on the (35)Cl direct observation method which has its own limitations in terms of the sensitivity and resolution due to the quadrupolar nature and the low gyromagnetic ratio of (35)Cl. In this contribution we demonstrate the two-dimensional (2D) (35)Cl/(1)H correlation measurement by using the proton detection-based (indirect observation of (35)Cl via(1)H) approach under fast magic angle sample spinning (MAS: 70 kHz). The main advantages of this approach over the direct observation method are highlighted in the present study. We have employed heteronuclear magnetization transfer through the recoupling of (35)Cl-(1)H heteronuclear dipolar interactions. The applicability of (35)Cl indirect detection method is first demonstrated on hydrochloride salts of amino acids, l-tyrosine·HCl and l-histidine·HCl·H2O following which the 2D (35)Cl/(1)H correlations are obtained for HCl APIs, procainamide HCl (Proc) and aminoguanidine HCl (Amin). On the basis of separation between the central transition (CT) and satellite transition (ST) peaks, and the shape/width of CT powder patterns, it is also shown that the quadrupolar parameters which are useful for the elucidation of the molecular structure can be determined. Moreover, the (35)Cl/(1)H correlations provide the precise determination of (1)H chemical shifts of nearby (35)Cl nuclei.

Proton–nitrogen-14 overtone two-dimensional correlation NMR spectroscopy of solid-sample at very fast magic angle sample spinning

1H–14N overtone (OT) heteronuclear multiple quantum coherence (HMQC) experiment at very fast magic angle spinning (MAS) is reported. The 14N OT coherence is excited and reconverted by 14N OT pulses at twice the 14N Larmor frequency. The OT coherence is free from the first order quadrupolar broadening. MAS further removes the broadening due to chemical shift anisotropy (CSA). With a small 0.75mm MAS rotor and coil system, very fast MAS up to 90kHz and very strong rf field are achieved, enhancing the sensitivity of indirect 14N OT observation via protons. In comparison with 1H–14N double-quantum HMQC, an enhancement factor of 1.8 is obtained for glycine with the 14N OT irradiation. The bandwidth in the 14N OT dimension is limited due to long 14N OT pulses.

Fast Magic Angle Sample Spinning NMR Yields a View of the F-actin - Cofilin Complex with Atomic Resolution

Proteins of the ADF/cofilin family are vital regulators of the actin cytoskeleton in eukaryotes. Binding of cofilin results in dramatic reorganization of the F-actin structure and potentiates filament severing, and its accelerated depolymerization from the pointed end. Although the X-ray structure of monomeric actin with cofilin-homology domain of twinfilin has been recently solved, F-actin evades crystallization and therefore the analysis of cofilin interaction with F-actin at the atomic level calls for alternative approaches. While considerable insight into the cofilin F-actin complex has been gained through chemical cross-linking and radiolytic footprinting, these approaches were unable to generate high resolution information on this interaction While considerable insight into the cofilin F-actin complex has been gained through chemical cross-linking and radiolytic footprinting, novel approaches are still desirable. Here we demonstrate that Fast-MAS Solid State NMR is a high sensitivity approach to studying this system. Isotopically labeled S. cerevisieae yeast cofilin, in complex with polymerized yeast actin, allows for an atomic resolution view of cofilin within the complex. Intramolecular conformational changes occurring in cofilin upon binding to actin can be deduced from dipolar and scalar coupling based spectra. Additional studies of the free cofilin have been performed in the solution and solid states for comprehensive comparisons. Therefore, our data demonstrates the general feasibility of the SS-NMR approach for studying the interaction between F-actin and actin binding proteins.

1H?15N correlation spectroscopy of nanocrystalline proteins

The limits of resolution that can be obtained in 1H-15N 2D NMR spectroscopy of isotopically enriched nanocrystalline proteins are explored. Combinations of frequency switched Lee-Goldburg (FSLG) decoupling, fast magic angle sample spinning (MAS), and isotopic dilution via deuteration are investigated as methods for narrowing the amide 1H resonances. Heteronuclear decoupling of 15N from the 1H resonances is also studied. Using human ubiquitin as a model system, the best resolution is most easily obtained with uniformly 2H and 15N enriched protein where the amides have been exchanged in normal water, MAS at approximately 20 kHz, and WALTZ-16 decoupling of the 15N nuclei. The combination of these techniques results in average 1H lines of only approximately 0.26 ppm full width at half maximum. Techniques for optimizing instrument stability and 15N decoupling are described for achieving the best possible performance in these experiments.

Solid-State NMR Spectroscopy under Periodic Modulation by Fast Magic-Angle Sample Spinning and Pulses: A Review

Fast magic-angle spinning (MAS) holds promise for new approaches to pulsed high-resolution NMR in solids where homogeneous interactions dominate. Prerequisite for developing new pulse methods is the understanding of signal encoding by spin interactions under MAS conditions and of interferences between MAS and pulses. This review discusses corresponding strategies and techniques in a coherent way with particular concentration on homonuclear decoupling techniques for line-narrowing in solids.

Solid-State Proton NMR Characterization of Ethylene Oxide and Propylene Oxide Random and Block Copolymer Composites with Poly(methyl silsesquioxanes)

Solid-state proton NMR with fast magic-angle sample spinning has been used to study the structure and dynamics of block and random ethylene oxide/propylene oxide copolymers in the bulk and as 30 wt % composites with poly(methyl silsesquioxane) which are precursors to nanoporous ultralow dielectric constant films. The NMR results for the bulk diblock copolymer show that the propylene oxide block is relatively mobile and the proton line widths are effectively narrowed with 12 kHz magic-angle sample spinning, while most of the signals for the semicrystalline ethylene oxide block are too broad to be observed under these conditions. Both the ethylene oxide and propylene oxide signals are observed in the 30 wt % composite, demonstrating that the formation of crystalline domains in the ethylene oxide block is inhibited in the composite. Dipolar filter NMR experiments show that the propylene oxide is less mobile than the ethylene oxide in the block copolymer composites, suggesting that the propylene oxide is near...

Fast magic-angle spinning proton NMR studies of polymers at surfaces and interfaces

Solid-state proton NMR with fast magic-angle sample spinning has been used to study the structure and dynamics of polymers and the water interface in porous glass composites. The composites were prepared by photopolymerization of poly(ethyl acrylate) and other acrylate formulations in a high surface-area rigid glass matrix with 40-Å interconnected pores. High resolution solid-state proton spectra were obtained for polymer films and composites with 15 kHz magic-angle sample spinning at temperatures above the polymer glass transition temperature. The solid-state proton spectra can be detected with high sensitivity and used to determine the composition of polymer and water filling the pores. These results and spin diffusion studies using 1H– 29Si 2D heteronuclear correlation and wideline separation NMR show that the polymer fills the central 30 Å of the pore, and that the remaining volume is filled with surface hydroxyl groups and water.

Controlling the chain dynamics of polymers at interfaces

Solid state proton NMR with fast magic angle spinning has been used to study a low Tg acrylate polymer in bulk and polymerized in a vycor glass with 40 Å pores. The combination of chain motion and fast magic angle sample spinning averages the proton–proton dipolar interactions such that high resolution spectra can be observed at ambient temperature. Multiple-quantum NMR shows that some of the chains are severely restricted relative to the bulk material by incorporation into the porous glass. These restricted chains can be returned to their bulk-like state by surface treatment of the porous glass.

Fast Magic-Angle Proton NMR Studies of Polymer Interfaces in Poly(ethyl acrylate)/Vycor Composites

Fast magic-angle spinning solid-state proton NMR has been used to study the structure and dynamics of poly(ethyl acrylate) in bulk and polymerized in a Vycor glass with 40 A pores. Bulk poly(ethyl acrylate) is a low-T g material, and high-resolution solid-state proton NMR spectra are observed at ambient temperature because the dipolar interactions are averaged by the combination of chain motion and fast magic-angle sample spinning. The spectrum of the poly(ethyl acrylate)/Vycor composite shows signals in addition to those of the polymer that are assigned to water and surface hydroxyl groups in the porous glass. Integration of the composite and bulk poly(ethyl acrylate) signals reveals that the volume fraction of polymer in the pores is 0.56 and that the polymer occupies the central 30 A in a 40 A pore. Spin diffusion studies demonstrate that filling of the poly(ethyl acrylate)/Vycor is incomplete, and a fraction of the water in the pores is isolated from the polymer. Multiple-quantum NMR studies reveal a large difference in chain dynamics between the bulk poly(ethyl acrylate) and the porous glass composite, and 2D wide line separation NMR shows that the polymer is restricted at the surface of the pores but has bulklike mobility in the center of the pore.

Adiabatic passage Hartmann-Hahn cross polarization in NMR under magic angle sample spinning

We have recently demonstrated that polarization transfer using an adiabatic passage through the Hartmann-Hahn condition (APHH-CP) by a variation of the radio-frequency amplitude can substantially improve the transfer efficiency over Hartmann-Hahn cross polarization. Here we show that APHH-CP can be combined with fast magic angle sample spinning (MAS). The heteronuclear dipolar order, established in the course of the transfer, can indeed be created and preserved.

Rotor-synchronized amplitude-modulated nuclear magnetic resonance spin-lock sequences for improved cross polarization under fast magic angle sample spinning

We demonstrate that in solid-state nuclear magnetic resonance the Hartmann–Hahn cross-polarization efficiency in the presence of fast magic angle sample spinning can be improved by the application of rotor-synchronized amplitude-modulated radio-frequency irradiation during the mixing time (AMCP pulse sequences). The modulation reintroduces time-independent coupling elements between dressed eigenstates of the Floquet Hamiltonian which lead to flip–flop transitions. By numerical optimization, an optimal AMCP pulse sequence is designed which provides a significant improvement over conventional cross polarization. The matching condition can be broadened by rotor-synchronized 180° phase shifts to render the transfer less sensitive to exact matching of the applied radio-frequency field strengths.

Direct detection of connectivities in glasses by 2D NMR

Abstract 31 P two-dimensional (2D) nuclear magnetic resonance (NMR) is proposed a powerful tool for direct investigation of the connectivities between like or different Q n units in glasses or the medium-range order. The entire experiment was carried out under conditions of fast magic angle sample spinning (MAS). The magnetization transfer was obtained by recoupling of the dipole-dipole interaction despite the fast MAS. The mixing time dependence of the 2D spectra for a binary Na 2 OP 2 O 5 glass suggests an almost regular alternation of Q 2 and Q 3 units for that glass.

Cross polarization under fast magic angle sample spinning using amplitude-modulated spin-lock sequences

Amplitude-modulated cross-polarization (AMCP) schemes that can greatly improve the transfer efficiency under fast magic angle spinning are discussed. A novel class of pulse sequences (S-AMCP) that employ an amplitude-modulated spin-lock field on the S-spin channel only and cw irradiation on the I-spin channel is introduced. It leads to efficient cross polarization at the standard Hartmann-Hahn condition while maintaining good spin-lock properties. S-AMCP does, however, not decrease the sensitivity to exact Hartmann-Hahn matching that can become critical under fast rotation. A double amplitude modulation scheme (D-AMCP) that uses amplitude modulation on both radiofrequency channels and significantly broadens the Hartmann-Hahn matching condition is presented. It can, however, be applied only to spin systems with relatively low I-spin homonuclear dipolar interactions due to the absence of an effective spin-locking field. S-AMCP pulse sequences, in contrast, are generally applicable and yield cross-polarization efficiencies that are always better than or equal to the standard continuous-wave cross-polarization experiment.

Cross polarization under fast magic angle spinning: thermodynamical considerations

Thermodynamic aspects of cross polarization under fast magic angle sample spinning are studied. An extension of the static cross-polarization model is presented which applies to the “sidebands” of the Hartmann—Hahn condition. The cross-polarization process for the “sidebands” is not spin-energy conserving but energy is supplied or adsorbed by the sample rotation. For a static sample, cross polarization proceeds through heteronuclear zero-quantum transitions. For a rotating sample, zero-quantum as well as double-quantum cross polarization is observed experimentally. This behavior is explained by the extended model.

14N effects on the 29Si solid-state NMR spectra of silicon nitride

29Si high-resolution solid-state NMR spectra of the α and β forms of silicon nitride show the effect of residual dipolar coupling to 14N nuclei which is not averaged out by fast magic-angle sample spinning. The influence of 14N is most noticeable in the low-field signal of α-Si 3N 4 and leads to a complex lineshape with field-dependent partitions. The use of a previously reported first-order perturbative approach allows a successful simulation of 29Si lineshapes at different magnetic fields. As an outcome of the simulation, a value of J( 29 Si, 14N ) = 15 Hz is derived. Comparison with solution scalar coupling constants in Si,N-containing compounds reveals that this value lies within the expected range. Additionally, a 14N quadrupole coupling constant of −2.1 MHz is obtained for Si 3N 4. This result confirms the fact that NSi σ bond polarization and ( d-p)π bonding result in a considerable decrease in χ, as compared with compounds where the nitrogen is bonded to carbon or hydrogen.

Characterization of some coals and coal intercalations by high-resolution 1H (BR-24/MAS at 270 MHz) and 13C (CP/MAS) n.m.r. in solids

High resolution 1H n.m.r. studies combining the BR-24 multiple-pulse method after BURUM and RHIM with fast magic-angle sample spinning (MAS), and high resolution solid-state 13C n.m.r. measurements with cross polarization (CP) and MAS have been used for investigating: a number of coals of different ranks; GDR soft brown coals characterized by textural differences; and typical intercalations to be found in such soft brown coals. The resolution level of 1H n.m.r. spectra achieved so far at a resonance frequency of 270 MHz permits determination of essential functional groups. The coalification series ranging from soft brown coal to anthracite shows an increase of the aromaticity values ƒ a C and also an increase of the ƒ a H values, which is analogous to the increases in reflectance and carbon content. There are marked differences between the functional groups to be found in detrital and xylite-containing soft brown coals. Tertiary (Miocene) brown coals from the 2nd Lower Lusatian seam (GDR) (i.e. unbedded, poorly bedded, bedded, xylite-containing unbedded, and gelified bedded coals) differ significantly due to variations in micropetrographical composition. Varying amounts of cellulose and lignin are contained in the xylites of the 2nd Lower Lusatian seam. 13C and 1H n.m.r. measurements are employed to determine the main constituents of substances such as pyropissite, retinite, fusite and ‘monkey hairs’ (fossilized rubber).