Radio Frequency-driven Recoupling Research Articles

Heteronuclear and homonuclear radio-frequency-driven recoupling.

The radio-frequency-driven recoupling (RFDR) pulse sequence is used in magic-angle spinning (MAS) NMR to recouple homonuclear dipolar interactions. Here we show simultaneous recoupling of both the heteronuclear and homonuclear dipolar interactions by applying RFDR pulses on two channels. We demonstrate the method, called HETeronuclear RFDR (HET-RFDR), on microcrystalline SH3 samples at 10 and 55.555 kHz MAS. Numerical simulations of both HET-RFDR and standard RFDR sequences allow for better understanding of the influence of offsets and paths of magnetization transfers for both HET-RFDR and RFDR experiments, as well as the crucial role of XY phase cycling.

Selectively Enhanced 1H-1H Correlations in Proton-Detected Solid-State NMR under Ultrafast MAS Conditions.

Proton-detected solid-state NMR has emerged as a powerful analytical technique in structural elucidation via 1H-1H correlations, which are mostly established by broadband methods. We propose a new class of frequency-selective homonuclear recoupling methods to selectively enhance 1H-1H correlations of interest under ultrafast magic-angle spinning (MAS). These methods, dubbed as selective phase-optimized recoupling (SPR), can provide a sensitivity enhancement by a factor of ∼3 over the widely used radio-frequency-driven recoupling (RFDR) to observe 1HN-1HN contacts in a protonated tripeptide N-formyl-Met-Leu-Phe (fMLF) under 150 kHz MAS and are successfully utilized to probe a long-range 1H-1H contact in a pharmaceutical molecule, the hydrochloride form of pioglitazone (PIO-HCl). SPR is not only highly efficient in frequency-selective recoupling but also easy to implement, imparting to it great potential to probe 1H-1H contacts for the structural elucidation of organic solids such as proteins and pharmaceuticals under ultrafast MAS conditions.

Local atomic and electronic structure in the LiVPO4 (F,O) tavorite-type materials from solid-state NMR combined with DFT calculations.

7 Li, 31 P, and 19 F solid-state nuclear magnetic resonance (NMR) spectroscopy was used to investigate the local arrangement of oxygen and fluorine in LiVPO4 F1-y Oy materials, interesting as positive electrode materials for Li-ion batteries. From the evolution of the 1D spectra versus y, 2D 7 Li radiofrequency-driven recoupling (RFDR) experiments combined, and a tentative signal assignment based on density functional theory (DFT) calculations, it appears that F and O are not randomly dispersed on the bridging X position between two X-VO4 -X octahedra (X = O or F) but tend to segregate at a local scale. Using DFT calculations, we analyzed the impact of the different local environments on the local electronic structure. Depending on the nature of the VO4 X2 environments, vanadium ions are either in the +III or in the +IV oxidation state and can exhibit different distributions of their unpaired electron(s) on the d orbitals. Based on those different local electronic structures and on the computed Fermi contact shifts, we discuss the impact on the spin transfer mechanism on adjacent nuclei and propose tentative signal assignments. The O/F clustering tendency is discussed in relation with the formation of short VIV O vanadyl bonds with a very specific electronic structure and possible cooperative effect along the chain.

Can proton-proton recoupling in fully protonated solids provide quantitative, selective and efficient polarization transfer?

Dipolar recoupling sequences have been used to probe spatial proximity of nuclear spins and were traditionally designed to probe rare spins such as 13C and/or 15N nuclei. The multi-spin dipolar-coupling network of the rare spins is weak due to smaller couplings and large chemical shift dispersion. Therefore, the recoupling approaches were tailored to design offset compensated or broadband sequences. In contrast, protons have a substantially stronger dipolar-coupling network and much narrower chemical shift range. Broadband recoupling sequences such as radio-frequency driven recoupling (RFDR), back-to-back (BABA), and lab frame proton-proton spin diffusion have been routinely used to characterize the structures of protein/macromolecules and small molecules. Recently selective 1H-1H recoupling sequences have been proposed that combine chemical shift offset of the resolved proton spectrum (at fast MAS) with first- and second-order dipolar recoupling Hamiltonians to obtain quantitative and qualitative proton distances, respectively. Herein, we evaluate the performances of broadband and selective proton recoupling sequences such as finite pulse RFDR (fp-RFDR), band-selective spectral spin diffusion (BASS-SD), second-order cross-polarization (SOCP), and selective recoupling of proton (SERP) in terms of the selectivity and efficiency of 1H-1H polarization transfers in a dense network of proton spins and explore the possibility of measuring 1H-1H distances. We use theoretical considerations, numerical simulations, and experiments to support the distinct advantages and disadvantages of each recoupling sequence. Experiments were performed on L-histidine.HCl.H2O at a MAS frequency of 71.43 kHz. This study rationalizes the proper selection of 1H-1H recoupling sequences when working with fully protonated solids.

Selective detection of active pharmaceutical ingredients in tablet formulations using solid-state NMR spectroscopy

Atomic-level characterization of active pharmaceutical ingredients (API) is crucial in pharmaceutical industry because APIs play an important role in physicochemical properties of drug formulations. However, the analysis of targeted APIs in intact tablet formulations is less straightforward due to the coexistence of excipients as major components and different APIs at dilute concentrations (often below 10 ​wt% loading). Although solid-state (ss) NMR spectroscopy is widely used to investigate short-range order, polymorphism, and pseudo-polymorphism in neat pharmaceutical compounds, the analysis of complex drug formulations is often limited by overlapped signals that originate from structurally different APIs and excipients. In particular, such examples are frequently encountered in the analysis of 1H ssNMR spectra of pharmaceutical formulations. While the high-resolution in 1H ssNMR spectra can be attained by, for example, high magnetic fields accompanied by fast magic-angle spinning (MAS) approaches, the spectral complexity associated with the mixtures of compounds hinders the accurate determination of chemical shifts and through-space proximities. Here we propose a fast MAS (70 ​kHz) NMR experiment for the selective detection of 1H signals associated with an API from a severely overlapped NMR spectrum of a tablet formulation. Spectral simplification is achieved by combining (i) symmetry-based dipolar recoupling (SR412) rotational-echo saturation-pulse double-resonance (RESPDOR) with phase-modulate (PM) saturation pulses, (ii) radio frequency-driven recoupling (RFDR), and (iii) double-quantum excitation using Back-to-Back (BaBa) pulse sequence elements. First, 1H sites in close proximities to 14N nuclei of an API are excited using a PM-S-RESPDOR sequence, and simultaneously, the other unwanted 1H signals of excipients are suppressed. Then, 1H magnetization transfer to adjacent 1H sites in the API is achieved by spin diffusion process using a RFDR sequence, which polarizes to 1H sites within the crystalline API regions of the drug formulation. Next, a PM-S-RESPDOR-RFDR sequence is combined with a Back-to-Back (BaBa) sequence to elucidate local-structures and 1H–1H proximities of the API in a dosage form. The PM-S-RESPDOR-RFDR-BaBa experiment is employed in one- (1D) and two-dimensional (2D) versions to selectively detect the 1H ssNMR spectrum of l-cysteine (10.6 ​wt% or 0.11 ​mg) in a commercial formulation, and compared with the spectra of neat l-cysteine recorded using a standard BaBa experiment. The 2D 1H double-quantum−single-quantum (DQ-SQ) spectrum of the API (l-cysteine)-detected pharmaceutical tablet is in good agreement with the 2D 1H DQ-SQ spectrum obtained from the pure API molecule. Furthermore, the sensitivity and robustness of the experiment is examined by selectively detecting 1H{14N} signals in an amino acid salt, l-histidine.H2O.HCl.

Forcing the 'lazy' protons to work.

The combination of cross-polarization (CP) with flip-back (FB) pulse has enabled in NMR the enhancement of 13C sensitivity and the decrease of the recycling delay at both moderate and fast magic-angle spinning (MAS) frequencies. However, only continuous-wave (CW) decoupling is presently compatible with FB-pulse (FB-CW), and depending on the CW radio-frequency (rf) field, either an insignificant sensitivity gain or an acquisition time-dependent gain and a low 13C resolution are obtained. In this study, we propose a new FB-pulse method in which radio frequency-driven recoupling (RFDR) is used as the 1H-13C decoupling scheme to overcome these drawbacks. The performances of FB-RFDR in terms of decoupling efficiency and sensitivity gain are tested on both natural abundance (NA) and uniformly 13C-15N labeled l-histidine·HCl·H2O (Hist) samples at a MAS frequency of νR = 70 kHz. The results show the superiority of RFDR over the CW decoupling with respect to these criteria. Importantly, they reveal that the sensitivity gain offered by FB-RFDR is nearly independent of the decoupling/acquisition duration. The application of FB-RFDR on NA-Hist and sucrose yields a sensitivity gain between 60 and 100% compared to conventional FB-CW and CPMAS-SPINAL experiments. Moreover, we compare the 13C sensitivities of NA-Hist obtained by our 1D FB-RFDR method and 2D 1H-{13C} double-CP acquisition. Both methods provide similar 13C sensitivity and are complementary. Indeed, the 2D method has the advantage of also providing the 1H-13C spatial proximities, but its sensitivity for quaternary carbons is limited; whereas our 1D FB-RFDR method is more independent of the type of carbon, and can provide a 13C 1D spectrum in a shorter experimental time. We also test the feasibility of FB-RFDR at a moderate frequency of νR = 20 kHz, but the experimental results demonstrate a poor resolution as well as a negligible sensitivity gain.

Transient effects in π-pulse sequences in MAS solid-state NMR

Dipolar recoupling techniques that use isolated rotor-synchronized π pulses are commonly used in solid-state NMR spectroscopy to gain insight into the structure of biological molecules. These sequences excel through their simplicity, stability towards radio-frequency (rf) inhomogeneity, and low rf requirements. For a theoretical understanding of such sequences, we present a Floquet treatment based on an interaction-frame transformation including the chemical-shift offset dependence. This approach is applied to the homonuclear dipolar-recoupling sequence Radio-Frequency Driven Recoupling (RFDR) and the heteronuclear recoupling sequence Rotational Echo Double Resonance (REDOR). Based on the Floquet approach, we show the influence of effective fields caused by pulse transients and discuss the advantages of pulse-transient compensation. We demonstrate experimentally that the transfer efficiency for homonuclear recoupling can be doubled in some cases in model compounds as well as in simple peptides if pulse-transient compensation is applied to the π pulses. Additionally, we discuss the influence of various phase cycles on the recoupling efficiency in order to reduce the magnitude of effective fields. Based on the findings from RFDR, we are able to explain why the REDOR sequence does not suffer in the recoupling efficiency despite the presence of effective fields.

Analysis of the Electronic Structure of the Special Pair of a Bacterial Photosynthetic Reaction Center by 13 C Photochemically Induced Dynamic Nuclear Polarization Magic-Angle Spinning NMR Using a Double-Quantum Axis.

The origin of the functional symmetry break in bacterial photosynthesis challenges since several decades. Although structurally very similar, the two branches of cofactors in the reaction center (RC) protein complex act very differently. Upon photochemical excitation, an electron is transported along one branch, while the other remains inactive. Photochemically induced dynamic nuclear polarization (photo-CIDNP) magic-angle spinning (MAS) 13 C NMR revealed that the two bacteriochlorophyll cofactors forming the "Special Pair" donor dimer are already well distinguished in the electronic ground state. These previous studies are relying solely on 13 C-13 C correlation experiments as radio-frequency-driven recoupling (RFDR) and dipolar-assisted rotational resonance (DARR). Obviously, the chemical-shift assignment is difficult in a dimer of tetrapyrrole macrocycles, having eight pyrrole rings of similar chemical shifts. To overcome this problem, an INADEQUATE type of experiment using a POST C7 symmetry-based approach is applied to selectively isotope-labeled bacterial RC of Rhodobacter (R.) sphaeroides wild type (WT). We, therefore, were able to distinguish unresolved sites of the macromolecular dimer. The obtained chemical-shift pattern is in-line with a concentric assembly of negative charge within the common center of the Special Pair supermolecule in the electronic ground state.

Revealing structure and dynamics in host–guest supramolecular crystalline polymer electrolytes by solid-state NMR: Applications to β-CD-polyether/Li+ crystal

The detailed knowledge of the structure and dynamics in molecular level is very relevant for our understanding of the conductivity mechanism of solid state polymer electrolytes. In this work, we have synthesized two conductive β-CD-polyether/Li+ inclusion crystals, which both have the channel-like structure formed by the β-CDs, but differ by the chemical nature of the assembled polymer chains. 1H, 7Li and 19F solid state NMR have been performed to study the dynamics of the polymer chains, cations and anions. 1H NMR of the samples reveals the clear difference in the segmental mobility of the polymer chains threaded inside the β-CDs, depending on the chemical nature of the polymer chains. Temperature dependent solid-state 2D 7Li-7Li exchange NMR combined with 7Li finite-pulse radio frequency-driven recoupling (fp-RFDR) NMR reveals two different Li+ local exchange dynamics, namely, the Li+ exchange process between the different polymer chain segments and the Li+ exchange process between the polymer chains and the channels (i.e., the assembled β-CDs). 19F NMR of the samples reveals that the anions in the samples are in a relatively immobile state, indicating that the anions are well separated from the mobile Li+ ions in the samples. The implications of these NMR results for understanding of the conductivity mechanism of the materials are discussed.

Improved transfer efficiencies in radio-frequency-driven recoupling solid-state NMR by adiabatic sweep through the dipolar recoupling condition.

The homonuclear radio-frequency driven recoupling (RFDR) experiment is commonly used in solid-state NMR spectroscopy to gain insight into the structure of biological samples due to its ease of implementation, stability towards fluctuations/missetting of radio-frequency (rf) field strength, and in general low rf requirements. A theoretical operator-based Floquet description is presented to appreciate the effect of having a temporal displacement of the π-pulses in the RFDR experiment. From this description, we demonstrate improved transfer efficiency for the RFDR experiment by generating an adiabatic passage through the zero-quantum recoupling condition. We have compared the performances of RFDR and the improved sequence to mediate efficient (13)CO to (13)Cα polarization transfer for uniformly (13)C,(15)N-labeled glycine and for the fibril forming peptide SNNFGAILSS (one-letter amino acid codes) uniformly (13)C,(15)N-labeled at the FGAIL residues. Using numerically optimized sweeps, we get experimental gains of approximately 20% for glycine where numerical simulations predict an improvement of 25% relative to the standard implementation. For the fibril forming peptide, using the same sweep parameters as found for glycine, we have gains in the order of 10%-20% depending on the spectral regions of interest.

Revealing Defects in Crystalline Lithium-Ion Battery Electrodes by Solid-State NMR: Applications to LiVPO4F

Identifying and characterizing defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple stable oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO4F and establish the relative subnanometer-scale proximities between them. Well-crystallized LiVPO4F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solid-state 7Li nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic 7Li environments that can account for up to 20% of the total lithium content. Multidimensional and site-selective solid-state 7Li NMR experiments using finite-pulse radio frequency-driven recoupling (fp-RFDR) establish unambiguously that the unexpected lithium environmen...

Proton-detected 3D 14N/14N/1H isotropic shift correlation experiment mediated through 1H–1H RFDR mixing on a natural abundant sample under ultrafast MAS

In this contribution, we have demonstrated a proton detection-based approach on a natural abundant powdered l-Histidine HCl–H2O sample at ultrafast magic angle spinning (MAS) to accomplish 14N/14N correlation from a 3D 14N/14N/1H isotropic shift correlation experiment mediated through 1H finite-pulse radio frequency-driven recoupling (fp-RFDR). Herein the heteronuclear magnetization transfer between 14N and 1H has been achieved by HMQC experiment, whereas 14N/14N correlation is attained through enhanced 1H–1H spin diffusion process due to 1H–1H dipolar recoupling during the RFDR mixing. While the use of ultrafast MAS (90kHz) provides sensitivity enhancement through increased 1H transverse relaxation time (T2), the use of micro-coil probe which can withstand strong 14N radio frequency (RF) fields further improves the sensitivity per unit sample volume.

Determination of relative orientation between 1H CSA tensors from a 3D solid-state NMR experiment mediated through 1H/1H RFDR mixing under ultrafast MAS

To obtain piercing insights into inter and intramolecular H-bonding, and π-electron interactions measurement of 1H chemical shift anisotropy (CSA) tensors is gradually becoming an obvious choice. While the magnitude of CSA tensors provides unique information about the local electronic environment surrounding the nucleus, the relative orientation between these tensors can offer further insights into the spatial arrangement of interacting nuclei in their respective three-dimensional (3D) space. In this regard, we present a 3D anisotropic/anisotropic/isotropic proton chemical shift (CSA/CSA/CS) correlation experiment mediated through 1H/1H radio frequency-driven recoupling (RFDR) which enhances spin diffusion through recoupled 1H–1H dipolar couplings under ultrafast magic angle spinning (MAS) frequency (70kHz). Relative orientation between two interacting 1H CSA tensors is obtained by fitting two-interacting 1H CSA tensors by fitting two-dimensional (2D) 1H/1H CSA/CSA spectral slices through extensive numerical simulations. To recouple 1H CSAs in the indirect frequency dimensions of a 3D experiment we have employed γ-encoded radio frequency (RF) pulse sequence based on R-symmetry (R1887) with a series of phase-alternated 2700°–90180° composite-180° pulses on citric acid sample. Due to robustness of applied 1H CSA recoupling sequence towards the presence of RF field inhomogeneity, we have successfully achieved an excellent 1H/1H CSA/CSA cross-correlation efficiency between H-bonded sites of citric acid.

Proton-detected 2D radio frequency driven recoupling solid-state NMR studies on micelle-associated cytochrome-b5

Solid-state NMR spectroscopy is increasingly used in the high-resolution structural studies of membrane-associated proteins and peptides. Most such studies necessitate isotopically labeled (13C, 15N and 2H) proteins/peptides, which is a limiting factor for some of the exciting membrane-bound proteins and aggregating peptides. In this study, we report the use of a proton-based slow magic angle spinning (MAS) solid-state NMR experiment that exploits the unaveraged 1H–1H dipolar couplings from a membrane-bound protein. We have shown that the difference in the buildup rates of cross-peak intensities against the mixing time – obtained from 2D 1H–1H radio frequency-driven recoupling (RFDR) and nuclear Overhauser effect spectroscopy (NOESY) experiments on a 16.7-kDa micelle-associated full-length rabbit cytochrome-b5 (cytb5) – can provide insights into protein dynamics and could be useful to measure 1H–1H dipolar couplings. The experimental buildup curves compare well with theoretical simulations and are used to extract relaxation parameters. Our results show that due to fast exchange of amide protons with water in the soluble heme-containing domain of cyb5, coherent 1H–1H dipolar interactions are averaged out for these protons while alpha and side chain protons show residual dipolar couplings that can be obtained from 1H–1H RFDR experiments. The appearance of resonances with distinct chemical shift values in 1H–1H RFDR spectra enabled the identification of residues (mostly from the transmembrane region) of cytb5 that interact with micelles.

Rapid measurement of multidimensional 1H solid-state NMR spectra at ultra-fast MAS frequencies

A novel method to realize rapid repetition of 1H NMR experiments at ultra-fast MAS frequencies is demonstrated. The ultra-fast MAS at 110kHz slows the 1H–1H spin diffusion, leading to variations of 1H T1 relaxation times from atom to atom within a molecule. The different relaxation behavior is averaged by applying 1H–1H recoupling during relaxation delay even at ultra-fast MAS, reducing the optimal relaxation delay to maximize the signal to noise ratio. The way to determine optimal relaxation delay for arbitrary relaxation curve is shown. The reduction of optimal relaxation delay by radio-frequency driven recoupling (RFDR) was demonstrated on powder samples of glycine and ethenzamide with one and multi-dimensional NMR measurements.

Insights into Atomic-Level Interaction between Mefenamic Acid and Eudragit EPO in a Supersaturated Solution by High-Resolution Magic-Angle Spinning NMR Spectroscopy

The intermolecular interaction between mefenamic acid (MFA), a poorly water-soluble nonsteroidal anti-inflammatory drug, and Eudragit EPO (EPO), a water-soluble polymer, is investigated in their supersaturated solution using high-resolution magic-angle spinning (HRMAS) nuclear magnetic resonance (NMR) spectroscopy. The stable supersaturated solution with a high MFA concentration of 3.0 mg/mL is prepared by dispersing the amorphous solid dispersion into a d-acetate buffer at pH 5.5 and 37 °C. By virtue of MAS at 2.7 kHz, the extremely broad and unresolved (1)H resonances of MFA in one-dimensional (1)H NMR spectrum of the supersaturated solution are well-resolved, thus enabling the complete assignment of MFA (1)H resonances in the aqueous solution. Two-dimensional (2D) (1)H/(1)H nuclear Overhauser effect spectroscopy (NOESY) and radio frequency-driven recoupling (RFDR) under MAS conditions reveal the interaction of MFA with EPO in the supersaturated solution at an atomic level. The strong cross-correlations observed in the 2D (1)H/(1)H NMR spectra indicate a hydrophobic interaction between the aromatic group of MFA and the backbone of EPO. Furthermore, the aminoalkyl group in the side chain of EPO forms a hydrophilic interaction, which can be either electrostatic or hydrogen bonding, with the carboxyl group of MFA. We believe these hydrophobic and hydrophilic interactions between MFA and EPO molecules play a key role in the formation of this extremely stable supersaturated solution. In addition, 2D (1)H/(1)H RFDR demonstrates that the molecular MFA-EPO interaction is quite flexible and dynamic.

2D 1H/1H RFDR and NOESY NMR Experiments on a Membrane-Bound Antimicrobial Peptide Under Magic Angle Spinning

There is significant interest in solving high-resolution dynamic structures of membrane-associated peptides using solid-state NMR spectroscopy. Previous solid-state NMR studies have provided valuable insights into the functional properties of an exciting class of biomacromolecules such as antimicrobial peptides and amyloid peptides. However, it has been a major challenge to apply solid-state NMR techniques to study peptides or proteins that are not labeled with specific isotopes such as (13)C, (15)N, and/or (2)H. This study utilizes 2D (1)H/(1)H radio frequency-driven recoupling (RFDR) and nuclear Overhauser effect spectroscopy (NOESY) pulse sequences under magic angle spinning to study a membrane-bound antimicrobial peptide MSI-78 (or also known as pexiganan). We demonstrate that proton resonances can be assigned and structural constraints, NOE and (1)H-(1)H dipolar couplings, can be measured without the need for any isotopic enrichment. The buildup curves, showing the dependence of the cross peak intensity against the mixing time, obtained from 2D (1)H/(1)H NOESY and RFDR experiments are compared. Our results reveal that the RFDR-recovered (1)H-(1)H dipolar couplings associated with alpha and side chain protons are larger than that with the amide-protons. This study provides a means to measure residual (1)H-(1)H dipolar couplings for the investigation of structure, dynamics, and aggregation of peptides using a suitable model membrane like micelles or bicelles.

Solid state NMR studies and chemical shift calculations of a gold(I) complex with a diphosphacyclobutadiene cobaltate sandwich anion

The 31P and 13C solid state nuclear magnetic resonance spectra of a neutral gold(I) complex with bis(diphosphacyclobutadiene) cobaltate anions, [Au{Co(P2C2tPent2)2}2(PMe3)2], are reported. Complete 31P resonance assignments have been derived from saturation transfer, radio-frequency driven recoupling (RFDR) and RTOBSY experiments and confirmed further by ab-initio calculations of magnetic shielding tensors by density functional theory, with consideration of relativistic effects. Coordination of the diphosphacyclobutadiene ring with gold(I) results in a high-frequency shift of the 31P signal of the directly coordinated P atom, whereas a low-frequency shift is observed for the P atom at the opposite end of that ring. Based on these results, a previous assignment made for the complex salts [Au(PMe3)4][Au{Co(P2C2Ad2)2}2] and [K(18-crown-6)(thf)2][Au{Co(P2C2Ad2)2}2] (Ad=adamantyl) must be corrected.

Structural and dynamic characterization of Li12Si7 and Li12Ge7 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicide Li12Si7, and the analogous germanium compound have been characterized by detailed 6Li, 7Li, and 29Si variable temperature static and magic-angle spinning (MAS) NMR experiments. In the MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200K, whereas at higher temperatures partial site averaging is observed on the kHz timescale. The observed lithium chemical shift ranges of up to 60ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl–Klemm–Bussmann concept used for the theoretical description of lithium silicides. Furthermore the strongly diamagnetic chemical shifts observed for the lithium ions situated directly above the five-membered Si5 rings suggest the possibility of aromatic ring currents in these structural elements. This assignment is confirmed further by 29Si{7Li} CPMAS-heteronuclear correlation experiments. The 29Si MAS-NMR spectra of Li12Si7, aided by 2-D J-resolved spectroscopy, are well suited for differentiating between the individual sites within the silicon framework, while further detailed connectivity information is available on the basis of 2-D INADEQUATE and radio frequency driven recoupling (RFDR) spectra. Variable temperature static 7Li NMR spectra reveal the onset of strong motional narrowing effects, illustrating high lithium ionic mobilities in both of these compounds.

The Functional Curli Amyloid Is Not Based on In-register Parallel β-Sheet Structure

The extracellular curli proteins of Enterobacteriaceae form fibrous structures that are involved in biofilm formation and adhesion to host cells. These curli fibrils are considered a functional amyloid because they are not a consequence of misfolding, but they have many of the properties of protein amyloid. We confirm that fibrils formed by CsgA and CsgB, the primary curli proteins of Escherichia coli, possess many of the hallmarks typical of amyloid. Moreover we demonstrate that curli fibrils possess the cross-beta structure that distinguishes protein amyloid. However, solid state NMR experiments indicate that curli structure is not based on an in-register parallel beta-sheet architecture, which is common to many human disease-associated amyloids and the yeast prion amyloids. Solid state NMR and electron microscopy data are consistent with a beta-helix-like structure but are not sufficient to establish such a structure definitively.