Raiker Witter

Solid-State 19F NMR Spectroscopy Reveals That Trp41 Participates in the Gating Mechanism of the M2 Proton Channel of Influenza A Virus

The integral membrane protein M2 of influenza A virus assembles as a tetrameric bundle to form a proton-conducting channel that is activated by low pH. The side chain of His37 in the transmembrane alpha-helix is known to play an important role in the pH activation of the proton channel. It has also been suggested that Trp41, which is located in an adjacent turn of the helix, forms part of the gating mechanism. Here, a synthetic 25-residue peptide containing the M2 transmembrane domain was labeled with 6F-Trp41 and studied in lipid membranes by solid-state 19F NMR. We monitored the pH-dependent differences in the 19F dipolar couplings and motionally narrowed chemical shift anisotropies of this 6F-Trp41 residue, and we discuss the pH activation mechanism of the H+ channel. At pH 8.0, the structural parameters implicate an inactivated state, while at pH 5.3 the tryptophan conformation represents the activated state. With the aid of COSMOS force field simulations, we have obtained new side-chain torsion angles for Trp41 in the inactivated state (chi1 = -100 degrees +/- 10 degrees , chi2 = +110 degrees +/- 10 degrees ), and we predict a most probable activated state with chi1 = -50 degrees +/- 10 degrees and chi2 = +115 degrees +/- 10 degrees . We have also validated the torsion angles of His37 in the inactivated state as chi1 = -175 degrees +/- 10 degrees and chi2 = -170 degrees +/- 10 degrees .

Solid state 19F NMR parameters of fluorine-labeled amino acids. Part II: Aliphatic substituents

A representative set of amino acids with aliphatic 19F-labels has been characterized here, following up our previous compilation of NMR parameters for single 19F-substituents on aromatic side chains. Their isotropic chemical shifts, chemical shift tensor parameters, intra-molecular 19F dipole-dipole couplings and temperature-dependent T1 and T2 relaxation times were determined by solid state NMR on twelve polycrystalline amino acid samples, and the corresponding isotropic 19F chemical shifts and scalar couplings were obtained in solution. Of particular interest are amino acids carrying a trifluoromethyl-group, because not only the 19F chemical shift but also the intra-CF3 homonuclear dipolar coupling can be used for structural studies of 19F-labeled peptides and proteins. The CF3-groups are further compared with CH2F-, CD2F-, and CD3-groups, using both 19F and 2H NMR to describe their motional behavior and to examine the respective linebroadening effects of the protonated and deuterated neighbors. We have also characterized two unnatural amino acids in which a CF3-label is rigidly connected to the backbone by a phenyl or bicyclopentyl moiety, and which are particularly well suited for structure analysis of membrane-bound polypeptides. The 19F NMR parameters of the polycrystalline amino acids are compared with data from the correspondingly labeled side chains in synthetic peptides.

Solid Electrolytes for Fluoride Ion Batteries: Ionic Conductivity in Polycrystalline Tysonite-Type Fluorides

Batteries based on a fluoride shuttle (fluoride ion battery, FIB) can theoretically provide high energy densities and can thus be considered as an interesting alternative to Li-ion batteries. Large improvements are still needed regarding their actual performance, in particular for the ionic conductivity of the solid electrolyte. At the current state of the art, two types of fluoride families can be considered for electrolyte applications: alkaline-earth fluorides having a fluorite-type structure and rare-earth fluorides having a tysonite-type structure. As regard to the latter, high ionic conductivities have been reported for doped LaF3 single crystals. However, polycrystalline materials would be easier to implement in a FIB due to practical reasons in the cell manufacturing. Hence, we have analyzed in detail the ionic conductivity of La(1-y)Ba(y)F(3-y) (0 ≤ y ≤ 0.15) solid solutions prepared by ball milling. The combination of DC and AC conductivity analyses provides a better understanding of the conduction mechanism in tysonite-type fluorides with a blocking effect of the grain boundaries. Heat treatment of the electrolyte material was performed and leads to an improvement of the ionic conductivity. This confirms the detrimental effect of grain boundaries and opens new route for the development of solid electrolytes for FIB with high ionic conductivities.

Altered reaction pathways of eutectic LiBH4–Mg(BH4)2 by nanoconfinement

The effects of nanoconfinement on the dehydrogenation rate and reaction pathways of the eutectic LiBH4–Mg(BH4)2 have been comprehensively investigated. By means of thermal analysis, mass spectroscopy and solid state 11B MAS NMR, it has been revealed that the multistep thermal decomposition pattern of the binary LiBH4–Mg(BH4)2 has been altered in a two-step reaction and the desorption kinetics has also been significantly improved after infiltration. The formation of diborane and stable MnB12H12 intermediates of the bulk LiBH4–Mg(BH4)2 has been found to be inhibited by nanoconfinement.

3D Structure Elucidation Using NMR Chemical Shifts

Abstract The NMR chemical shift is virtually always available from conventional NMR experiments. In contrast to X-ray diffraction it is caused by the density distribution of the valence electrons, hence it contains genuine information about the valence structure of the molecular system. This paper reviews the available theoretical, empirical and semi-empirical methods to obtain 3D structure information from chemical shifts. Besides direct empirical correlations of chemical shifts an overview of computational quantum chemical methods is presented. A critical survey is given how these methods can be used in structure refinement procedures. Special attention is paid to methods for protein and peptide structure analysis using chemical shifts. Computed and empirical chemical shift maps are discussed and compared to direct refinement methods. Chemical shift tensors or their principal components can provide additional data to characterise structural motifs in proteins. Furthermore, methods are discussed to extract orientational constraints from chemical shift tensors in macroscopically aligned samples. Applications are presented for structure elucidation in solution and in the solid state, including the first applications of chemical shifts to crystal structure refinements.

Spectral assignments and anisotropy data of cellulose Iα: 13C-NMR chemical shift data of cellulose Iα determined by INADEQUATE and RAI techniques applied to uniformly 13C-labeled bacterial celluloses of different Gluconacetobacter xylinus strains

Solid‐state 13C‐NMR spectroscopy was used to characterize native cellulose pellicles from two strains of Gluconacetobacter xylinus (ATCC 53582, ATCC 23769), which had been statically cultivated in Hestrin–Schramm (HS) medium containing fully 13C‐labeled β‐D‐glucose‐U‐13C6 as the sole source of carbon. For both samples, the 13C‐NMR chemical shifts were completely assigned for each 13C‐labeled site of cellulose Iα with the aid of 2D refocused INADEQUATE NMR. To determine the principal chemical shift tensor components, a pulse sequence based on the recoupling of anisotropy information (RAI) was applied at 10 kHz MAS. The detailed 13C tensors of cellulose Iα from different bacterial celluloses are thus available now for the first time, and these results have been compared with previously published data of nonenriched material and with theoretical predictions. Copyright

All-atom molecular dynamics simulations using orientational constraints from anisotropic NMR samples

Orientational constraints obtained from solid state NMR experiments on anisotropic samples are used here in molecular dynamics (MD) simulations for determining the structure and dynamics of several different membrane-bound molecules. The new MD technique is based on the inclusion of orientation dependent pseudo-forces in the COSMOS-NMR force field. These forces drive molecular rotations and re-orientations in the simulation, such that the motional time-averages of the tensorial NMR properties approach the experimentally measured parameters. The orientational-constraint-driven MD simulations are universally applicable to all NMR interaction tensors, such as chemical shifts, dipolar couplings and quadrupolar interactions. The strategy does not depend on the initial choice of coordinates, and is in principle suitable for any flexible molecule. To test the method on three systems of increasing complexity, we used as constraints some deuterium quadrupolar couplings from the literature on pyrene, cholesterol and an antimicrobial peptide embedded in oriented lipid bilayers. The MD simulations were able to reproduce the NMR parameters within experimental error. The alignment of the three membrane-bound molecules and some aspects of their conformation were thus derived from the NMR data, in good agreement with previous analyses. Furthermore, the new approach yielded for the first time the distribution of segmental orientations with respect to the membrane and the order parameter tensors of all three systems.

Beneficial effects of stoichiometry and nanostructure for a LiBH4–MgH2 hydrogen storage system

The hydrogen storage system [MgH2–2LiBH4] shows attractive properties such as favorable thermodynamics, high hydrogen capacity and reversibility. However, there exists an incubation period that amounts up to 10 hours in the dehydrogenation steps, which restricts this system as a practical material. In this study, the influences of stoichiometry and the nanoscale MgH2 were investigated for the system. Considerably shortened incubation times were achieved with deficit amounts of LiBH4 or by using nanoscale MgH2. In addition, the application of nanoscale MgH2 prevented or suppressed the formation of [B12H12]2− in the dehydrogenation, which is otherwise an issue concerning the re-cyclability.

1H line width dependence on MAS speed in solid state NMR –comparison of experiment and simulation

Recent developments in magic angle spinning (MAS) technology permit spinning frequencies of ≥100 kHz. We examine the effect of such fast MAS rates upon nuclear magnetic resonance proton line widths in the multi-spin system of β-Asp-Ala crystal. We perform powder pattern simulations employing Fokker-Plank approach with periodic boundary conditions and 1H-chemical shift tensors calculated using the bond polarization theory. The theoretical predictions mirror well the experimental results. Both approaches demonstrate that homogeneous broadening has a linear-quadratic dependency on the inverse of the MAS spinning frequency and that, at the faster end of the spinning frequencies, the residual spectral line broadening becomes dominated by chemical shift distributions and susceptibility effects even for crystalline systems.

Lithiation-driven structural transition of VO2F into disordered rock-salt LixVO2F

We synthesize a new vanadium oxyfluoride VO2F (rhombohedral, Rc) through a simple one-step ball-milling route and demonstrate its promising lithium storage properties with a high theoretical capacity of 526 mA h g−1. Similar to V2O5, VO2F transfers into an active disordered rock-salt (Fmm) phase after initial cycling against the lithium anode, as confirmed by diffraction and spectroscopic experiments. The newly formed nanosized LixVO2F remains its crystal structure over further cycling between 4.1 and 1.3 V. A high capacity of 350 mA h g−1 at 2.5 V was observed at 25 °C and 50 mA g−1. Furthermore, superior performance was observed for VO2F in comparison with a commercial crystalline V2O5, in terms of discharge voltage, voltage hysteresis and reversible capacity.