Powder Pattern Line Shapes Research Articles

Crystalline Bis-urea Nanochannel Architectures Tailored for Single-File Diffusion Studies

Urea is a versatile building block that can be modified to self-assemble into a multitude of structures. One-dimensional nanochannels with zigzag architecture and cross-sectional dimensions of only ∼3.7 Å × 4.8 Å are formed by the columnar assembly of phenyl ether bis-urea macrocycles. Nanochannels formed by phenylethynylene bis-urea macrocycles have a round cross-section with a diameter of ∼9.0 Å. This work compares the Xe atom packing and diffusion inside the crystalline channels of these two bis-ureas using hyperpolarized Xe-129 NMR. The elliptical channel structure of the phenyl ether bis-urea macrocycle produces a Xe-129 powder pattern line shape characteristic of an asymmetric chemical shift tensor with shifts extending to well over 300 ppm with respect to the bulk gas, reflecting extreme confinement of the Xe atom. The wider channels formed by phenylethynylene bis-urea, in contrast, present an isotropic dynamically average electronic environment. Completely different diffusion dynamics are revealed in the two bis-ureas using hyperpolarized spin-tracer exchange NMR. Thus, a simple replacement of phenyl ether with phenylethynylene as the rigid linker unit results in a transition from single-file to Fickian diffusion dynamics. Self-assembled bis-urea macrocycles are found to be highly suitable materials for fundamental molecular transport studies on micrometer length scales.

Membrane protein structure from rotational diffusion

The motional averaging of powder pattern line shapes is one of the most fundamental aspects of sold-state NMR. Since membrane proteins in liquid crystalline phospholipid bilayers undergo fast rotational diffusion, all of the signals reflect the angles of the principal axes of their dipole–dipole and chemical shift tensors with respect to the axis defined by the bilayer normal. The frequency span and sign of the axially symmetric powder patterns that result from motional averaging about a common axis provide sufficient structural restraints for the calculation of the three-dimensional structure of a membrane protein in a phospholipid bilayer environment. The method is referred to as rotationally aligned (RA) solid-state NMR and demonstrated with results on full-length, unmodified membrane proteins with one, two, and seven trans-membrane helices. RA solid-state NMR is complementary to other solid-state NMR methods, in particular oriented sample (OS) solid-state NMR of stationary, aligned samples. Structural distortions of membrane proteins from the truncations of terminal residues and other sequence modifications, and the use of detergent micelles instead of phospholipid bilayers have also been demonstrated. Thus, it is highly advantageous to determine the structures of unmodified membrane proteins in liquid crystalline phospholipid bilayers under physiological conditions. RA solid-state NMR provides a general method for obtaining accurate and precise structures of membrane proteins under near-native conditions. This article is part of a Special Issue entitled: NMR Spectroscopy for Atomistic Views of Biomembranes and Cell Surfaces. Guest Editors: Lynette Cegelski and David P. Weliky.

Analysis of 2H NMR spectra of water molecules on the surface of nano-silica material MCM-41: Deconvolution of the signal into a Lorentzian and a powder pattern line shapes

Water 2H NMR signal on the surface of nano-silica material MCM-41 consists of two overlapping resonances. The 2H water spectrum shows a superposition of a Lorentzian line shape and the familiar NMR powder pattern line shape, indicating the existence of two spin components. Exchange occurs between these two groups. Decomposition of the two signals is a crucial starting point to study the exchange process. In this article we have determined these spin component populations along with other important parameters for the 2H water NMR signal over a temperature range between 223 K and 343 K.

High field Pb207 spin-lattice relaxation in solid lead nitrate and lead molybdate

Spin-lattice relaxation rates of lead have been measured at 17.6 T (156.9 MHz) as a function of temperature in polycrystalline lead nitrate and lead molybdate. Comparing the results with relaxation rates measured at lower fields, it is found that at high fields and low temperature, chemical shift anisotropy (CSA) makes small but observable contributions to lead relaxation in both materials. At 17.6 T and 200 K, CSA accounts for about 15% of the observed relaxation rate. Above 300 K, the dominant relaxation mechanism even at 17.6 T is an indirect Raman process involving modulation of the (207)Pb spin-rotation tensor, as first proposed by Grutzner et al. [J. Am. Chem. Soc. 123, 7094 (2001)] and later treated theoretically in more detail by Vega et al. [Phys. Rev. B 74, 214420 (2006)]. The improved signal to noise ratio at high fields makes it possible to quantify relaxation time anisotropy by analyzing saturation-recovery functions for individual frequencies on the powder pattern line shape. No orientation dependence is found for the spin-lattice relaxation rate of either material. It is argued from examination of the appropriate theoretical expressions, derived here for the first time, that the lack of observable relaxation time anisotropy is probably a general feature of this indirect Raman mechanism.

Determination of chemical shift anisotropies of unresolved carbonyl sites by C-α detection under magic-angle spinning

We demonstrate that the static powder pattern line shape of chemical shift anisotropy (CSA) can be obtained for unresolved carbonyl sites of polypeptides under magic-angle spinning. The CSA interaction is first recoupled at the carbonyl site. The phase factors associated with the CSA recoupling are then transferred to the adjacent α carbon by an isotropic polarization transfer based on scalar spin–spin coupling. Because α carbons of polypeptides are usually better resolved, we can then obtain the CSA static powder pattern line shapes of the carbonyl sites after Fourier transformation in the second dimension. We validate our approach experimentally by measurements on [U– 13C, 15N]- l-alanine, [U– 13C, 15N]- l-valine and prion fibrils with uniform 13C and 15N labels on selected residues.

Recoupling of chemical-shift anisotropy powder patterns in MAS NMR

A comparison of three different implementations of the chemical-shift recoupling experiment of Tycko et al. [R. Tycko, G. Dabbagh, P.A. Mirau, Determination of chemical-shift-anisotropy lineshapes in a two-dimensional magic-angle-spinning NMR experiment, J. Magn. Reson. 85 (1989) 265–274] is presented. The methods seek to reduce the effects of artefacts resulting from pulse imperfections and residual C-H dipolar coupling in organic solids. An optimised and constant time implementation are shown to give well-defined and artefact free powder pattern lineshapes in the indirectly observed dimension for both sp2 and sp3 carbon sites. Experimental setup is no more demanding than for the original experiment, and can be implemented using standard commercial hardware.

Correlation of 129Xe NMR shielding data with the pore structures of various aluminophosphate molecular sieves

129Xe NMR spectra of two different amounts of xenon sealed inside sample tubes containing various microporous aluminophosphate (AlPO-5, -11, -31, and -41) and silicoaluminophosphate (SAPO-5, -11, -31, and -41) molecular sieves are recorded at different temperatures and analysed. For most cases, the 129Xe shielding tensor of adsorbed xenon is asymmetric and distinct spectral differences between circular and elliptical cross-sections, framework dimensions, and xenon loading levels of sieve channels are detected based on the powder pattern lineshape fittings. The roles of Xe–wall and Xe–Xe interactions and different adsorbant diffusion modes on spectral characteristics are discussed. The shielding tensor axes are associated with the sieve channel planes by comparing the data with previously published limiting case models of lineshapes. Shielding tensor span is found to be a promising indicator of sieve channel dimensions. Small differences between AlPO and SAPO molecular sieves belonging to the same framework structure type are also detected. In AlPO-31, fast exchange effects between Xe in the free intercrystallite volume and Xe physisorbed on the external crystallite surfaces are observed and the intercrystallite cavity sizes are estimated.

Magnetic-field dependent optically detected ESR in the photoexcited triplet states of bridged Zr(IV) metallocenes

We report on the magnetic-field-dependent optically detected magnetic resonance (ODMR) spectra for polycrystalline samples of the bridged Zr(IV) metallocenes, Me2Si<(Cp2)ZrCl2 ( (dimethylsilylbis(cyclopentadienyl)zirconium-dichloride) and Me2C<(Cp2)ZrCl2 (iso-propylidenebis(cyclopentadienyl)zirconium-dichloride). ODMR spectra at zero magnetic field were recorded by frequency sweeping a microwave source from 0.1 to 10 GHz with the sample contained in a microwave helix. ODMR spectra at finite magnetic fields were recorded with the sample contained in either a helix or a slotted-tube resonator with a fixed microwave frequency and sweeping the magnetic field. For all experiments, the sample and microwave probes were contained in an immersion dewar cryostat, and the temperature was held at about 2 K. All three zero field ODMR transitions (2|E|, and |D| − |E| and |D|+|E|) were observed in the frequency-swept ODMR spectra recorded at zero and small magnetic fields. The zero-field frequency-swept spectra allowed the determination ofD andE values uniquely. For frequency-swept small-field ODMR spectra recorded at successively higher magnetic fields, each of the ODMR line intensities was observed to increase with increasing magnetic field. This intensity increase was observed for all three ODMR lines, reflecting an increase in the total intensity rather than simply a change in the polarization of the triplet sublevels. The latter would result in a change in the relative intensities of the ODMR lines but would not change simultaneously the intensities of all three lines. The ODMR line intensities increase in proportion toB n, wheren<1. This field dependence is weaker than the expected proportionalB 2 dependence from the Zeeman effect, which likely originates from the magnetic field dependence of the spin relaxation rates between the triplet sublevels. Magnetic-field-swept ODMR spectra recorded at fixed microwave frequencies in the X-band frequency range (9.8 GHz) do not show all three expected classic Pake powder pattern line shape profiles, exhibited by the molecules with their magneticZ, Y, andX axes parallel to the external magnetic field. In particular, the intensity for molecular magneticY-axes parallel to the external magnetic field is completely suppressed. In addition, an external magnetic field dependence in field-swept ODMR spectra was observed, which results in a linear decrease of the ODMR intensity with increasing strength of the external magnetic field over and above that would be expected in a polycrystalline spectrum. The data are analyzed by simulation of the continuous-wave ESR spectrum with the eigenvalues and eigenvectors of the spin Hamiltonian matrix characterizing the triplet state exhibiting the ODMR spectrum, in conjunction with homotopy, as a function of the orientations of the magnetic axes of the various molecules in a polycrystalline sample. This approach is useful to interpret the experimentally observed ODMR transition frequencies andg-values but does not take the amplitudes in the ODMR spectrum. The corrections required to modify the continuous-wave ESR spectral amplitudes that reproduce the observed ODMR amplitudes are effects associated with the ODMR processes.

Recoupling of chemical shift anisotropies in solid-state NMR under high-speed magic-angle spinning and in uniformly C13-labeled systems

We demonstrate the possibility of recoupling chemical shift anisotropy (CSA) interactions in solid-state nuclear magnetic resonance (NMR) under high-speed magic-angle spinning (MAS) while retaining a static CSA powder pattern line shape and simultaneously attenuating homonuclear dipole–dipole interactions. CSA recoupling is accomplished by a rotation-synchronized radio-frequency pulse sequence with symmetry properties that permit static CSA line shapes to be obtained. We suggest a specific recoupling sequence, which we call ROCSA, for which the scaling factors for CSA and homonuclear dipole–dipole interactions are 0.272 and approximately 0.05, respectively. This sequence is suitable for high-speed C13 MAS NMR experiments on uniformly C13-labeled organic compounds, including biopolymers. We demonstrate the ROCSA sequence experimentally by measuring the C13 CSA patterns of the uniformly labeled, polycrystalline compounds L-alanine and N-acetyl-D,L-valine at MAS frequencies of 11 and 20 kHz. We also present experimental data for amyloid fibrils formed by a 15-residue fragment of the β-amyloid peptide associated with Alzheimer’s disease, in which four amino acid residues are uniformly labeled, demonstrating the applicability to biochemical systems of high molecular weight and significant complexity. Analysis of the CSA patterns in the amyloid fibril sample demonstrates the utility of ROCSA measurements as probes of peptide and protein conformation in noncrystalline solids.

Paramagnetic Centers in Microcrystalline Silicon

AbstractWe have studied hydrogenated microcrystalline silicon, μc-Si:H, using dark and lightinduced electron spin resonance (ESR). In dark ESR measurements only one center is observed. The g values obtained empirically, assuming axial symmetry, from powder pattern lineshape simulations are g⊥∥ = 2.0096 and gτ = 2.0031. We suggest that this center is related to defects in the crystalline phase. Another signal, which occurs only after illumination, at low temperatures, is best described by two powder patterns indicating the presence of two centers. One center is slightly asymmetric (g∥ = 1.999, g⊥ = 1.996) and the other has a large unresolved broadening such that unique g-values cannot be obtained. The average g value for this center is 1.998. We interpret the light-induced electron spin resonance (LESR) signal as coming from electrons and holes trapped in the band tails at the crystalline grain boundaries.

Broadband dipolar recoupling in rotating solids: a numerical comparison of some pulse schemes

A numerical comparison of the dipolar recoupling performance of several previously published homonuclear recoupling schemes under magic angle-spinning conditions is presented. Emphasis is put on the recoupled polarization transfer in a two-spin system where the efficiency is studied as a function of resonance offsets in the presence and absence of chemical-shielding anisotropies. In addition, the effect of the rf field strength is investigated. Powder pattern line shapes are shown in the on-resonance case that reveal the distribution of dipolar couplings for each recoupling scheme. These results are compared to data computed with a purposely misset rf field strength to estimate the pulse scheme sensitivity to rf-inhomogeneity and experimental missettings.

Structure and Dynamics of the Antibiotic Peptide PGLa in Membranes by Solution and Solid-State Nuclear Magnetic Resonance Spectroscopy

PGLa, a 21-residue member of the magainin family of antibiotic peptides, is shown to be helical between residues 6 and 21 when associated with detergent micelles by multidimensional solution nuclear magnetic resonance (NMR) spectroscopy. Solid-state NMR experiments on specifically 15N-labeled peptides in oriented phospholipid bilayer samples show that the helix axis is parallel to the plane of the bilayers. 15N solid-state NMR powder pattern line shapes obtained on unoriented samples demonstrate that the amino-terminal residues are highly mobile and that the fluctuations of backbone sites decrease from Ala6 toward the carboxy terminus. The powder pattern observed for 15N-labeled Ala20 is essentially that expected for a rigid site. These findings are similar to those for the 23-residue magainin2 peptide in membrane environments.

19F NMR relaxation studies on 5-fluorotryptophan- and tetradeutero-5-fluorotryptophan-labeled E. coli glucose/galactose receptor

19F NMR relaxation studies have been carried out on a fluorotryptophan-labeled E. coli periplasmic glucose/galactose receptor (GGR). The protein was derived from E. coli grown on a medium containing a 50:50 mixture of 5-fluorotryptophan and [2,4,6,7-2H4]-5-fluorotryptophan. As a result of the large gamma-isotope shift, the two labels give rise to separate resonances, allowing relaxation contributions of the substituted indole protons to be selectively monitored. Spin-lattice relaxation rates were determined at field strengths of 11.75 T and 8.5 T, and the results were analyzed using a model-free formalism. In order to evaluate the contributions of chemical shift anisotropy to the observed relaxation parameters, solid-state NMR studies were performed on [2,4,6,7-2H4]-5-fluorotryptophan. Analysis of the observed 19F powder pattern lineshape resulted in anisotropy and asymmetry parameters of delta sigma = -93.5 ppm and eta = 0.24. Theoretical analyses of the relaxation parameters are consistent with internal motion of the fluorotryptophan residues characterized by order parameters S2 of approximately 1, and by correlation times for internal motion approximately 10(-11)s. Simultaneous least squares fitting of the spin-lattice relaxation and line-width data with tau i set at 10 ps yielded a molecular correlation time of 20 ns for the glucose-complexed GGR, and a mean order parameter S2 = 0.89 for fluorotryptophan residues 183, 127, 133, and 195. By contrast, the calculated order parameter for FTrp284, located on the surface of the protein, was 0.77. Significant differences among the spin-lattice relaxation rates of the five fluorotryptophan residues of glucose-complexed GGR were also observed, with the order of relaxation rates given by: R1F183 > R1F127 approximately R1F133 approximately R1F195 > R1F284. Although such differences may reflect motional variations among these residues, the effects are largely predicted by differences in the distribution of nearby hydrogen nuclei, derived from crystal structure data. In the absence of glucose, spin-lattice relaxation rates for fluorotryptophan residues 183, 127, 133, and 195 were found to decrease by a mean of 13%, while the value for residue 284 exhibits an increase of similar magnitude relative to the liganded molecule. These changes are interpreted in terms of a slower overall correlation time for molecular motion, as well as a change in the internal mobility of FTrp284, located in the hinge region of the receptor.

13C NMR hyperfine couplings, T1 anisotropy, and Korringa relations in Rb2CsC60: Search for effects of strong correlation.

Initial considerations lead one to suspect that effects of strong correlation might be present in the alkali fulleride superconductors. We report direct measurements of $^{13}\mathrm{C}$ ${T}_{1}$ anisotropy at 80 K in ${\mathrm{Rb}}_{2}$Cs${\mathrm{C}}_{60}$ and compare, in the context of the Korringa relation, the inferred spin-dipolar contribution to $\frac{1}{{T}_{1}}$ with the widths of the measured powder pattern line shape. The results demonstrate that the Korringa relation, valid in the limit of noninteracting electrons, holds in this case. Taken together with other normal-state NMR behavior this result makes important effects of strong correlation appear unlikely.

Characterization of enzyme-bound ligand dynamics by solid-state NMR in the presence of ligand exchange: L-phenylalanine on carboxypeptidase A

Deuterium NMR spectra were obtained for L-phenylalanine-d5, deuterated on the phenyl ring, in cross-linked polycrystalline samples of carboxypeptidase A containing different amounts of water. The deuterium powder pattern line shapes are simulated by extension of the theory to include both a local reorientational motion of the bound L-phenylalanine phenyl ring and exchange of the L-phenylalanine with an intracrystalline isotropic environment. The spectral simulations are consistent with the phenyl ring of the phenylalanine executing pi-flips in the bound environment at rates that vary from 3 x 10(4) Hz at 6% water content to 1 x 10(5) Hz at 21% water content. At all water contents studied, the ligand exchanges with an essentially isotropic environment in the crystal with a rate constant of approximately 2.5 x 10(-3) Hz. Although the dissociation constant for the L-phenylalanine is only 18 mM, the spectral simulations that reproduce the experimental line shape well do not require significant wobble of the phenyl ring rotation axis, which is consistent with the binding interactions identified by x-ray crystallography.

Side-chain structure and dynamics at the lipid-protein interface: Val1 of the gramicidin A channel

High resolution dynamics and structural information has been resolved from 2H solid-state NMR spectra of the Val-1 side-chain of the gramicidin channel in a lipid bilayer. Both powder pattern lineshapes and spectra from uniformly aligned samples of gramicidin in lipid bilayers have been analyzed to achieve a fully consistant interpretation of the data. Torsional motions about the C alpha C beta axis (chi 1) are shown to be three-state jumps in which the occupancy of the states is given by the ratio, 75:15:10 for the chi 1 angles of 184 degrees:304 degrees:64 degrees. The dominant conformer is also the most common conformation observed for valines in well defined protein structures. The distribution of conformational substates that represents the chi 1 dynamics appears to be largely independent of the lipid phase transition and the hydration of the sample. However, there is evidence that the residence time between jumps is dependent on the lipid phase transition. Although this time is shown to be approximately 1 microseconds below the phase transition temperature, it is in the fast exchange limit above the transition temperature.

Pseudo-spin rotational resonance and homonuclear dipolar N.M.R. of rotating solids

The theory of homonuclear dipolar N.M.R. of rotating solids is presented. A pseudo-spin model is introduced to simplify the problem, and several special cases are discussed using average hamiltonian theory. For a homonuclear spin pair with a large isotropic shift difference (Δv iso), a resonance phenomenon occurs if the condition Δv iso = nωr is satisfied. Off resonance, a rotational Block-Sigert shift is expected. In the fast spinning limit, both the powder pattern lineshape and width can be analysed by this theory. In addition, under slow spinning, where the adiabatic approximation is valid for the motion of the pseudo-spin, the sample rotation experiment provides a means to demonstrate the geometrical Berry phase.

High-pressure deuterium solid-state NMR study of the dynamics of pyridine intercalated CdPS 3

High-pressure deuterium solid state NMR techniques have been used to study pyridine-d 5 intercalated CdPS 3. The effect of host compression on the dynamics of the intercalated molecule was observed. Deuterium powder pattern lineshapes were collected at four isotherms (270 K, 300 K, 330 K, 360 K) and at pressures up to 4500 kbar. The application of pressure to the intercalated lamellar host material caused a suppression of the large-amplitude reorientational motion seen for the pyridine molecule at room temperature. As expected, with increasing isothermal temperatures higher pressures were required to freeze out the rotational diffusion of the pyridine. Our results show that an increase in pressure acts much like a decrease in temperature for the motional dynamics of an intercalated molecule.

Overtone NMR spectroscopy

The theoretical background and experimental implementation of overtone NMR spectroscopy of quadrupolar nuclei, i.e., the direct observation of NMR signals at multiples of the Larmor frequency, are described. Perturbation theory is used to derive expressions for the dependence of overtone NMR frequencies, transition moments, and dipole–dipole splittings on molecular orientation for a nucleus with arbitrary spin. Experimental techniques for overtone cross-polarization and for two-dimensional spectroscopic measurements of transition moments and dipole–dipole splittings are demonstrated with 14N overtone spectra of single crystals. Overtone powder pattern line shapes are analyzed, both for spin S=1 and spin S=3/2 nuclei, and examples of 14N overtone powder patterns are presented. Overtone spectroscopy can be used to characterize quadrupole and dipole–dipole interactions in polycrystalline samples. The sensitivity, resolution, and spectral width characteristics of overtone NMR spectra are discussed and contrasted with those of conventional NMR spectra. The sensitivity and resolution of overtone spectra are shown to be comparable to those of conventional spectra, while the reduced spectral width of overtone spectra is a significant advantage in studies of single crystals and, in particular, polycrystalline materials.

Nuclear magnetic resonance in Al-rich quasiperiodic crystals

The 27Al and the 55Mn nuclear magnetic resonance powder pattern lineshapes have been obtained in icosahedral and decagonal (T phase) Al-Mn quasiperiodic crystals, and are compared to that of orthorhombic Al6Mn. The quasiperiodic crystals yield much broader spectra with little resolved structure. The quadrupole and Knight shift parameters for the 55Mn resonance in orthorhombic Al6Mn have been determined as |vQ| = 0.76 MHz,Kax = −2.7 × 10−4, Kiso = + 5 × 10−3. The results imply that Al6Mn and the quasiperiodic crystals have similar electronic and magnetic properties.