Rotating-frame Spin-lattice Relaxation Research Articles

Microsecond Motion of the Bacterial Transporter EmrE in Lipid Bilayers.

The bacterial transporter EmrE is a homo-dimeric membrane protein that effluxes cationic polyaromatic substrates against the concentration gradient by coupling to proton transport. As the archetype of the small multidrug resistance family of transporters, EmrE structure and dynamics provide atomic insights into the mechanism of transport by this family of proteins. We recently determined high-resolution structures of EmrE in complex with a cationic substrate, tetra(4-fluorophenyl)phosphonium (F4-TPP+), using solid-state NMR spectroscopy and an S64V-EmrE mutant. The substrate-bound protein exhibits distinct structures at acidic and basic pH, reflecting changes upon binding or release of a proton from residue E14, respectively. To obtain insight into the protein dynamics that mediate substrate transport, here we measure 15N rotating-frame spin-lattice relaxation (R1ρ) rates of F4-TPP+-bound S64V-EmrE in lipid bilayers under magic-angle spinning (MAS). Using perdeuterated and back-exchanged protein and 1H-detected 15N spin-lock experiments under 55 kHz MAS, we measured 15N R1ρ rates site-specifically. Many residues show spin-lock field-dependent 15N R1ρ relaxation rates. This relaxation dispersion indicates the presence of backbone motions at a rate of about 6000 s-1 at 280 K for the protein at both acidic and basic pH. This motional rate is 3 orders of magnitude faster than the alternating access rate but is within the range estimated for substrate binding. We propose that these microsecond motions may allow EmrE to sample different conformations to facilitate substrate binding and release from the transport pore.

Solid-state NMR study of spin dynamics and local disorder in smart polymers: PDMAEMA

The solid-state 1H and 13C NMR spectra as well as the 1H–13C cross-polarization upon magic angle spinning (CP MAS) kinetics were studied for poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), i.e. a smart pH- and thermo-responsive polymer. The stereochemical content of PDMAEMA was determined from the complex shaped 13C MAS signal of CH3 group. The kinetic data were processed using the Hirschinger and Raya spin dynamics model that includes the complete scheme of rotating frame spin-lattice relaxation pathways. The general solution was adapted for the spin cluster treatment. The earlier studied experimental CP MAS kinetics data of poly [2-(methacryloyloxy)ethyl trimethylammonium chloride] (PMETAC), i.e. one of its quaternized form, were revisited and newly processed applying this model. The spin-lattice relaxation of protons in the rotating frame in PDMAEMA and PMETAC occurs in the same scale from one to tens of milliseconds. Very high anisotropy of spin-diffusion was found for both polymers. However, the local disorder of various spin sites in PDMAEMA is significantly higher than in PMETAC. It is characterized by the order parameters 0.71–0.77 and 0.87–0.91, respectively. The main chain in PDMAEMA is also more disordered and more flexible than in PMETAC.

Exploring Molecular Dynamics of Adsorbed CO2 Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation.

Previous studies on CO2 adsorbents have mainly addressed the identification and quantification of adsorbed CO2 species in amine-modified porous materials. Investigation of molecular motion of CO2 species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO2 species chemi- and physisorbed at amine-modified silica materials through the determination of the rotating frame spin–lattice relaxation times (T1ρ) by solid-state NMR. Rotational correlation times (τC) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen–Purcell–Pound theories. As expected, the τC values for the two physisorbed CO2 species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO2 species (162, 62, and 123 μs). The differences in molecular dynamics between the different chemisorbed species correlate well with the structures previously proposed. In the case of the physisorbed CO2 species, the τC values of the CO2 species displaying faster molecular dynamics falls in the range of viscous liquids, whereas the species presenting slower dynamics exhibit T1ρ and τC values compatible with a CO2 layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and 1H–13C heteronuclear dipolar couplings have also been estimated from T1ρ measurements, for each adsorbed CO2 species. The CSA tensor parameters obtained from fitting the relaxation data agree with the experimentally measured CSA values, thus showing that the theories are well suited to study CO2 dynamics in silica surfaces.

Solid-State NMR and Impedance Spectroscopy Study of Spin Dynamics in Proton-Conducting Polymers: An Application of Anisotropic Relaxing Model.

The 1H–13C cross-polarization (CP) kinetics in poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) was studied under moderate (10 kHz) magic-angle spinning (MAS). To elucidate the role of adsorbed water in spin diffusion and proton conductivity, PMETAC was degassed under vacuum. The CP MAS results were processed by applying the anisotropic Naito and McDowell spin dynamics model, which includes the complete scheme of the rotating frame spin–lattice relaxation pathways. Some earlier studied proton-conducting and nonconducting polymers were added to the analysis in order to prove the capability of the used approach and to get more general conclusions. The spin-diffusion rate constant, which describes the damping of the coherences, was found to be strongly depending on the dipolar I–S coupling constant (DIS). The spin diffusion, associated with the incoherent thermal equilibration with the bath, was found to be most probably independent of DIS. It was deduced that the drying scarcely influences the spin-diffusion rates; however, it significantly (1 order of magnitude) reduces the rotating frame spin–lattice relaxation times. The drying causes the polymer hardening that reflects the changes of the local order parameters. The impedance spectroscopy was applied to study proton conductivity. The activation energies for dielectric relaxation and proton conductivity were determined, and the vehicle-type conductivity mechanism was accepted. The spin-diffusion processes occur on the microsecond scale and are one order faster than the dielectric relaxation. The possibility to determine the proton location in the H-bonded structures in powders using CP MAS technique is discussed.

UV Pretreatment Impairs the Enzymatic Degradation of Polyethylene Terephthalate.

The biocatalytic degradation of polyethylene terephthalate (PET) emerged recently as a promising alternative plastic recycling method. However, limited activity of previously known enzymes against post-consumer PET materials still prevents the application on an industrial scale. In this study, the influence of ultraviolet (UV) irradiation as a potential pretreatment method for the enzymatic degradation of PET was investigated. Attenuated total reflection Fourier transform infrared (ATR-FTIR) and 1H solution nuclear magnetic resonance (NMR) analysis indicated a shortening of the polymer chains of UV-treated PET due to intra-chain scissions. The degradation of UV-treated PET films by a polyester hydrolase resulted in significantly lower weight losses compared to the untreated sample. We also examined site-specific and segmental chain dynamics over a time scale of sub-microseconds to seconds using centerband-only detection of exchange, rotating-frame spin-lattice relaxation (T1ρ), and dipolar chemical shift correlation experiments which revealed an overall increase in the chain rigidity of the UV-treated sample. The observed dynamic changes are most likely associated with the increased crystallinity of the surface, where a decreased accessibility for the enzyme-catalyzed hydrolysis was found. Moreover, our NMR study provided further knowledge on how polymer chain conformation and dynamics of PET can mechanistically influence the enzymatic degradation.

Ion Dynamics in Al-Doped Cubic Li7La3Zr2O12 Garnet-Type Single Crystals

Li7La3Zr2O12 (LLZO) garnet-type materials are regarded as promising candidates for solid electrolytes and have been extensively investigated in the past few years.[1,2] Several studies report on an increase in conductivity by doping with ions to stabilize the cubic structure.[3,4] In the present study we focus on the ionic transport processes in Al-doped LLZO single crystals. To investigate ionic motions we used 7Li nuclear magnetic resonance (NMR) spectroscopy and impedance spectroscopy. With the help of rotating-frame spin-lock NMR spin-lattice relaxation (SLR) we looked at localized and medium-ranged ionic motions, respectively. Impedance spectroscopy was used to shed light on the long range ion dynamics. At room temperature the total conductivity turned out to be 0.082 mS cm- 1 at, which is remarkably good for LLZO with an Al-content of only 0.34 wt%.[5] Most importantly, 7Li NMR spin-lock NMR of revealed two overlapping diffusion-induced processes that share almost the same activation energy. A similar behavior has recently been seen for Mo-bearing LLZO.[6] Overall, activation energies gathered from spin-lock NMR and impedance measurements are in good agreement with each other; both techniques yield values around 0.36 eV. The solid-state coefficient (D) and the self-diffusion coefficient (Dsd), calculated from impedance spectroscopy and NMR spin-lock SLR, respectively, almost coincide. This excellent agreement points out that the two methods are sensitive to very similar motional correlations functions probing magnetic spin fluctuations and electrical relaxation processes, respectively.

Structural Polymorphism of Alzheimer's β-Amyloid Fibrils as Controlled by an E22 Switch: A Solid-State NMR Study.

The amyloid-β (Aβ) peptide of Alzheimer's disease (AD) forms polymorphic fibrils on the micrometer and molecular scales. Various fibril growth conditions have been identified to cause polymorphism, but the intrinsic amino acid sequence basis for this polymorphism has been unclear. Several single-site mutations in the center of the Aβ sequence cause different disease phenotypes and fibrillization properties. The E22G (Arctic) mutant is found in familial AD and forms protofibrils more rapidly than wild-type Aβ. Here, we use solid-state NMR spectroscopy to investigate the structure, dynamics, hydration and morphology of Arctic E22G Aβ40 fibrils. (13)C, (15)N-labeled synthetic E22G Aβ40 peptides are studied and compared with wild-type and Osaka E22Δ Aβ40 fibrils. Under the same fibrillization conditions, Arctic Aβ40 exhibits a high degree of polymorphism, showing at least four sets of NMR chemical shifts for various residues, while the Osaka and wild-type Aβ40 fibrils show a single or a predominant set of chemical shifts. Thus, structural polymorphism is intrinsic to the Arctic E22G Aβ40 sequence. Chemical shifts and inter-residue contacts obtained from 2D correlation spectra indicate that one of the major Arctic conformers has surprisingly high structural similarity with wild-type Aβ42. (13)C-(1)H dipolar order parameters, (1)H rotating-frame spin-lattice relaxation times and water-to-protein spin diffusion experiments reveal substantial differences in the dynamics and hydration of Arctic, Osaka and wild-type Aβ40 fibrils. Together, these results strongly suggest that electrostatic interactions in the center of the Aβ peptide sequence play a crucial role in the three-dimensional fold of the fibrils, and by inference, fibril-induced neuronal toxicity and AD pathogenesis.

2H nuclear magnetic resonance study of deuterated water dynamics in perfluorosulfonic acid ionomer Nafion

We have employed deuteron nuclear magnetic resonance (NMR) spectroscopy in order to study the dynamics of the deuterated water (D2O) molecules introduced into a perfluorosulfonic acid ionomer Nafion (NR-211) film. According to the 2H NMR spectral analysis, the deuterated water molecules at low temperatures occupied either relatively rigid or mobile sites up to the temperature TM=240K where all the deuterated water molecules became mobile. The temperature-dependent NMR linewidths sensitively reflected the motional narrowing of the rigid and mobile sites, and the NMR chemical shift reflected significant changes in the hydrogen bonds of the deuterated water. While a slow- to fast-limit motional transition was manifested at TM in the laboratory-frame NMR spin–lattice relaxation, the rotating-frame spin–lattice relaxation indicated no bulk liquid water state down to 200K.

Interchannel Hopping in Single Crystalline Lithium Triborate Probed by 7Li NMR: Spin Relaxation, Line Shape Analysis, Selective-Inversion Spin Alignment, and Two-Dimensional Exchange Spectra

The ion dynamics of single crystalline lithium triborate, LiB3O5, an important material in nonlinear optics, is studied using various 7Li and 11B nuclear magnetic resonance (NMR) techniques at temperatures from about 480 to 780 K in order to elucidate the apparent discrepancies underlying previous interpretations of NMR line shape analyses and results from dielectric spectroscopy. Rotating frame spin–lattice relaxation as well as line shape measurements are carried out and are combined with selective-inversion spin alignment as well as two-dimensional chemical exchange spectroscopy to track the temperature-dependent Li ion motion. From symmetry considerations the latter is clearly identified as interchannel hopping. By combining the present results with those from the published, yet so far not fully analyzed, dielectric loss spectra, it is shown how seeming differences in energy barriers hindering the ion motion and in the evolution of the distribution of correlation times can be reconciled.

NMR Observation of Mobile Protons in Proton-Implanted ZnO Nanorods.

The diffusion properties of H+ in ZnO nanorods are investigated before and after 20 MeV proton beam irradiation by using 1H nuclear magnetic resonance (NMR) spectroscopy. Herein, we unambiguously observe that the implanted protons occupy thermally unstable site of ZnO, giving rise to a narrow NMR line at 4.1 ppm. The activation barrier of the implanted protons was found to be 0.46 eV by means of the rotating-frame spin-lattice relaxation measurements, apparently being interstitial hydrogens. High-energy beam irradiation also leads to correlated jump diffusion of the surface hydroxyl group of multiple lines at ~1 ppm, implying the presence of structural disorder at the ZnO surface.

Capturing fast relaxing spins with SWIFT adiabatic rotating frame spin-lattice relaxation (T1ρ) mapping.

Rotating frame spin-lattice relaxation, with the characteristic time constant T1ρ, provides a means to access motion-restricted (slow) spin dynamics in MRI. As a result of their restricted motion, these spins are sometimes characterized by a short transverse relaxation time constant T2 and thus can be difficult to detect directly with conventional image acquisition techniques. Here, we introduce an approach for three-dimensional adiabatic T1ρ mapping based on a magnetization-prepared sweep imaging with Fourier transformation (MP-SWIFT) sequence, which captures signal from almost all water spin populations, including the extremely fast relaxing pool. A semi-analytical procedure for T1ρ mapping is described. Experiments on phantoms and musculoskeletal tissue specimens (tendon, articular and epiphyseal cartilages) were performed at 9.4 T for both the MP-SWIFT and fast spin echo (FSE) read outs. In the phantom with liquids having fast molecular tumbling and a single-valued T1ρ time constant, the measured T1ρ values obtained with MP-SWIFT and FSE were similar. Conversely, in normal musculoskeletal tissues, T1ρ values measured with MP-SWIFT were much shorter than the values obtained with FSE. Studies of biological tissue specimens demonstrated that T1ρ-weighted SWIFT provides higher contrast between normal and diseased tissues relative to conventional acquisitions. Adiabatic T1ρ mapping with SWIFT readout captures contributions from the otherwise undetected fast relaxing spins, allowing more informative T1ρ measurements of normal and diseased states.

1H nuclear magnetic resonance study of hydrated water dynamics in perfluorosulfonic acid ionomer Nafion

We have studied the dynamics of hydrated water molecules in the proton exchange membrane of Nafion by means of high-resolution 1H nuclear magnetic resonance (NMR) measurements. “Bound” and “free” states of hydrated water clusters as well as the exchange protons were identified from the NMR chemical shift measurements, and their activation energies were obtained from the temperature-dependent laboratory- and rotating-frame spin-lattice relaxation measurements. Besides, a peculiar motional transition in the ultralow frequency region was observed at 373 K for the “free” hydrated water from the rotating-frame NMR spin-lattice relaxation time measurements.

Effects of diffusion in magnetically inhomogeneous media on rotating frame spin–lattice relaxation

In an aqueous medium containing magnetic inhomogeneities, diffusion amongst the intrinsic susceptibility gradients contributes to the relaxation rate R1ρ of water protons to a degree that depends on the magnitude of the local field variations ΔBz, the geometry of the perturbers inducing these fields, and the rate of diffusion of water, D. This contribution can be reduced by using stronger locking fields, leading to a dispersion in R1ρ that can be analyzed to derive quantitative characteristics of the material. A theoretical expression was recently derived to describe these effects for the case of sinusoidal local field variations of a well-defined spatial frequency q. To evaluate the degree to which this dispersion may be extended to more realistic field patterns, finite difference Bloch–McConnell simulations were performed with a variety of three-dimensional structures to reveal how simple geometries affect the dispersion of spin-locking measurements. Dispersions were fit to the recently derived expression to obtain an estimate of the correlation time of the field variations experienced by the spins, and from this the mean squared gradient and an effective spatial frequency were obtained to describe the fields. This effective spatial frequency was shown to vary directly with the second moment of the spatial frequency power spectrum of the ΔBz field, which is a measure of the average spatial dimension of the field variations. These results suggest the theory may be more generally applied to more complex media to derive useful descriptors of the nature of field inhomogeneities. The simulation results also confirm that such diffusion effects disperse over a range of locking fields of lower amplitude than typical chemical exchange effects, and should be detectable in a variety of magnetically inhomogeneous media including regions of dense microvasculature within biological tissues.

Rotating-frame nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4

The proton and phosphorus dynamics of the superprotonic conduction in the LiH2PO4 system has been studied by means of 1H and 31P NMR rotating-frame nuclear magnetic resonance (NMR). The temperature-dependent 1H NMR rotating-frame spin–lattice relaxation time measurements revealed in a sensitive manner the evolution of the microscopic environment and proton dynamics of the two distinct hydrogen-bond types in the system, which is quite compatible with the electrical conductivity and the impedance spectroscopy measurements. Besides, the proton dynamics was found to be coupled to the reorientational motion of the PO4 tetrahedra.

Rotating-frame nuclear magnetic resonance study of the distinct dynamics of hydrogen donors in ZnO

The rotating-frame spin-lattice relaxation of two types of the hydrogen donors was well distinguished in the 1H nuclear magnetic resonance measurements in a sol-gel prepared ZnO system, providing a unique opportunity to study the distinct proton dynamics. Our study indicates interconversion of the interstitial H (Hi). The population of the mobile Hi showed decrease above ∼370 K, apparently being trapping into the oxygen vacancies resulting in the more stable oxygen-substitutional H (HO). The activation barrier for migration of Hi and the binding energy of HO were found to be 0.27 eV and 0.51 eV, respectively.

Long-range Li+ dynamics in the lithium argyrodite Li7PSe6 as probed by rotating-frame spin–lattice relaxation NMR

Lithium-rich argyrodites belong to a relatively new group of fast ion conducting solids. They might serve as powerful electrolytes in all-solid-state lithium-ion batteries being, from a medium-term point of view, the key technology when safe energy storage systems have to be developed. Spin-lattice relaxation (SLR) nuclear magnetic resonance (NMR) measurements carried out in the rotating frame of reference turned out to be the method of choice to study Li dynamics in argyrodites. When plotted as a function of the inverse temperature, the SLR rates log10(R1ρ) reveal an asymmetric diffusion-induced rate peak. The rate peak contains information on the Li jump rate, the activation energy of the hopping process as well as correlation effects. In particular, considering the high-temperature flank of the SLR NMR rate peak recorded in the rotating frame of reference, an activation energy of approximately 0.49 eV is found. This value represents long-range lithium jump diffusion in crystalline Li7PSe6. As an example, at 325 K the Li jump rate determined from SLR NMR is in the order of 1.4 × 10(5) s(-1). The pronounced asymmetry of the rate peak R1ρ(1/T) points to correlated Li motion. It is comparable to that which is typically found for structurally disordered materials showing a broad range of correlation times.

Modified lipid and protein dynamics in nanodiscs

For membrane protein studies, nanodiscs have been shown to hold great potential in terms of preparing soluble samples while maintaining a lipid environment. Here, we describe the differences in lipid order and protein dynamics in MSP1 nanodiscs compared to lamellar preparations by solid-state NMR. For DMPC, an increase of the dipolar C-H lipid acyl chain order parameters in nanodiscs is observed in both gel- and liquid crystalline phases. Incorporating proteorhodopsin in these nanodiscs resulted in a significantly longer rotating frame spin-lattice relaxation time for 13C leerzeichen and better cross polarisation efficiency due to restricted protein dynamics. A comparison of 13C–13C correlation spectra revealed no structural differences. The incorporation of proteorhodopsin into nanodiscs has been optimised with respect to detergent and to protein/scaffold protein/lipid stoichiometries. Its functional state was probed by time-resolved optical spectroscopy revealing only minor differences between lamellar and nanodisc preparations. Our observations show remarkable dynamic effects between membrane proteins, lipids and scaffold protein. The potential use of nanodiscs for solid-state NMR applications is discussed.

Ex situ NMR Measurements of Li Dynamics in TiO2 Anodes with an Ordered Hierarchical Pore Structure

Materials with nanometer-sized dimensions turned out to show a number of advantages significantly improving the performance of Li-ion batteries.[1] For instance, short diffusion length of nanostructured electrode materials lead to facile Li insertion. The mesoporous morphology of the present sample investigated ensures a high lithium storage capacity. Even after massive cycling the mesoporous structure is preserved throughout illustrating the high stability of the electrode material.[2] Here, we used complementary 7Li NMR techniques to study the self-diffusion behavior of Li+ in LixTiO2 samples at different state of charges. Both 7Li spin-alignment echo NMR and rotating-frame 7Li spin-lattice relaxation NMR measurements reveal that, despite some slight structural changes during Li insertion,[2] the Li diffusion parameters do not strongly depend on composition, see also Ref. [3]. This might also contribute to the excellent cycleability observed recently.[2]

Multidimensional solid‐state NMR studies of the structure and dynamics of pectic polysaccharides in uniformly 13C‐labeled Arabidopsis primary cell walls

Plant cell wall (CW) polysaccharides are responsible for the mechanical strength and growth of plant cells; however, the high-resolution structure and dynamics of the CW polysaccharides are still poorly understood because of the insoluble nature of these molecules. Here, we use 2D and 3D magic-angle-spinning (MAS) solid-state NMR (SSNMR) to investigate the structural role of pectins in the plant CW. Intact and partially depectinated primary CWs of Arabidopsis thaliana were uniformly labeled with (13)C and their NMR spectra were compared. Recent (13)C resonance assignment of the major polysaccharides in Arabidopsis thaliana CWs allowed us to determine the effects of depectination on the intermolecular packing and dynamics of the remaining wall polysaccharides. 2D and 3D correlation spectra show the suppression of pectin signals, confirming partial pectin removal by chelating agents and sodium carbonate. Importantly, higher cross peaks are observed in 2D and 3D (13)C spectra of the depectinated CW, suggesting higher rigidity and denser packing of the remaining wall polysaccharides compared with the intact CW. (13)C spin-lattice relaxation times and (1)H rotating-frame spin-lattice relaxation times indicate that the polysaccharides are more rigid on both the nanosecond and microsecond timescales in the depectinated CW. Taken together, these results indicate that pectic polysaccharides are highly dynamic and endow the polysaccharide network of the primary CW with mobility and flexibility, which may be important for pectin functions. This study demonstrates the capability of multidimensional SSNMR to determine the intermolecular interactions and dynamic structures of complex plant materials under near-native conditions.

High-resolution solid-state NMR study of isotactic polypropylenes

The high-resolution solid-state 13 C NMR spectra were recorded for metallocene (m) and Ziegler-Natta (ZN) iso- tactic polypropylenes (iPP) in pelletized form using cross polarization (CP) and magic angle spinning (MAS) techniques within the temperature range of 20-160°C. Besides the CP MAS experiments also the MAS 13 C NMR spectra (without CP), MAS 1 H NMR spectra and rotating frame spin-lattice relaxation times T1 ( 13 C) were measured at elevated temperatures. With the rise of temperature the splitting of CH2, CH and CH3 signals into two components was detected in 13 C NMR spec- tra and assigned to amorphous and crystalline phases. The temperature dependences of chemical shifts and integral intensi- ties obtained from the deconvoluted spectra provided information on the main chain and CH3 groups motions in amorphous and crystalline regions of studied samples. While T1 ( 13 C) values show that the rate of segmental motion in amorphous regions in m-iPP and ZN-iPP is virtually the same, larger linewidths in 13 C and 1 H NMR spectra indicate somewhat larger restraints of the motion in amorphous regions of ZN-iPP.