Pulsed NMR Techniques Research Articles

ATP‐Independent Turnover of Dinitrogen Intermediates Captured on the Nitrogenase Cofactor

AbstractNitrogenase reduces N2 to NH3 at its active‐site cofactor. Previous studies of an N2‐bound Mo‐nitrogenase from Azotobacter vinelandii suggest binding of three N2 species via asymmetric belt‐sulfur displacements in the two cofactors of its catalytic component (designated Av1*), leading to the proposal of stepwise N2 reduction involving all cofactor belt‐sulfur sites; yet, the evidence for the existence of multiple N2 species on Av1* remains elusive. Here we report a study of ATP‐independent, EuII/SO32−‐driven turnover of Av1* using GC‐MS and frequency‐selective pulse NMR techniques. Our data demonstrate incorporation of D2‐derived D by Av1* into the products of C2H2‐ and H+‐reduction, and decreased formation of NH3 by Av1* concomitant with the release of N2 under H2; moreover, they reveal a strict dependence of these activities on SO32−. These observations point to the presence of distinct N2 species on Av1*, thereby providing strong support for our proposed mechanism of stepwise reduction of N2 via belt‐sulfur mobilization.

ATP-Independent Turnover of Dinitrogen Intermediates Captured on the Nitrogenase Cofactor.

Nitrogenase reduces N2 to NH3 at its active-site cofactor. Previous studies of an N2-bound Mo-nitrogenase from Azotobacter vinelandii suggest binding of three N2 species via asymmetric belt-sulfur displacements in the two cofactors of its catalytic component (designated Av1*), leading to the proposal of stepwise N2 reduction involving all cofactor belt-sulfur sites; yet, the evidence for the existence of multiple N2 species on Av1* remains elusive. Here we report a study of ATP-independent, EuII/SO3 2--driven turnover of Av1* using GC-MS and frequency-selective pulse NMR techniques. Our data demonstrate incorporation of D2-derived D by Av1* into the products of C2H2- and H+-reduction, and decreased formation of NH3 by Av1* concomitant with the release of N2 under H2; moreover, they reveal a strict dependence of these activities on SO3 2-. These observations point to the presence of distinct N2 species on Av1*, thereby providing strong support for our proposed mechanism of stepwise reduction of N2 via belt-sulfur mobilization.

Mobility of Li+, Na+, and Cs+ cations in Nafion membrane, as studied by NMR techniques

The local mobility and diffusion of Li+, Na+, and Cs+ cations in Nafion 117 membrane were explored by 7Li, 23Na, and 133Cs spin relaxation and pulsed field gradient NMR techniques. It was shown that the macroscopic mass transfer of cations is controlled by ion motion near sulfonate groups. Lithium and sodium cations, whose hydrated energy is higher than the water hydrogen bond energy, are moving together with water molecules, but cesium cations possessing a low hydrated energy are jumping directly between the neighboring sulfonate groups.

Possibility of Measurement of the Three-Spin Dipolar Contribution to Nuclear Spin–Lattice Relaxation in Solids by the Conventional Pulse NMR Technique

Possibility of Measurement of the Three-Spin Dipolar Contribution to Nuclear Spin–Lattice Relaxation in Solids by the Conventional Pulse NMR Technique

Ion and Molecular Transport in Solid Electrolytes Studied by NMR.

NMR is the method of choice for molecular and ionic structures and dynamics investigations. The present review is devoted to solvation and mobilities in solid electrolytes, such as ion-exchange membranes and composite materials, based on cesium acid sulfates and phosphates. The applications of high-resolution NMR, solid-state NMR, NMR relaxation, and pulsed field gradient 1H, 7Li, 13C, 19F, 23Na, 31P, and 133Cs NMR techniques are discussed. The main attention is paid to the transport channel morphology, ionic hydration, charge group and mobile ion interaction, and translation ions and solvent mobilities in different spatial scales. Self-diffusion coefficients of protons and Li+, Na+, and Cs+ cations are compared with the ionic conductivity data. The microscopic ionic transfer mechanism is discussed.

NMR spectroscopy of lipidic cubic phases.

Lipidic cubic phase (LCP) structures have been used for stabilisation and crystallisation of membrane proteins and show promising properties as drug carriers. In this mini-review, we present how NMR spectroscopy has played a major role in understanding the physico-chemical properties of LCPs and how recent advances in pulsed field gradient NMR techniques open new perspectives in characterising encapsulated molecules.

Molecular and Ionic Diffusion in Ion Exchange Membranes and Biological Systems (Cells and Proteins) Studied by NMR.

The results of NMR, and especially pulsed field gradient NMR (PFG NMR) investigations, are summarized. Pulsed field gradient NMR technique makes it possible to investigate directly the partial self-diffusion processes in spatial scales from tenth micron to millimeters. Modern NMR spectrometer diffusive units enable to measure self-diffusion coefficients from 10−13 m2/s to 10−8 m2/s in different materials on 1 H, 2 H, 7 Li, 13 C, 19 F, 23 Na, 31 P, 133 Cs nuclei. PFG NMR became the method of choice for reveals of transport mechanism in polymeric electrolytes for lithium batteries and fuel cells. Second wide field of application this technique is the exchange processes and lateral diffusion in biological cells as well as molecular association of proteins. In this case a permeability, cell size, and associate lifetime could be estimated. The authors have presented the review of their research carried out in Karpov Institute of Physical Chemistry, Moscow, Russia; Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia; Kazan Federal University, Kazan, Russia; Korea University, Seoul, South Korea; Yokohama National University, Yokohama, Japan. The results of water molecule and Li+, Na+, Cs+ cation self-diffusion in Nafion membranes and membranes based on sulfonated polystyrene, water (and water soluble) fullerene derivative permeability in RBC, casein molecule association have being discussed.

Hydration, self-diffusion and ionic conductivity of Li+, Na+ and Cs+ cations in Nafion membrane studied by NMR

Hydration of Nafion 117 perfluorinated sulfonic cation-exchange membrane in alkaline ion forms was investigated by high resolution 1H NMR. Hydration numbers of Li+, Na+ and Cs+ cations were 5 ± 1, 6 ± 1 and 1 ± 0.2, correspondingly for membrane equilibrated with water vapor at 98% RH. As opposed to Li+ and Na+, which form separate ion pair, Cs+ cation directly contacts with membrane sulfonate group. Cation self-diffusion coefficients were measured by pulsed field gradient NMR technique on 7Li, 23Na and 133Cs nuclei for the first time. Self-diffusion coefficients are changed in the next rows Li+ ≤ Na+ > Cs+. Self-diffusion activation energies of Li+ and Na+ cations are about 20 kJ/mol which is close to water self-diffusion activation energy in these membranes, but Cs+ self-diffusion activation energy is distinctly more (25 kJ/mol). Ionic conductivities calculated on the basis of Nernst–Einstein equation from cation self-diffusion coefficients 1.6∙10−2, 2∙10−2, 6∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, are closely approximating to conductivities measured by impedance spectroscopy: 1.3∙10−2, 1.1∙10−2, 2.3∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, but calculated values are appreciably more compared with experimental meanings.

A Practical Review of NMR Lineshapes for Spin-1/2 and Quadrupolar Nuclei in Disordered Materials.

NMR is a powerful spectroscopic method that can provide information on the structural disorder in solids, complementing scattering and diffraction techniques. The structural disorder in solids can generate a dispersion of local magnetic and electric fields, resulting in a distribution of isotropic chemical shift δiso and quadrupolar coupling CQ. For spin-1/2 nuclei, the NMR linewidth and shape under high-resolution magic-angle spinning (MAS) reflects the distributions of isotropic chemical shift, providing a rich source of disorder information. For quadrupolar nuclei, the second-order quadrupolar broadening remains present even under MAS. In addition to isotropic chemical shift, structural disorder can impact the electric field gradient (EFG) and consequently the quadrupolar NMR parameters. The distributions of quadrupolar coupling and isotropic chemical shift are superimposed with the second-order quadrupolar broadening, but can be potentially characterized by MQMAS (multiple-quantum magic-angle spinning) spectroscopy. We review analyses of NMR lineshapes in 2D DQ–SQ (double-quantum single-quantum) and MQMAS spectroscopies, to provide a guide for more general lineshape analysis. In addition, methods to enhance the spectral resolution and sensitivity for quadrupolar nuclei are discussed, including NMR pulse techniques and the application of high magnetic fields. The role of magnetic field strength and its impact on the strategy of determining optimum NMR methods for disorder characterization are also discussed.

NMR Studies of the Dynamics of 1D $$^3\hbox {He}$$ in $$^4\hbox {He}$$ Plated MCM-41

Pulsed NMR techniques have been used to study the dynamics of $$^3\hbox {He}$$ confined to the interior of the hexagonal nanochannels of MCM-41 for which the walls were coated with a monolayer of $$^4\hbox {He}$$ as determined by isotherm measurements. The $$^3\hbox {He}$$ was added afterward to form a 1D $$^3\hbox {He}$$ line density of about 0.1 $$\hbox {A}^{-1}$$ , corresponding to a Fermi temperature of $$T_{\mathrm{F}} \sim$$ 120 mK. A distinct and appreciable departure from the Curie law was observed for the nuclear spin magnetization below 0.5 K. The temperature dependence of the nuclear spin–lattice relaxation times, $$T_1$$ , for temperatures $$0.05<T<2.5$$ K, followed the expected linear behavior at low temperatures, and a peak was observed at $$T\sim 2T_{\mathrm{F}}$$ consistent with the Luttinger liquid theory as predicted by Polini et al. (Phys Rev Lett 98:266403, 2007). The observed temperature dependence of the nuclear spin–spin relaxation times, $$T_2$$ , differed considerably from that observed for $$T_1$$ , with a minimum at $$T= 0.8$$ K, similar to the tendency reported by Matsushita and colleagues.

Investigation of Non-Thermodynamical CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Adsorption Behavior for Amine-Modified Nanoporous Silica

Non-thermodynamical CO2 adsorption behavior for amine-modified nanoporous silica is clarified by evaluating the mobility of organic functional group inside mesopores by using pulsed NMR technique. CO2 adsorption behavior of nanoporous silica modified with amino-propyl silane (AP) changes significantly depending on the amount of AP loaded. A low AP loaded sample shows normal adsorption behavior; the amount of CO2 adsorbed decreases with increasing temperature. In contrast, a high AP loaded sample possesses non-thermodynamic CO2 adsorption behavior in which the amount of CO2 adsorbed increases with increasing temperature in within a certain temperature range. To address the mechanism, a pulsed NMR technique was employed to clarify the mobility of AP molecules, and it was found that the mobility of mobile components in a high AP loaded sample increased drastically with increasing temperature while the mobility in a low AP loaded sample remained unchanged. It is understood that the enhancement of the diffusion of CO2 inside nanopores leads to the non-thermodynamic adsorption behavior.

Improving the methodology of sample preparation of sunflower seeds when determining the content of oleic acid by NMR

Developing modern instrumental express methods for monitoring quality parameters requires special attention to the comparability of measurement results with reference methods. This article includes the result of improving the method for determining oleic acid content in sunflower seed oil using pulsed NMR technique. In the conditions of industrial laboratories, different seed quality and temperature have the greatest influence on the accuracy of measuring oleic acid content. It is possible to reduce the influence of different quality by increasing the number of simultaneously analyzed samples. A method for preliminary SHF treatment of seeds was proposed that could significantly reduce the time of seed sample preparation with the temperature below 15°C. Due to the special characteristics of SHF treatment, seeds are heated under certain parameters to the temperature of 20-25°C without loss of moisture contained in the analyzed sample what is especially important for the comprehensive determination of quality parameters of sunflower seeds using pulsed NMR method. To eliminate temperature influence, it was proposed to measure the temperature of each analyzed sample before the analysis. This allowed significant increasing the accuracy of measuring oleic acid content. These solutions were implemented in a sampling device that made a sample with a volume of 25 cm3 and at the same time measured its temperature.

Effect of Temperature and Water Content on the Properties of Solid Protic Electrolyte of Tetra Calixarene Sulfonic Acid: NMR Study

The methods of 1H NMR and pulsed field gradient NMR are used to study the protic solid electrolyte of calix[4]arene-para-sulfonic acid in the temperature range of –15 to 24°C and water content λ of 1.6 to 5.7 H2O molecules per SO3H. Analysis of the 1H signal intensity in NMR spectra showed that no ice phase is formed in the whole studied temperature and water content range. The hydration numbers of H+(H2O)h complexes are calculated on the basis of the temperature dependences of chemical shift values. The pulsed field gradient NMR technique is used to determine self-diffusion coefficients. Activation energies Eact are calculated on the basis of the temperature dependences of self-diffusion coefficients. Good agreement of the data on diffusion Eact and protic conductivity are shown. A corollary of the Nernst–Einstein equation is used to calculate the values of protic conductivity on the basis of self-diffusion data. The calculated values of protic conductivity in the water content λ range of 1.6 to 5.7 agree with the data obtained experimentally.

Self-diffusion of water-soluble fullerene derivatives in mouse erythrocytes

Self-diffusion of water-soluble fullerene derivative (WSFD) C60[S(CH2)3SO3Na]5H in mouse red blood cells (RBC) was characterized by 1H pulsed field gradient NMR technique. It was found that a fraction of fullerene molecules (~13% of the fullerene derivative added in aqueous RBC suspension) shows a self-diffusion coefficient of (5.5 ± 0.8)·10−12 m2/s, which is matching the coefficient of the lateral diffusion of lipids in the erythrocyte membrane (DL = (5.4 ± 0.8)·10−12 m2/s). This experimental finding evidences the absorption of the fullerene derivative by RBC. Fullerene derivative molecules are also absorbed by RBC ghosts and phosphatidylcholine liposomes as manifested in self-diffusion coefficients of (7.9 ± 1.2)·10−12 m2/s and (7.7 ± 1.2)·10−12 m2/s, which are also close to the lateral diffusion coefficients of (6.5 ± 1.0)·10−12 m2/s and (8.5 ± 1.3)·10−12 m2/s, respectively. The obtained results suggest that fullerene derivative molecules are, probably, fixed on the RBC surface. The average residence time of the fullerene derivative molecule on RBC was estimated as 440 ± 70 ms. Thus, the pulsed field gradient NMR was shown to be a versatile technique for investigation of the interactions of the fullerene derivatives with blood cells providing essential information, which can be projected on their behavior in-vivo after intravenous administration while screening as potential drug candidates.

Intrinsic Proton NMR Studies of Mg(OH)2 and Ca(OH)2

We studied the short proton free induction decay signals and the broad ^1H NMR spectra of Mg(OH)2 and Ca(OH)2 powders at 77-355 K and 42 MHz using pulsed NMR techniques. Using a Gaussian-type back extrapolation procedure for the obscured data of the proton free induction decay signals, we obtained more precise values of the second moments of the Fourier-transformed broad NMR spectra than those in a previous report [Y. Itoh and M. Isobe, J. Phys. Soc. Jpn. Vol. 84, 113601 (2015)] and compared with the theoretical second moments. The decrease in the second moment could not account for the large decrease in the magnitude of the intrinsic proton spin-lattice relaxation rate 1/T_1 from Mg(OH)2 to Ca(OH)2. The analysis of 1/T_1 $propto$ exp(-E_g/k_BT) with E_g~0.01 eV points to a local hopping mechanism, and that of 1/T_1 $propto$ T^n with n~0.5 points to an anharmonic rattling mechanism.

Extrinsic Proton NMR Studies of Mg(OH)2 and Ca(OH)2

We studied narrow $^1$H NMR spectra of Mg(OH)$_2$ and Ca(OH)$_2$ powders at 100$-$355 K and 42$-$59 MHz using pulsed NMR techniques. The Fourier transformed NMR spectra of the proton free-induction signals show the superposition of broad and narrow components, which can be assigned to immobile protons and extrinsic mobile protons, respectively. We found that a narrow spectrum develops on heating above about $T_c$ = 260 K and widens above a Larmor frequency of about $f_c$ = 50 MHz for Mg(OH)$_2$. The temperature-induced NMR spectrum and the characteristic frequency $f_c$ of 50 MHz are the noteworthy features of the nuclear spin fluctuation spectra of the extrinsic protons.

Self-organization of a water-soluble fullerene derivative studied by pulsed field gradient NMR spectroscopy

The self-organization of a water-soluble sulfur-containing fullerene adduct with captopril in deuterated water was studied by a pulsed field gradient NMR technique. Two types of fullerene particles characterized by the self-diffusion coefficients Ds1 and Ds2 of (4.0±0.5)×10–10 and (6.5±0.3)×10–11 m2 s–1, respectively, were revealed.

Self-diffusion of Water and Blood Lipids in Mouse Erythrocytes

The self-diffusion of water and lipids in water—mouse RBCs’ suspension was investigated by pulsed field gradient NMR technique at different diffusion times in temperature range from 5 to 35 °C. For the experimental data interpretation, the phenomenological scaling approach was applied. The intracellular water restriction size (2.1 μm), erythrocyte water permeability (about 10−5 m/s), intracellular residence time (about 20 ms), and water permeability activation energy (24.1 ± 1.9 kJ/mol) were calculated. The lipid self-diffusion coefficients are varied from 3 × 10−12 to 10−11 m2/s depending on temperature and diffusion time. The lipid lateral self-diffusion activation energy is about 25 ± 2.9 kJ/mol.

Analysis of the interaction between rubber polymer and carbon black surfaces by efficient removal of physisorbed polymer from carbon-rubber composites

The purpose of the present study is to provide insight into the structure of “bound rubber”, which is responsible for the reinforcing effect of carbon black (CB) in a rubber composite. When styrene-butadiene rubber (SBR)-CB composites were treated with toluene at a high temperature (150 °C) in an autoclave (AC) vessel, it was found that the AC treatment extracted a much larger amount of physisorbed rubber than the conventional Soxhlet extraction. Moreover, the pulsed 1H NMR technique reveals that the AC treatment can extract almost all of “loosely bound rubber” but leave “tightly bound rubber”. An attempt was then made to correlate the amount of tightly bound rubber with the carbon surface chemistry in CB. As a result, a good linear relationship was found between the amount of tightly bound rubber and the hydrogen content of CB among four SBR composites with different types of CBs. Based on the above linear relationship, the origin of the tightly bound rubber is discussed in terms of the nature of the interaction (chemical and physical) between the rubber polymer chains and the CB surface.

Solvation structure and dynamics of Li+ in Lewis-basic ionic liquid of 1-octyl-4-aza-1-azoniabicyclo[2.2.2]octane bis(trifluoromethanesulfonyl)amide

1H, 7Li, 13C, 15N, and 19F NMR spectra of 1-octyl-4-aza-1-azoniabicyclo[2.2.2]octane bis(trifluoromethanesulfonyl)amide ([C8dabco][TFSA]) solutions with and without LiTFSA (lithium mole fractions of xLi=0 and 0.1) were measured at 313.2K. The chemical shift measured for each nucleus was corrected for volume magnetic susceptibility of the solution. For comparison, those NMR spectra of 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide ([C8mim][TFSA]) solutions were recorded under the same condition. The peak shifts for the nuclei induced by the dissolution of Li+ showed that Li+ interacts with not only TFSA− but also C8dabco+ in [C8dabco][TFSA] solution. Self-diffusion coefficients of the cation, anion, and Li+ in the ionic liquid solutions were determined by a pulsed field gradient NMR technique. Correlation times for jump motion of Li+ were estimated from 7Li longitudinal relaxation times with two different magnetic fields. The diffusion and jump motions of Li+ in [C8dabco][TFSA] and [C8mim][TFSA] solutions are discussed in terms of interionic interactions.