The structural and dynamic properties of 1‐bromodecane in urea inclusion compounds investigated by solid‐state 1H, 13C and 2H NMR spectroscopy

For asymmetric guest molecules in urea, the end‐groups of two adjacent guest molecules may arrange in three different ways: head–head, head–tail and tail–tail. Solid‐state ¹H and ¹³C NMR spectroscopy is used to study the structural properties of 1‐bromodecane in urea. It is found that the end groups of the guest molecules are randomly arranged. The dynamic characteristics of 1‐bromodecane in urea inclusion compounds are probed by variable‐temperature solid‐state ²H NMR spectroscopy (line shapes, spin–spin relaxation: T₂, spin‐lattice relaxation: T_1Z and T_1Q) between 120 K and room temperature. The comparison between the simulation and experimental data shows that the dynamic properties of the guest molecules can be described in a quantitative way using a non‐degenerate three‐site jump process in the low‐temperature phase and a degenerate three‐site jump in the high‐temperature phase, in combination with the small‐angle wobbling motion. The kinetic parameters can be derived from the simulation. Copyright © 2011 John Wiley & Sons, Ltd.     

Metal‐Chelated Polymer Nanodiscs for NMR Studies

Paramagnetic relaxation enhancement (PRE) is commonly used to speed up spin lattice relaxation time (T₁) for rapid data acquisition in NMR structural studies. Consequently, there is significant interest in novel paramagnetic labels for enhanced NMR studies on biomolecules. Herein, we report the synthesis and characterization of a modified poly(styrene‐co‐maleic acid) polymer which forms nanodiscs while showing the ability to chelate metal ions. Cu²⁺‐chelated nanodiscs are demonstrated to reduce the T₁ of protons for both polymer and lipid‐nanodisc components. The chelated nanodiscs also decrease the proton T₁ values for a water‐soluble DNA G‐quadruplex. These results suggest that polymer nanodiscs functionalized with paramagnetic tags can be used to speed‐up data acquisition from lipid bilayer samples and also to provide structural information from water‐soluble biomolecules.     

NMR microscopy - limitations and applications in material sciences -

Alongside the numerous applications of NMR spectroscopy in analytical chemistry, materials sciences and morphological studies by magnetic resonance imaging (MRI), NMR microscopy makes possible a whole new range of applications in materials sciences such as the development and non destructive testing of polymers and ceramic materials. This includes imaging of microscopic structures and structural changes in such materials. The contrast in the images is determined by the NMR specific parameters chemical shift δ, spin density ρ, spin lattice relaxation time T₁, spin spin relaxation time T₂ and spin lattice relaxation time in the rotating frame T_1ρ. The numerous well developed methods available make it possible to study dynamic processes by fast imaging, the measurement of diffusion constants of solvents or liquids, the mobility of fluids in polymers or ceramics or the three dimensional evaluation of pore sizes in porous materials.     

Micellar Formation Study of Crown Ether Surfactants

Fluorescence probe and nuclear magnetic resonance (NMR) methods were employed to investigate the micellation of prepared crown ether surfactants, e.g. decyl 15‐crown‐5 and decyl 18‐crown‐6. Pyrene was employed as the fluorescence probe to evaluate the critical micellar concentration (CMC) of these surfactants in aqueous solutions while spin lattice relaxation times (T₁) and chemical shifts of H‐1 NMR were applied in non‐aqueous solutions. Decyl 15‐crown‐5 with lower CMC forms micelles much easier than decyl 18‐crown‐6 with higher CMC in aqueous solutions, whereas decyl 18‐crown‐6 forms micelles easier than decyl 15‐crown‐5 in nonaqueous solutions. Comparison of the CMC of crown ether surfactants and other polyoxyethylene surfactants such as decylhexaethylene glycol was made. Effects of salts and solvents on the micellar formation were also investigated. In general, additions of both alkali metal salts and polar organic solvents into the aqueous surfactant solutions increased in the CMC of these surfactants. The formation of micelles in organic solvents such as methanol and acetonitrile was successfully observed by the NMR method while it was difficult to study these surfactants in organic solutions by the pyrene fluorescence probe method. The NMR study revealed that the formation of micelles resulted in the decrease in all H‐1 spin lattice relaxation times (T₁) of hydrophobic groups, e.g. CH₃ and CH₂, and hydrophilic group OCH₂ of these surfactants. However, upon the micellar formation, the H‐1 chemical shifts (δ) of these surfactant hydrophobic groups were found to shift to downfield (increased δ) while the chemical shift of the hydrophilic group OCH₂ moved to up‐field. Comparison of the spin lattice relaxation time and H‐1 chemical shift methods was also made and discussed.     

19F Paramagnetic Relaxation Enhancement: A Valuable Tool for Distance Measurements in Proteins

Fluorine NMR paramagnetic relaxation enhancement was evaluated as a versatile approach for extracting distance information in selectively F‐labeled proteins. Proof of concept and initial applications are presented for the HIV‐inactivating lectin cyanovirin‐N. Single F atoms were introduced at the 4‐, 5‐, 6‐ or 7 positions of Trp49 and the 4‐position of Phe4, Phe54, and Phe80. The paramagnetic nitroxide spin label was attached to Cys residues that were placed into the protein at positions 50 or 52. ¹⁹F‐T₂ NMR spectra with different relaxation delays were recorded and the transverse ¹⁹F‐PRE rate, ¹⁹F‐Γ₂, was used to determine the average distance between the F nucleus and the paramagnetic center. Our data show that experimental ¹⁹F PRE‐based distances correspond to 0.93 of the ¹H_N‐PRE distances, in perfect agreement with the gyromagnetic γ¹⁹F/γ¹H ratio, thereby demonstrating that ¹⁹F PREs are excellent alternative parameters for quantitative distance measurements in selectively F‐labeled proteins.     

How to measure absolute P3HT crystallinity via 13C CPMAS NMR

We outline the details of acquiring quantitative ¹³C cross‐polarization magic angle spinning (CPMAS) nuclear magnetic resonance on the most ubiquitous polymer for organic electronic applications, poly(3‐hexylthiophene) (P3HT), despite other groups' claims that CPMAS of P3HT is strictly nonquantitative. We lay out the optimal experimental conditions for measuring crystallinity in P3HT, which is a parameter that has proven to be critical in the electrical performance of P3HT‐containing organic photovoltaics but remains difficult to measure by scattering/diffraction and optical methods despite considerable efforts. Herein, we overview the spectral acquisition conditions of the two P3HT films with different crystallinities (0.47 and 0.55) and point out that because of the chemical similarity of P3HT to other alkyl side chain, highly conjugated main chain polymers, our protocol could straightforwardly be extended to other organic electronic materials. Variable temperature ¹H NMR results are shown as well, which (i) yield insight into the molecular dynamics of P3HT, (ii) add context for spectral editing techniques as applied to quantifying crystallinity, and (iii) show why T_1ρ^H, the ¹H spin–lattice relaxation time in the rotating frame, is a more optimal relaxation filter for distinguishing between crystalline and noncrystalline phases of highly conjugated alkyl side‐chain polymers than other relaxation times such as the ¹H spin–spin relaxation time, T₂^H, and the spin–lattice relaxation time in the toggling frame, T_1xz^H. A 7 ms T_1ρ^H spin lock filter, prior to CPMAS, allows for spectroscopic separation of crystalline and noncrystalline ¹³C nuclear magnetic resonance signals. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.     

Identification of the Chromophore in the Apatite Pigment [Sr10(PO4)6(CuxOH1−x−y)2]: Linear OCuO− Featuring a Resonance Raman Effect,an Extreme Magnetic Anisotropy,and Slow Spin Relaxation

A new chromophore has been identified in copper‐doped apatite pigments having the general composition [Sr₁₀(PO₄)₆(Cu_xOH_1?x?y)₂], in which x=0.1, 0.3 and y=0.01–0.42. By using X‐ray absorption spectroscopy, low‐temperature magnetization measurements, and synchrotron X‐ray powder structure refinement, it has been shown that the oxygenated compounds contain simultaneously diamagnetic Cu¹⁺ and paramagnetic Cu³⁺ with S=1. Cu³⁺ is located at the same crystallographic position as Cu¹⁺, being linearly coordinated by two oxygen atoms and forming the OCuO^? anion. The Raman spectroscopy study of [A₁₀(PO₄)₆(Cu_xOH_1?x?y)₂,], in which A=Ca, Sr, Ba, reveals resonance bands at 651–656 cm^?1 assigned to the symmetric stretching vibration (ν₁) of OCuO^?. The strontium apatite pigment exhibits a strong paramagnetic anisotropy with an unprecedentedly large negative zero‐field splitting parameter (D) of ≈?400 cm^?1. The extreme magnetic anisotropy causes slow magnetization relaxation with relaxation times (τ) up to 0.3 s at T=2 K, which relates the compounds to single‐ion magnets. At low temperature, τ is limited by a spin quantum‐tunneling, whereas at high temperature a thermally activated relaxation prevails with U_eff≈48 cm^?1. Strong dependence of τ on the paramagnetic center concentration at low temperature suggests that the spin‐spin relaxation dominates in the spin quantum‐tunneling process. The compound is the first example of a d‐metal‐based single‐ion magnet with S=1, the smallest spin at which an energy barrier arises for the spin flipping.     

Electron spin relaxation time of Ni(II) ion in hexapyrazole zinc(II) dinitrate at 300 K

Understanding the electron spin relaxation properties of paramagnetic species is a fundamental requirement to use them as a probe to measure distances between sites in biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Even though Ni(II) ion is an essential trace element for many species, relaxation properties are not well understood. Herein, the polycrystalline sample of Ni(II) ion magnetically diluted in Zn(Pyrazole)₆(NO₃)₂ (Ni/ZPN) has been studied in detail by EPR spectroscopy to explore the electron spin relaxation time. Progressive continuous-wave (CW) EPR power saturation study on Ni/ZPN at 300 K yielded 907 mW as the P_1/2 value. The cavity constant (K_Q) has been calculated using tempol in PVA-BA glass matrix and the product of electron spin-lattice relaxation time (T₁) and spin–spin relaxation time (T₂) for Ni/ZPN at 300 K has been reported for the first time.     

Simultaneous measurements of T1 and T2 during fast polymerization reaction using continuous wave‐free precession NMR method

Continuous wave‐free precession (CWFP) pulse sequence employing time domain nuclear magnetic resonance spectroscopy (TD‐NMR) was used to measure longitudinal (T₁) and transverse relaxation times (T₂), during the cure of a commercial epoxy resin (Araldite^TM) with a 10‐min solidification time. The intensity of the NMR signal after the first pulse and in the CWFP regime were used to monitor the concentration of the monomers, and the relaxation times were used to monitor the chain mobility. The main advantage of CWFP over the standard methods to measure relaxation times, inversion recovery (inv‐rec) for T₁ and Carr‐Purcell‐Meiboom‐Gill (CPMG) for T₂, is that the measurement of both relaxation times can be performed in a fast and single NMR experiment and, therefore, using a single reaction batch. CWFP is also as fast as the CPMG measurement but at least fivefold faster than the method to obtain T₁ using null point approximation in the inv‐rec method. Therefore, the CWFP sequence can be used as a fast and general method to measure relaxation times in polymerization reactions, even with fast solidification time. As a TD‐NMR technique, CWFP can be employed in any low‐cost bench top TD‐NMR equipment commonly used in an academic or industrial laboratory. Copyright © 2012 John Wiley & Sons, Ltd.     

Spin‐Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

A potentially biocompatible class of spin‐labeled macromolecules, spin‐labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser‐type dynamic nuclear polarization (DNP). All presented SL‐heparins show high ¹H DNP enhancement factors up to E=?110, which validates that effectively more than one hyperfine line can be saturated even for spin‐labeled polarizing agents. The parameters for the Overhauser‐type DNP are determined and discussed. A striking result is that for spin‐labeled heparins, the off‐resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non‐negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin‐nuclear spin relaxation (T_1ne). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.     

Studies on the phase structure of ethylene‐vinyl acetate copolymers by solid‐state 1H and 13C NMR

The phase structure of a series of ethylene‐vinyl acetate copolymers has been investigated by solid‐state wide‐line ¹H NMR and solid‐state high‐resolution ¹³C NMR spectroscopy. Not only the degree of crystallinity but the relative contents of the monoclinic and orthorhombic crystals within the crystalline region varied with the vinyl acetate (VA) content. Biexponential ¹³C NMR spin–lattice relaxation behavior was observed for the crystalline region of all samples. The component with longer ¹³C NMR spin–lattice relaxation time (T₁) was attributed to the internal part of the crystalline region, whereas the component with shorter ¹³C NMR T₁ to the mobile crystalline component was located between the noncrystalline region and the internal part of the crystalline region. The content of the mobile crystalline component relative to the internal part of the crystalline region increased with the VA content, showing that the ¹³C NMR spin–lattice relaxation behavior is closely related to the crystalline structure of the copolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2199–2207, 2002     

Electron spin relaxation times and rapid scan EPR imaging of pH‐sensitive amino‐substituted trityl radicals

Carboxy‐substituted trityl (triarylmethyl) radicals are valuable in vivo probes because of their stability, narrow lines, and sensitivity of their spectroscopic properties to oxygen. Amino‐substituted trityl radicals have the potential to monitor pH in vivo, and the suitability for this application depends on spectral properties. Electron spin relaxation times T₁ and T₂ were measured at X‐band for the protonated and deprotonated forms of two amino‐substituted triarylmethyl radicals. Comparison with relaxation times for carboxy‐substituted triarylmethyl radicals shows that T₁ exhibits little dependence on protonation or the nature of the substituent, which makes it useful for measuring O₂ concentration, independent of pH. Insensitivity of T₁ to changes in substituents is consistent with the assignment of the dominant contribution to spin lattice relaxation as a local mode that involves primarily atoms in the carbon and sulfur core. Values of T₂ vary substantially with pH and the nature of the aryl group substituent, reflecting a range of dynamic processes. The narrow spectral widths for the amino‐substituted triarylmethyl radicals facilitate spectral‐spatial rapid scan electron paramagnetic resonance imaging, which was demonstrated with a phantom. The dependence of hyperfine splittings patterns on pH is revealed in spectral slices through the image. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantification of multiple compounds containing heterogeneous elements in the mixture by one‐dimensional nuclear magnetic resonance spectroscopy of different nuclei using a single universal concentration reference

One‐dimensional (1D) quantitative NMR (qNMR) is a useful tool for concentration determination due to its experimental simplicity and the direct proportionality of the integrated signal area to the number of nuclei spin. For complex mixtures, however, signal overlapping often in one‐dimensional quantitative ¹H NMR (1D ¹H qNMR) spectrum limits the accurate quantification of individual compound. Here, we introduced employing joint 1D qNMR methods of different nuclei, such as ¹H and ³¹P (or/and ¹⁹F), to quantify multiple compounds in a complex mixture using a single universal concentration reference. When the concentration ratio of several compounds containing different elements in a complex mixture is of interest, the result calculated from measured intensities from 1D qNMR of different nuclei is independent of the gravimetric error from the reference. In this case, the common reference also serves as a ‘quantitative bridge’ among these 1D qNMR of different nuclei. Quantitative analysis of choline, phosphocholine, and glycerophosphocholine mixture is given as an example using trimethylphosphine oxide ((CH₃)₃P(O)) as concentration reference. Compounds containing multiple elements, such as tetramethylammonium hexafluorophosphate (N⁺(CH₃)₄PF₆^?), are proposed as the common concentration reference for ¹H, ¹³C, ¹⁵N, ³¹P, and ¹⁹F qNMR for the quantitative analysis of complex mixture containing these different elements. We anticipate that the proposed joint 1D qNMR approach using a universal concentration reference will be a valuable alternative for simultaneous quantification of multiple compounds in a complex mixture due to its accuracy and single and simple sample preparation. Copyright © 2014 John Wiley & Sons, Ltd.     

Keeping PASE with WEFT: SHWEFT–PASE pulse sequences for 1H NMR spectra of highly paramagnetic molecules

Metalloproteins are a category of biomolecules in which the metal site is usually the locus of activity or function. In many cases, the metal ions are paramagnetic or have accessible paramagnetic states, many of which can be studied using NMR spectroscopy. Extracting useful information from ¹H NMR spectra of highly paramagnetic proteins can be difficult because the paramagnetism leads to large resonance shifts (~400 ppm), extremely broad lines, extreme baseline nonlinearity, and peak shape distortion. It is demonstrated that employing polychromatic and adiabatic shaped pulses in simple pulse sequences, then combining existing sequences, leads to significant spectral improvement for highly paramagnetic proteins. These sequences employ existing technology, with available hardware, and are of short duration to accommodate short nuclear T₁ and T₂. They are shown to display uniform excitation over large spectral widths (~75 kHz), accommodate high repetition rates, produce flat baselines over 75 kHz while maintaining peak shape fidelity, and can be used to reduce spectral dynamic range. High‐spin (S = 5/2) metmyoglobin, a prototypical highly paramagnetic protein, was used as the test molecule. The resulting one‐dimensional (1D) pulse sequences combine shaped pulses with super‐water elimination Fourier transform, which can be further combined with paramagnetic spectroscopy to give shaped pulses with super‐water elimination Fourier transform–paramagnetic spectroscopy. These sequences require, at most, direct current offset correction and minimal phasing. The performance of these sequences in simple ¹H 1D, 1D NOE, and two‐dimensional NOESY experiments is demonstrated for metmyoglobin and Paracoccus denitrificans Co²⁺‐amicyanin (S = 3/2), and employed to make new heme hyperfine resonance assignments for high‐spin metBjFixLH_151–256, the heme sensing domain of Bradyrhizobium japonicum FixL. Copyright © 2013 John Wiley & Sons, Ltd.     

Slow Magnetic Relaxation of Ni(III) Complexes toward Molecular Spin Qubits

Molecule-based magnetic materials are promising candidates for molecular spin qubits, which utilize spin relaxation behavior. Various kinds of transition metal complexes with S=1/2 have been reported to act as spin qubits with long spin-spin relaxation times (T₂). However, the spin qubit properties of low-spin Ni(III) complexes are not as well known since Ni(III) compounds are often unstable. We report here the slow magnetic relaxation behavior and T₂ values for three kinds of low-spin Ni(III) based complexes with S=1/2 under magnetically diluted conditions. [Ni(cyclam)X₂]Y (cyclam=1,4,8,11-tetraazacyclotetradecane) with octahedral structures and [Ni(mnt)₂]⁻ (mnt=maleonitriledithiolate) with a square-planar structure underwent slow magnetic relaxations in the presence of a dc magnetic bias field. From electron spin resonance (ESR) spectroscopy, the Ni(III) complexes exhibited observable T₂, indicating that Ni(III) complexes are promising candidates for use as molecule-based spin qubits.     

2H NMR spin relaxation study of the soft segment motion in alternating ethylene–propylene copolymers

We synthesized three partially deuterated polymer samples, namely a poly(ethylene‐alt‐propylene) (EP) alternating copolymer, a poly(styrene‐b‐EP) diblock copolymer (SEP) and a poly(styrene‐b‐EP‐b‐styrene) triblock copolymer (SEPS). The ²H spin–lattice relaxation time, T₁, of EP soft segments above their glass transition temperature was measured by solid‐state ²H NMR spectroscopy. It was found that the block copolymers had a fast and a slow T₁ component whereas EP copolymer had only a fast component. The fast T₁ components for SEP and SEPS are similar to the T₁ value of EP above ca 20°C. The slow T₁ component for SEP and SEPS exhibited a minimum at 60°C and approached the value of the fast component near the T_g of polystyrene. The motional behavior of the EP units for SEP is similar to that of SEPS over the entire range of temperature. Copyright © 2001 John Wiley & Sons, Ltd.     

Ultrafast Two‐Dimensional NMR Relaxometry for Investigating Molecular Processes in Real Time

Nuclear spin–lattice (T₁) and spin–spin (T₂) relaxation times provide versatile information about the dynamics and structure of substances, such as proteins, polymers, porous media, and so forth. Multidimensional experiments increase the information content and resolution of NMR relaxometry, but they also multiply the measurement time. To overcome this issue, we present an efficient strategy for a single‐scan measurement of a 2D T₁–T₂ correlation map. The method shortens the experimental time by one to three orders of magnitude as compared to the conventional method, offering an unprecedented opportunity to study molecular processes in real‐time. We demonstrate that, despite the tremendous speed‐up, the T₁–T₂ correlation maps determined by the single‐scan method are in good agreement with the maps measured by the conventional method. The concept of the single‐scan T₁–T₂ correlation experiment is applicable to a broad range of other multidimensional relaxation and diffusion experiments.     

Nuclear magnetic relaxation in polyolefin resins

Nuclear magnetic resonance (NMR) spin–lattice relaxation times (T₁) in various polyethylene and polypropylene resins were measured at 20 MHz and at temperatures of 130–490 K. At each temperature and for all resins, only a single value of T₁ was found, which was consistent with the occurrence of rapid spin diffusion throughout the protons attached to the polymer chains. The data were analyzed for the estimation of activation energies corresponding to molecular motion causing spin–lattice relaxation. Two well‐defined minima were found for log_e(T₁) plotted as a function of temperature for all of the polypropylene resins. Single very broad minima were found for all of the polyethylene samples. In contrast, the free induction decay signals from all of the resins following single radio‐frequency pulses were observed to contain a rapidly decaying component followed by a much more slowly decaying signal. These components were used to estimate the amount of rigid component present in the solid resins at room temperature. Samples of one high‐density polyethylene and one low‐density polyethylene were irradiated with γ radiation up to a 500‐kGy dose to examine the effects of crosslinking on the NMR relaxation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 572–584, 2002; DOI 10.1002/polb.10116     

Nuclear Magnetic Resonance Relaxivities: Investigations of Ultrahigh‐Spin Lanthanide Clusters from 10 MHz to 1.4 GHz

Paramagnetic relaxation enhancement is often explored in magnetic resonance imaging in terms of contrast agents and in biomolecular nuclear magnetic resonance (NMR) spectroscopy for structure determination. New ultrahigh‐spin clusters are investigated with respect to their NMR relaxation properties. As their molecular size and therefore motional correlation times as well as their electronic properties differ significantly from those of conventional contrast agents, questions about a comprehensive characterization arise. The relaxivity was studied by field‐dependent longitudinal and transverse NMR relaxometry of aqueous solutions containing Fe^III₁₀Dy^III₁₀ ultrahigh‐spin clusters (spin ground state 100/2). The high‐field limit was extended to 32.9 T by using a 24 MW resistive magnet and an ultrahigh‐frequency NMR setup. Interesting relaxation dispersions were observed; the relaxivities increase up to the highest available fields, which indicates a complex interplay of electronic and molecular correlation times.