1H Decoupling Research Articles

Heteronuclear Polarization Transfer under Steady-State Conditions: The INEPT-SSFP Experiment.

NMR finds a wide range of applications, ranging from fundamental chemistry to medical imaging. The technique, however, has an inherently low signal-to-noise ratio (SNR)─particularly when dealing with nuclei having low natural abundances and/or low γs. In these cases, sensitivity is often enhanced by methods that, similar to INEPT, transfer polarization from neighboring 1Hs via J-couplings. In 1958, Carr proposed an alternative approach to increase NMR sensitivity, which involves generating and continuously detecting a steady-state transverse magnetization, by applying a train of pulses on an ensemble of noninteracting spins. This study broadens Carr's steady-state free precession (SSFP) framework to encompass the possibility of adding onto it coherent polarization transfers, allowing one to combine the SNR-enhancing benefits of both INEPT and SSFP into a single experiment. Herein, the derivation of the ensuing INEPT-SSFP (ISSFP) sequences is reported. Their use in 13C NMR and MRI experiments leads to ca. 300% improvements in SNR/ over conventional J-driven polarization transfer experiments, and sensitivity gains of over 50% over 13C SSFP performed in combination with 1H decoupling and NOE. These enhancements match well with numerical simulations and analytical evaluations. The conditions needed to optimize these new methods in both spectroscopic and imaging studies are discussed; we also examine their limitations, and the valuable vistas that, in both analytical and molecular imaging NMR, could be opened by this development.

Toward structure and metabolism of glycogen C1-C6 in humans at 7T by localized 13C MRS using low-power bilevel broadband 1H decoupling.

This work aims to develop and implement a pulse-acquire sequence for three-dimensional (3D) single-voxel localized 13C MRS in humans at 7T, in conjunction with bilevel broadband 1H decoupling, and to test its feasibility in vitro and in vivo in human calf muscle with emphasis on the detection of glycogen C1-C6. A localization scheme suitable for measuring fast-relaxing 13C signals in humans at 7T was developed and implemented using the outer volume suppression (OVS) and one-dimensional image selected in vivo spectroscopy (ISIS-1D) schemes, similar to that which was previously reported in humans at 4T. The 3D 13C localization scheme was followed by uniform 13C adiabatic excitation, all complemented with an option for bilevel broadband 1H decoupling to improve both 13C sensitivity and spectral resolution at 7T. The performance of the pulse-acquire sequence was investigated in vitro on phantoms and in vivo in the human calf muscle of three healthy volunteers, while measuring glycogen C1-C6. In addition, T1 and T2 of glycogen C1-C6 were measured in vitro at 7T, as well as T1 of glycogen C1 in vivo. The glycerol C2 and C1,3 lipid resonances were efficiently suppressed in vitro at 7T using the OVS and ISIS-1D schemes, allowing distinct detection of glycogen C2-C6. While some glycerol remained in calf muscle in vivo, the intense lipid at 130 ppm was efficiently suppressed. The 13C sensitivity and spectral resolution of glycogen C1-C6 in vitro and glycogen C1 in vivo were improved at 7T using bilevel broadband 1H decoupling. The T1 and T2 of glycogen C1-C6 in vitro at 7T were consistent compared with those at 8.5T, while the T1 of glycogen C1 in vivo at 7T resulted similar to that in vitro. Localized 13C MRS is feasible in human calf muscle in vivo at 7T, and this will allow further extension of this method for 13C MRS measurements such as in the brain.

Quantitative analysis of the slow exchange process by 19F NMR in the presence of scalar and dipolar couplings: applications to the ribose 2'-19F probe in nucleic acids.

Solution NMR spectroscopy is a particularly powerful technique for characterizing the functional dynamics of biomolecules, which is typically achieved through the quantitative characterization of chemical exchange processes via the measurement of spin relaxation rates. In addition to the conventional nuclei such as 15N and 13C, which are abundant in biomolecules, fluorine-19 (19F) has recently garnered attention and is being widely used as a site-specific spin probe. While 19F offers the advantages of high sensitivity and low background, it can be susceptible to artifacts in quantitative relaxation analyses due to a multitude of dipolar and scalar coupling interactions with nearby 1H spins. In this study, we focused on the ribose 2'-19F spin probe in nucleic acids and investigated the effects of 1H-19F spin interactions on the quantitative characterization of slow exchange processes on the millisecond time scale. We demonstrated that the 1H-19F dipolar coupling can significantly affect the interpretation of 19F chemical exchange saturation transfer (CEST) experiments when 1H decoupling is applied, while the 1H-19F interactions have a lesser impact on Carr-Purcell-Meiboom-Gill relaxation dispersion applications. We also proposed a modified CEST scheme to alleviate these artifacts along with experimental verifications on self-complementary RNA systems. The theoretical framework presented in this study can be widely applied to various 19F spin systems where 1H-19F interactions are operative, further expanding the utility of 19F relaxation-based NMR experiments.

Efficient Access to Elusive 1D 13C NMR Spectra through Highly Resolved 1H, 13C-Long-Range Correlation Spectroscopy.

A method for obtaining 1D 13C NMR spectra from natural products or metabolites using proton detection is described. The approach delivers singlets for every 13C signal without conducting any broadband 1H decoupling (CPD) and is based on calculating 13C projections from constant-time HMBC and conventional HSQC experiments, recorded at high digital resolution and processed to pure phases. Paramount to the proposed method is the implication of nonuniform sampling and echo processing. The echo processing produces phase-sensitive 2D CT-HMBC spectra with narrow 13C signal line shapes. Two simple HMBC pulse sequences are utilized with the suppression of homo- and heteronuclear couplings. Due to the removal of the 1H multiplet structure in F1 (no tilt at higher digital resolution), 13C singlets arise. An overall increase in 13C signal-to-noise (SINO) for all types of carbon multiplicities is observed, making the proposed technique superior compared to direct 13C excitation. For otherwise difficult-to-measure quaternary carbon atoms, a SINO enhancement of up to 6 and 12 depending on F1 resolution (3 and 6 Hz/point) is reported. Echo/anti-Echo signal detection cleans up the spectrum. Nonuniform sampling (NUS) lays the groundwork to significantly reduce the total acquisition time. Final 1D 13C projections are obtained by combining the 13C projection from CT HMBC and conventional HSQC. This orthogonal concept of combining the 13C projections from different spectra inherently minimizes the risk of missing 13C cross-peaks by inappropriate setting of long-range nJHC coupling delays and the shortcoming of T2 relaxations. The advantages and some limitations of the concept are discussed.

Skin, soap, and spaghetti: investigations of co-existing solid and liquid phases in organic materials using solid-state NMR with dynamics-based spectral editing

Abstract Solid-state NMR methods incorporating dynamics-based spectral editing have a remarkable versatility for resolving and separately characterizing co-existing solid and liquid phases or domains in biologically and technically relevant organic materials. While 13C spectra acquired under magic-angle spinning and 1H decoupling provide atomic resolution, the signal intensities obtained with the CP and INEPT polarization transfer techniques give qualitative information about dynamics. This mini-review covers the basics of translational and rotational motion of atoms and molecules in organic materials, theoretical aspects of the relations between C–H bond reorientation and CP and INEPT signal intensities, and applications of the methods to a broad range of heterogeneous materials comprising hydrated assemblies of surfactants, lipids, proteins, and/or carbohydrates.

19F fast MAS (60–111 kHz) dipolar and scalar based correlation spectroscopy of organic molecules and pharmaceutical formulations

19F magic angle spinning (MAS) NMR spectroscopy is a powerful tool for characterization of fluorinated solids. The recent development of 19F MAS NMR probes, operating at spinning frequencies of 60–111 kHz, enabled analysis of systems spanning from organic molecules to pharmaceutical formulations to biological assemblies, with unprecedented resolution. Herein, we systematically evaluate the benefits of high MAS frequencies (60–111 kHz) for 1D and 2D 19F-detected experiments in two pharmaceuticals, the antimalarial drug mefloquine and a formulation of the cholesterol-lowering drug atorvastatin calcium. We demonstrate that 1H decoupling is essential and that scalar-based, heteronuclear single quantum coherence (HSQC) and heteronuclear multiple quantum coherence (HMQC) correlation experiments become feasible and efficient at the MAS frequency of 100 kHz. This study opens doors for the applications of high frequency 19F MAS NMR to a wide range of problems in chemistry and biology.

Determination of accurate 19F chemical shift tensors with R-symmetry recoupling at high MAS frequencies (60–100 kHz)

Fluorination is a versatile and valuable modification for numerous systems, and 19F NMR spectroscopy is the premier method for their structural characterization. 19F chemical shift anisotropy is a sensitive probe of structure and dynamics, even though 19F chemical shift tensors have been reported for only a handful of systems to date. Here, we explore γ-encoded R-symmetry based recoupling sequences for the determination of 19F chemical shift tensors in fully protonated organic solids at high, 60–100 kHz MAS frequencies. We show that the performance of 19F-RNCSA experiments improves with increasing MAS frequencies, and that 1H decoupling is required to determine accurate chemical shift tensor parameters. In addition, these sequences are tolerant to B1-field inhomogeneity making them suitable for a wide range of systems and experimental conditions.

Removal of 2H-decoupling sidebands in 13CHD2 13C-CEST profiles.

A unique aspect of NMR is its capacity to provide integrated insight into both the structure and intrinsic dynamics of biomolecules. Chemical exchange phenomena that often serve as probes of dynamic processes in biological macromolecules can be quantitatively investigated with chemical exchange saturation transfer (CEST) experiments. 2H-decoupling sidebands, however, always occur in the profiles of 13CHD2 13C-CEST experiments when using the simple CW (continuous wave) method, which may obscure the detection of minor dips of excited states. Traditionally, these sidebands are manually eliminated from the profiles before data analysis by removing experimental points in the range of 2H-decoupling field strength ±50Hz away from the major dips of the ground state on either side of the dips. Unfortunately, this may also eliminate potential minor dips if they overlap with the decoupling sidebands. Here, we developed methods that use pseudo-continuous waves with variable RF amplitudes distributed onto ramps for 2H decoupling. The new methods were thoroughly validated on Bruker spectrometers at a range of fields (1H frequencies of 600, 700, and 850MHz, and 1.1 GHz). By using these methods, we successfully removed the sidebands from the NMR profiles of 13CHD2 13C-CEST experiments.

Probing hepatic metabolism of [2-13C]dihydroxyacetone in vivo with 1H-decoupled hyperpolarized 13C-MR

ObjectivesTo enhance detection of the products of hyperpolarized [2-13C]dihydroxyacetone metabolism for assessment of three metabolic pathways in the liver in vivo. Hyperpolarized [2-13C]DHAc emerged as a promising substrate to follow gluconeogenesis, glycolysis and the glycerol pathways. However, the use of [2-13C]DHAc in vivo has not taken off because (i) the chemical shift range of [2-13C]DHAc and its metabolic products span over 144 ppm, and (ii) 1H decoupling is required to increase spectral resolution and sensitivity. While these issues are trivial for high-field vertical-bore NMR spectrometers, horizontal-bore small-animal MR scanners are seldom equipped for such experiments.MethodsReal-time hepatic metabolism of three fed mice was probed by 1H-decoupled 13C-MR following injection of hyperpolarized [2-13C]DHAc. The spectra of [2-13C]DHAc and its metabolic products were acquired in a 7 T small-animal MR scanner using three purpose-designed spectral-spatial radiofrequency pulses that excited a spatial bandwidth of 8 mm with varying spectral bandwidths and central frequencies (chemical shifts).ResultsThe metabolic products detected in vivo include glycerol 3-phosphate, glycerol, phosphoenolpyruvate, lactate, alanine, glyceraldehyde 3-phosphate and glucose 6-phosphate. The metabolite-to-substrate ratios were comparable to those reported previously in perfused liver.DiscussionThree metabolic pathways can be probed simultaneously in the mouse liver in vivo, in real time, using hyperpolarized DHAc.

Hydrogen-exchange kinetics studied through analysis of self-decoupling of nuclear magnetic resonance

Hydrogen exchange between solute and water molecules occurs across a wide range of timescales. Rapid hydrogen-exchange processes can effectively diminish 1H-15N scalar couplings. We demonstrate that the self-decoupling of 15N nuclear magnetic resonance can allow quantitative investigations of hydrogen exchange on a micro- to millisecond timescale, which is relatively difficult to analyze with other methods. Using a Liouvillian matrix incorporating hydrogen exchange as a mechanism for scalar relaxation, the hydrogen exchange rate can be determined from 15N NMR line shapes recorded with and without 1H decoupling. Self-decoupling offers a simple approach to analyze the kinetics of hydrogen exchange in a wide range of timescale.

3D 31 P MR spectroscopic imaging of the human brain at 3 T with a 31 P receive array: An assessment of 1 H decoupling, T1 relaxation times, 1 H-31 P nuclear Overhauser effects and NAD.

31P MR spectroscopic imaging (MRSI) is a versatile technique to study phospholipid precursors and energy metabolism in the healthy and diseased human brain. However, mainly due to its low sensitivity, 31P MRSI is currently limited to research purposes. To obtain 3D 31P MRSI spectra with improved signal‐to‐noise ratio on clinical 3 T MR systems, we used a coil combination consisting of a dual‐tuned birdcage transmit coil and a 31P eight‐channel phased‐array receive insert. To further increase resolution and sensitivity we applied WALTZ4 1H decoupling and continuous wave nuclear Overhauser effect (NOE) enhancement and acquired high‐quality MRSI spectra with nominal voxel volumes of ~ 17.6 cm3 (effective voxel volume ~ 51 cm3) in a clinically relevant measurement time of ~ 13 minutes, without exceeding SAR limits. Steady‐state NOE enhancements ranged from 15 ± 9% (γ‐ATP) and 33 ± 3% (phosphocreatine) to 48 ± 11% (phosphoethanolamine). Because of these improvements, we resolved and detected all 31P signals of metabolites that have also been reported for ultrahigh field strengths, including resonances for NAD+, NADH and extracellular inorganic phosphate. T1 times of extracellular inorganic phosphate were longer than for intracellular inorganic phosphate (3.8 ± 1.4s vs 1.8 ± 0.65 seconds). A comparison of measured T1 relaxation times and NOE enhancements at 3 T with published values between 1.5 and 9.4 T indicates that T1 relaxation of 31P metabolite spins in the human brain is dominated by dipolar relaxation for this field strength range. Even although intrinsic sensitivity is higher at ultrahigh fields, we demonstrate that at a clinical field strength of 3 T, similar 31P MRSI information content can be obtained using a sophisticated coil design combined with 1H decoupling and NOE enhancement.

Direct estimation of shale oil potential by the structural insight of Indian origin kerogen

Structural insight of Indian origin kerogen has been investigated by Solid State 13C Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (13C CPMAS NMR) and Infrared (IR) Spectroscopic techniques. Cross Polarization (CP) with decoupling using two pulse phase modulation (TPPM) and Bloch decay experiment with 1H decoupling have been performed on each kerogen sample. Direct estimation of oil potential present in Indian origin kerogen has been done utilizing developed method based on IR and 13C CPMAS NMR techniques. The IR methods have been developed using both transmission and DRIFT reflectance spectra by employing standard blends of oil shale (1–12%) with Vacuum Gas Oil (VGO) range products. Models were developed by correlating the IR intensity in 3000–2700 cm−1 region with the % oil shale in VGO. Validation has been done by estimating aliphatic carbon (Calip) and aromatic carbon (Carom) content obtained by 13C CPMAS NMR method and a comparison has been made between 13CMAS NMR with and without dipolar dephasing, with and without CP. 13C MAS NMR spectra of Indian origin have been compared with Estonia and US Green River origin oil shale, containing high oil potential. Both IR and NMR methods have been applied to number of samples from different Indian sources. Results from these comprehensive studies indicate that both 13C CPMAS NMR and IR techniques are quite useful for structural inputs and direct determination of oil potential from different origin oil shale.

Determination of Long-Range Distances by Fast Magic-Angle-Spinning Radiofrequency-Driven 19F-19F Dipolar Recoupling NMR.

Nanometer-range distances are important for restraining the three-dimensional structure and oligomeric assembly of proteins and other biological molecules. Solid-state NMR determination of protein structures typically utilizes 13C-13C and 13C-15N distance restraints, which can only be measured up to ∼7 Å because of the low gyromagnetic ratios of these nuclear spins. To extend the distance reach of NMR, one can harvest the power of 19F, whose large gyromagnetic ratio in principle allows distances up to 2 nm to be measured. However, 19F possesses large chemical shift anisotropies (CSAs) as well as large isotropic chemical shift dispersions, which pose challenges to dipolar coupling measurements. Here, we demonstrate 19F-19F distance measurements at high magnetic fields under fast magic-angle spinning (MAS) using radiofrequency-driven dipolar recoupling (RFDR). We show that 19F-19F cross-peaks for distances up to 1 nm can be readily observed in two-dimensional 19F-19F correlation spectra using less than 5 ms of RFDR mixing. This efficient 19F-19F dipolar recoupling is achieved using practically accessible MAS frequencies of 15-55 kHz, moderate 19F radio frequency field strengths, and no 1H decoupling. Experiments and simulations show that the fastest polarization transfer for aromatic fluorines with the highest distance accuracy is achieved using either fast MAS (e.g., 60 kHz) with large pulse duty cycles (>50%) or slow MAS with strong 19F pulses. Fast MAS considerably reduces relaxation losses during the RFDR π-pulse train, making finite-pulse RFDR under fast-MAS the method of choice. Under intermediate MAS frequencies (25-40 kHz) and intermediate pulse duty cycles (15-30%), the 19F CSA tensor orientation has a quantifiable effect on the polarization transfer rate; thus, the RFDR buildup curves encode both distance and orientation information. At fast MAS, the impact of CSA orientation is minimized, allowing pure distance restraints to be extracted. We further investigate how relayed transfer and dipolar truncation in multifluorine environments affect polarization transfer. This fast-MAS 19F RFDR approach is complementary to 19F spin diffusion for distance measurements and will be the method of choice under high-field fast-MAS conditions that are increasingly important for protein structure determination by solid-state NMR.

Physiochemical characterization and support interaction of alumina‐supported heteropolyacid catalyst for biodiesel production

Abstract12‐Tungstophosphoric acid (TPA)‐supported γ‐Al2O3 catalysts were synthesized with varying TPA loadings. Catalysts characteristics were determined using Brunauer–Emmett–Teller surface area analysis, thermogravimetric analyses, X‐ray diffraction, Raman, pyridine‐adsorbed Fourier‐transform infrared spectroscopy, and 13C hpdec (high‐power 1H decoupling) nuclear magnetic resonance, whereas surface morphology was studied using scanning electron microscope and transmission electron microscopy analyses. Surface defects were found in the catalyst with higher TPA loadings (55–65 wt.%). The presence of WOx was observed at higher loadings and later agglomerated into tungsten oxide crystals. The TPA impregnated catalysts were investigated for biodiesel synthesis from canola oil. The reaction was found to be independent of mass transfer limitation. The activation energy was 33.6 kJ mol−1, and pre‐exponential factor was 7.3*10−2 min−1. The turnover frequency for the supported catalysts was found in the range of 9.7*10−4–8.8*10−2 min−1. A conversion of 94.9 ± 2.3% was obtained under the optimized conditions, that is, 10 wt.% of the catalyst loading, 17.5 methanol to oil molar ratio, 4 MPa, and at 200°C in 10 hr.

The "Missing" Bicarbonate in CO2 Chemisorption Reactions on Solid Amine Sorbents.

We have identified a hydrated bicarbonate formed by chemisorption of 13CO2 on both dimethylaminopropylsilane (DMAPS) and aminopropylsilane (APS) pendant molecules grafted on SBA-15 mesoporous silica. The most commonly used sequence in solid-state NMR, 13C CPMAS, failed to detect bicarbonate in these solid amine sorbent samples; here, we have employed a Bloch decay ("pulse-acquire") sequence (with 1H decoupling) to detect such species. The water that is present contributes to the dynamic motion of the bicarbonate product, thwarting CPMAS but enabling direct 13C detection by shortening the spin-lattice relaxation time. Since solid-state NMR plays a major role in characterizing chemisorption reactions, these new insights that allow for the routine detection of previously elusive bicarbonate species (which are also challenging to observe in IR spectroscopy) represent an important advance. We note that employing this straightforward NMR technique can reveal the presence of bicarbonate that has often otherwise been overlooked, as demonstrated in APS, that has been thought to only contain adsorbed CO2 as carbamate and carbamic acid species. As in other systems (e.g., proteins), dynamic species that sample multiple environments tend to broaden as their motion is frozen out. Here, we show two distinct bicarbonate species upon freezing, and coupling to different protons is shown through preliminary 13C-1H HETCOR measurements. This work demonstrates that bicarbonates have likely been formed in the presence of water but have gone unobserved by NMR due to the nature of the experiments most routinely employed, a perspective that will transform the way the sorption community will view CO2 capture by amines.

Using Hyperpolarized [2-13C] Dihydroxyacetone to Detect Hepatic Gluconeogenesis

Diabetes afflicts millions of people worldwide and contributes to elevated risk of cardiovascular disease and stroke. Imaging of hepatic metabolic activity using MRI could prove advantageous for diagnosis, treatment monitoring, etc. With the advent of hyperpolarized (HP) carbon-13 based MRI, rapid imaging of metabolic flux is feasible without the use of ionizing radiation. In this work, we have utilized HP [2-13C]dihydroxyacetone (DHA) to observe hepatic metabolism in real time in a perfused mouse (C57BLKS/J) liver model. We have utilized fatty acids and a 1:10 ratio of pyruvate to lactate in the perfusate to mimic physiological conditions. Figure 1 shows the various metabolites observed following the injection of DHA. Ratios of integrals of three-carbon intermediates and hexose sugars (and phosphates) are used to determine the rate of gluconeogenesis. Hepatic gluconeogenic rates are measured in parallel with gold standard tracer methods, confirming the accuracy of HP imaging. Figure 1: Representative 13C NMR spectrum (sum) of hyperpolarized [2-13C] DHA metabolism in perfused liver at 14.1 T. Spectra were measured without 1H decoupling. Phosphoenolpyruvate (PEP) and glycerol-3-phosphate (G3P) are observed. Note that PEP is normally considered a low concentration metabolites, but it is easily detected here. Disclosure M. Merritt: None. M. Ragavan: None. T. Tsarova: None.

Removal of slow-pulsing artifacts in in-phase 15N relaxation dispersion experiments using broadband 1H decoupling

Understanding of the molecular mechanisms of protein function requires detailed insight into the conformational landscape accessible to the protein. Conformational changes can be crucial for biological processes, such as ligand binding, protein folding, and catalysis. NMR spectroscopy is exquisitely sensitive to such dynamic changes in protein conformations. In particular, Carr–Purcell–Meiboom–Gill (CPMG) relaxation dispersion experiments are a powerful tool to investigate protein dynamics on a millisecond time scale. CPMG experiments that probe the chemical shift modulation of 15N in-phase magnetization are particularly attractive, due to their high sensitivity. These experiments require high power 1H decoupling during the CPMG period to keep the 15N magnetization in-phase. Recently, an improved version of the in-phase 15N-CPMG experiment was introduced, offering greater ease of use by employing a single 1H decoupling power for all CPMG pulsing rates. In these experiments however, incomplete decoupling of off-resonance amide 1H spins introduces an artefactual dispersion of relaxation rates, the so-called slow-pulsing artifact. Here, we analyze the slow-pulsing artifact in detail and demonstrate that it can be suppressed through the use of composite pulse decoupling (CPD). We report the performances of various CPD schemes and show that CPD decoupling based on the 90x–240y–90x element results in high-quality dispersion curves free of artifacts, even for amides with high 1H offset.

The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR.

Carbon nanothreads are a new type of one-dimensional sp3-carbon nanomaterial formed by slow compression and decompression of benzene. We report characterization of the chemical structure of 13C-enriched nanothreads by advanced quantitative, selective, and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy. The width of the NMR spectral peaks suggests that the nanothread reaction products are much more organized than amorphous carbon. In addition, there is no evidence from NMR of a second phase such as amorphous mixed sp2/sp3-carbon. Spectral editing reveals that almost all carbon atoms are bonded to one hydrogen atom, unlike in amorphous carbon but as is expected for enumerated nanothread structures. Characterization of the local bonding structure confirms the presence of pure fully saturated "degree-6" carbon nanothreads previously deduced on the basis of crystal packing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise between 20% and 45% of the sample. Furthermore, 13C-13C spin exchange experiments indicate that the length of the fully saturated regions of the threads exceeds 2.5 nm. Two-dimensional 13C-13C NMR spectra showing bonding between chemically nonequivalent sites rule out enumerated single-site thread structures such as polytwistane or tube (3,0) but are consistent with multisite degree-6 nanothreads. Approximately a third of the carbon is in "degree-4" nanothreads with isolated double bonds. The presence of doubly unsaturated degree-2 benzene polymers can be ruled out on the basis of 13C-13C NMR with spin exchange rateconstants tuned by rotational resonance and 1H decoupling. A small fraction of the sample consists of aromatic rings within the threads that link sections with mostly saturated bonding. NMR provides the detailed bonding information necessary to refine solid-state organic synthesis techniques to produce pure degree-6 or degree-4 carbon nanothreads.

Improving quantitative 13C NMR performance by an adiabatic scheme

NMR is a primary method of measurement that provides quantitative traceable results without the need for a standard of the same measurand. Despite the advantages of the 1H nucleus, such as high sensitivity, short relaxation times and widespread distribution in organic compounds, some molecules present only labile H atoms, or the 1H peaks of a sample may overlap with impurities. With the CHORAD scheme – a combination of the pulse sequence CHORUS and adiabatic modulation for 1H decoupling – this study has shown that 13C NMR can be used for quantitative analyses with a bias smaller than 2% and expanded uncertainty of 0.7% (for a confidence level of 95%). Since 13C is found with significant non-statistic isotope fractionation, all the peaks of the sample and internal standard should be integrated and their bulk 13C abundances obtained by isotope ratio mass spectrometry (IRMS) must be considered to meet quantitative requirements. Future experimental work may be performed to test this procedure for complex matrix samples. This approach also proved to be suitable for studies of site-specific 13C distribution and led to better heteronuclear decoupling when compared with the use of adiabatic decoupling with ordinary hard 90° pulses, a combination extensively reported in the literature over the last decade. An examination of the molecules studied in this work and others from the literature revealed an overall trend in the 13C intramolecular profile – hydrogen-containing positions are often 13C-depleted. A potential contribution of 1JCH coupling to these results is discussed.

Proton-decoupled carbon magnetic resonance spectroscopy in human calf muscles at 7 T using a multi-channel radiofrequency coil

13C magnetic resonance spectroscopy is a viable, non-invasive method to study cell metabolism in skeletal muscles. However, MR sensitivity of 13C is inherently low, which can be overcome by applying a higher static magnetic field strength together with radiofrequency coil arrays instead of single loop coils or large volume coils, and 1H decoupling, which leads to a simplified spectral pattern. 1H-decoupled 13C-MRS requires RF coils which support both, 1H and 13C, Larmor frequencies with sufficient electromagnetic isolation between the pathways of the two frequencies. We present the development, evaluation, and first in vivo measurement with a 7 T 3-channel 13C and 4-channel 1H transceiver array optimized for 1H-decoupled 13C-MRS in the posterior human calf muscles. To ensure minimal cross-coupling between 13C and 1H arrays, several strategies were combined: mutual magnetic flux was minimized by coil geometry, two LCC traps were inserted into each 13C element, and band-pass and low-pass filters were integrated along the signal pathways. The developed coil array was successfully tested in phantom and in vivo MR experiments, showing a simplified spectral pattern and increase in signal-to-noise ratio of approximately a factor 2 between non-decoupled and 1H-decoupled spectra in a glucose phantom and the human calf muscle.