Chemical Shift Differences Research Articles

SABRE polarized low field rare-spin spectroscopy.

High-field nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for identification and characterization of chemicals and biomolecular structures. In the vast majority of NMR experiments, nuclear spin polarization arises from thermalization in multi-Tesla magnetic fields produced by superconducting magnets. In contrast, NMR instruments operating at low magnetic fields are emerging as a compact, inexpensive, and highly accessible alternative but suffer from low thermal polarization at a low field strength and consequently a low signal. However, certain hyperpolarization techniques create high polarization levels on target molecules independent of magnetic fields, giving low-field NMR a significant sensitivity boost. In this study, SABRE (Signal Amplification By Reversible Exchange) was combined with high homogeneity electromagnets operating at mT fields, enabling high resolution 1H, 13C, 15N, and 19F spectra to be detected with a single scan at magnetic fields between 1 mT and 10 mT. Chemical specificity is attained at mT magnetic fields with complex, highly resolved spectra. Most spectra are in the strong coupling regime where J-couplings are on the order of chemical shift differences. The spectra and the hyperpolarization spin dynamics are simulated with SPINACH. The simulations start from the parahydrogen singlet in the bound complex and include both chemical exchange and spin evolution at these mT fields. The simulations qualitatively match the experimental spectra and are used to identify the spin order terms formed during mT SABRE. The combination of low field NMR instruments with SABRE polarization results in sensitive measurements, even for rare spins with low gyromagnetic ratios at low magnetic fields.

Synthesis and microstructure analysis of stereoregular oligomers of CO<sub>2</sub>/<italic>cis</italic>-2,3-butene oxide alternating copolymer

The utilization of carbon dioxide (CO2) as a nontoxic, abundant, economical and renewable C1 building block for the production of valuable organic chemicals can contribute to a more sustainable chemical industry. In the chemical process using CO2 as a raw material, the synthesis of degradable aliphatic polycarbonates by alternating copolymerization with epoxides has attracted widespread attention. Notably, aliphatic polycarbonates with biocompatibility and biodegradability have gradually attracted great attention in the field of biomedicine in recent decades, since Inoue and co-workers first reported the copolymerization of CO2 and epoxides to produce biodegradable CO2-based polycarbonates by using heterogeneous catalyst in 1969. This method used non-toxic and inexpensive CO2 to replace the highly toxic phosgene in the traditional polycondensation as carbonylation reagent. Since then, many heterogeneous and homogeneous catalysts systems have been developed for this copolymerization. However, the commercial applications of aliphatic polycarbonates have very limited due to their poor thermal stability and the mechanical properties in comparison with polyolefins. Recently, the binary or bifunctional catalyst based on metal-salen complexes was found to be very efficient for this transformation in immortal polymerization mode, affording various polycarbonates with complete alternating nature, and excellent region- and stereo-selectivity. Along with the discovery of these highly stereoregular catalyst systems that can generate copolymers with high stereoregularity, study on the regiochemistry of epoxides ring-opening during the copolymerization and the stereochemistry of the carbonate unit sequence in the polymer main chain was also reported. The stereochemistry of a polymer significantly affects its physical property. For biodegradable polymers, e.g., polylactide, the degradation rate of isotactic polymers is much slower than that of amorphous polymers due to the orderly arrangement of building units in the former. Additionally, the relative stereochemistry in a polymeric chain also bears a memory of the reaction pathway leading to its formation. Therefore, the assignment of the stereochemistry information on a polymer chain is beneficial for understanding the polymerization mechanism and further designing more efficient catalyst systems. The microstructure information of a polymer can be obtained by analyzing its NMR spectrum. However, since the difference in chemical shifts of the same atom in different stereochemical environments is very small, it is quite difficult to assign the nuclear magnetic signals of polycarbonates. The signals of carbonyl and methine regions in the main chain in the 13C NMR spectra are significantly affected by stereochemistry of polycarbonates. As a consequence, the chemical shift and split of carbonyl and methane in the 13C NMR spectra are used to characterize the microstructure of polycarbonates. The accurate microstructure analysis of a polymer is performed by synthesizing the corresponding model oligomers and analysis of their 13C NMR spectra. In the present contribution, four kinds of model compounds of poly(2,3-butene carbonate): isotactic and syndiotactic tetramers and octamers, were synthesized by the use of chiral 2,3-butanediols as starting materials and triphosgene as carbonylation reagent, which not only avoided the degradation to cyclic carbonate during synthesis but also ensured a fast and reliable increase in the number of oligomer units. By comparing the signals in the 13C NMR spectra of steroregular oligomers, it could be concluded that the signals at 154.12, 75.23 and 15.71 ppm were attributed to carbonyl, methine and methyl of isotactic polymer, respectivey, while the signals at 154.09, 75.07 and 15.51 ppm were assigned to carbonyl, methine and methyl of syndiotactic polymer, respectively.

Arylpyran Pseudoacid Racemization: Rate Estimation and Structural Influences

Arylpyran pseudoacids showed slow ring-opening to the oxocarboxylic acids, and fast ring-closure to the lactols (pseudoacids) in a process that amounts to racemization. Compounds studied were patterned on “Cooper’s Pseudoacid” [3-hydroxy-4,4-dimethylisobenzopyran-1-one, 1], to which modifications were introduced to influence racemization rates by steric compression. Pseudoacid 1 in d6-DMSO had a barrier to racemization (ΔH‡) of +77.8(3) kJ/mol determined by dynamic nuclear magnetic resonance line-broadening measured from 25 °C to 100 °C, and coalescence of its diastereotopic gem-dimethyl signals at 78.5 °C. The solution half-life of 1 at room temperature is a few seconds. Introduction of various aryl 5-substituents lead to similar racemization barriers for the 5-fluoro-8-methyl derivative, and increasingly higher barriers for 5-methyl-8-methyl, 5-chloro-8-methyl, and 5-bromo-7-methyl derivatives. In 3-hydroxy-4,4,5,8-tetramethylisobenzopyran-1-one, the barrier was + 104(6) kJ/mol, and the solution enantiomer t1/2 at room temperature was of the order of hours. Chemical shift differences (ΔνMe) between diastereotopic methyls of 8-methyl-5-substituted enantiomeric pseudoacids were observed in the order F < CH3 < Cl, and generally decrease with increasing temperatures. The pseudoaxial and pseudoequatorial dispositions of the gem-dimethyl groups show smaller torsional differences in the presence of adjacent aryl 5-substituents. The crystal structures of 5-fluoro, 5-chloro and 5-methyl derivatives of 3-hydroxy-4,4,8-trimethylisobenzopyran-1-ones and of 5-bromo-3-hydroxy-4,4,7-trimethylisobenzopyran-1-one are reported, along with several relevant secondary endocyclic pseudoamide derivatives. Electronic energy computations (B3LYP/6-31 + (G(d,p) level of theory) generally support the steric compression model. Equilibration between diastereomeric pseudoacids 3-hydroxy-4-ethyl-4-methylisobenzopyran-1-one and 3-hydroxy-4,5,8-trimethyl-4-phenylisobenzopyran-1-one were rapid with trans isomers decreasingly favored as temperature increases, and cis isomers less stable by + 4.8(2) kJ/mol and + 7.0(4) kJ/mol, respectively. The first-order rate of cis → trans conversion was measureable for 3-hydroxy-4,5,8-trimethyl-4-phenylisobenzopyran-1-one at 25 °C was found to be 0.00403/min, t1/2 = 2.87 h. Attempts to produce 5-isopropyl, 5-t-butyl and 4-phenyl-5-methyl pseudoacid derivatives were frustrated by aryl substituent rearrangement upon ring closure to the intermediate indanones. Structures of rearranged 3-hydroxy-4-methyl-4-(2′-chloro-5′-methylphenyl)isobenzopyran-1-one (pseudoacid) and N-benzyl-6-isopropyl-4,4,8-trimethylisobenzopyrimidin-1-one (pseudoamide) are reported. Arylpyran pseudoacids undergo slow ring opening and fast ring closing in solution amounting to racemization, rates of which can be slowed considerably by steric compression between the 5-substituent and the adjacent gem-dimethyl groups.

A compact X-Band ODNP spectrometer towards hyperpolarized 1H spectroscopy

The demand for compact benchtop NMR systems that can resolve chemical shift differences in the ppm to sub-ppm range is growing. However due to material and size restrictions these magnets are limited in field strength and thus in signal intensity and quality. The implementation of standard hyperpolarization techniques is a next step in an effort to boost the signal. Here we present a compact Overhauser Dynamic Nuclear Polarization (ODNP) setup with a permanent magnet that can resolve 1H chemical shift differences in the ppm range. The assembly of the setup and its components are described in detail, and the functionality of the setup is demonstrated experimentally with ODNP enhanced relaxation measurements yielding a maximal enhancement of −140 for an aqueous 4-hydroxy-TEMPO solution. Additionally, 1H spectroscopic resolution and significant enhancements are demonstrated on acetic acid as a solvent.

An In Vivo Assessment of Regional Brain Temperature during Whole-Body Cooling for Neonatal Encephalopathy

To assess differences in regional brain temperatures during whole-body hypothermia and test the hypothesis that brain temperature profile is nonhomogenous in infants with hypoxic-ischemic encephalopathy. Infants with hypoxic-ischemic encephalopathy were enrolled prospectively in this observational study. Magnetic resonance (MR) spectra of basal ganglia, thalamus, cortical gray matter, and white matter (WM) were acquired during therapeutic hypothermia. Regional brain tissue temperatures were calculated from the chemical shift difference between water signal and metabolites in the MR spectra after performing calibration measurements. Overall difference in regional temperature was analyzed by mixed-effects model; temperature among different patterns and severity of injury on MR imaging also was analyzed. Correlation between temperature and depth of brain structure was analyzed using repeated-measures correlation. In total, 53 infants were enrolled (31 girls, mean gestational age: 38.6±2weeks; mean birth weight: 3243±613g). MR spectroscopy was acquired at mean age of 2.2±0.6days. A total of 201MR spectra were included in the analysis. The thalamus, the deepest structure (36.4±2.3mm from skull surface), was lowest in temperature (33.2±0.8°C, compared with basal ganglia: 33.5±0.9°C; gray matter: 33.6±0.7°C; WM: 33.8±0.9°C, all P < .001). Temperatures in more superficial gray matter and WM regions (depth: 21.9±2.4 and 21.5±2.2mm) were greater than the rectal temperatures (33.4±0.4°C, P < .03). There was a negative correlation between temperature and depth of brain structure (rrm=-0.36, P < .001). Whole-body hypothermia was effective in cooling deep brain structures, whereas superficial structures were warmer, with temperatures significantly greater than rectal temperatures.

19F NMR Studies of Cyclophilin a and its Interaction with HIV-1 Capsid

Human cyclophilin A (CypA) directly interacts with the HIV-1 capsid, the conical core within the virion that encloses the RNA genome, and modulates viral infectivity through ill-understood mechanism(s). To study the binding between CypA and capsid, we employed 19F NMR spectroscopy. The 100% naturally abundant spin-1/2 19F nucleus with high gyromagnetic ratio is absent from most naturally occurring biological molecules, and 19F chemical shifts are highly sensitive to the local conformation and chemical environment. As a result, 19F serves as a valuable NMR probe in biological systems. Using 4F-, 5F-, 6F-, or 7F-substituted indole as a precursor, single 19F labels were introduced into CypA at different positions of the sole tryptophan ring in the protein, which is conveniently located in the capsid binding site. X-ray crystallography and 1H-1H NOEs confirmed that CypA retains the same structure after fluorine labelling and only small chemical shift changes were present in the 1H-15N HSQC spectra of the F-CypA samples. Interestingly, all amides that exhibit chemical shift differences are connected through an extended hydrogen bonding network, and the topology of the networks differ between the differently labeled fluorinated F-CypA proteins. We hypothesize that the fluorine atom interacts with nearby polar groups and this electronic perturbation is passed on through the hydrogen bonding network to generate a unique, allosteric long-range effect. The solvent accessibility of the capsid binding site for the different positions on the 6-membered indole ring was probed by recording the effects of D2O content in D2O/H2O solutions on the 19F chemical shift. We also investigated the binding between capsid protein and CypA by 19F NMR titration experiment. Analysis of the titration shifts permitted the determination of individual binding constants for different fluorinated F-CypAs.

Improved Magnetization Transfers among Quadrupolar Nuclei in Two-Dimensional Homonuclear Correlation NMR Experiments Applied to Inorganic Network Structures.

We demonstrate that supercycles of previously introduced two-fold symmetry dipolar recoupling schemes may be utilized successfully in homonuclear correlation nuclear magnetic resonance (NMR) spectroscopy for probing proximities among half-integer spin quadrupolar nuclei in network materials undergoing magic-angle-spinning (MAS). These (SR2) (SR2) and (SR2)M recoupling sequences with and offer comparably efficient magnetization transfers in single-quantum–single-quantum (1Q–1Q) correlation NMR experiments under moderately fast MAS conditions, as demonstrated at 14.1 T and 24 kHz MAS in the contexts of B NMR on a NaO–CaO–BO–SiO glass and Al NMR on the open framework aluminophosphate AlPO-CJ19 [(NH)Al(PO)HPOHO]. Numerically simulated magnetization transfers in spin–3/2 pairs revealed a progressively enhanced tolerance to resonance offsets and rf-amplitude errors of the recoupling pulses along the series (SR2) (SR2) (SR2)M for increasing differences in chemical shifts between the two nuclei. Nonetheless, for scenarios of a relatively minor chemical-shift dispersions ( kHz), the (SR2)M supercycles perform best both experimentally and in simulations.

Experimental estimates of compression heating and decompression cooling in ethylene glycol.

The chemical shift difference, Δσ, between the methylene and hydroxyl protons in the high resolution 1 H nuclear magnetic resonance spectrum of ethylene glycol is shown to be pressure dependent. The equilibrium Δσ values for ethylene glycol are reported as a function of temperature and pressure between ambient conditions, 323 K and 2 kbar, respectively. This surface is used along with Δσ values measured in response to a rapid pressure increase to calculate a temperature rise that is used to infer a temperature change for water that is consistent with theoretical estimates. This work implies that compression heating and decompression cooling are not significant enough to interfere with pressure induced protein folding studies.

Polarization Effect on Regioselectivity of Pd‐Catalyzed Cyclization of 2‐Alkynylbenzaldehydes

We report a study on the relationship between the polarization of C≡C bond in a series of 2‐alkynylbenzaldehydes and the regioselectivity of their Pd‐catalyzed annulation reactions. The electrophilic and nucleophilic character of the triple bond carbon atoms was examined using 13C and HMBC NMR experiments. The direction of polarization is determined only by the formyl group and its position on an aromatic ring. On the other hand, the strength of the polarization expressed through chemical shift difference Δδ of acetylenic carbon atoms is a result of electron‐withdrawing or electron‐donating ability of the other substituent. Out of two possible regioisomers, the Pd‐catalyzed cyclization reactions of 2‐alkynylbenzaldehydes that were studied predominantly afforded 1H‐isochromenes suggesting that the polarization of a triple bond is one of the contributing factors in the regioselectivity of this process.

When not to rely on Boltzmann populations. Automated CASE-3D structure elucidation of hyacinthacines through chemical shift differences.

An Akaike Information Criterion (AIC) procedure (CASE-3D) has been successfully applied to the NMR based configurational assignment of reported hyacinthacines (1-3,5-8), recently target of configurational analysis using the popular DP4+ methodology. The present analysis makes use of reported 1 H and 13 C shifts and, in some particular cases, a few 3 JHH couplings. The difficulty in proper computational prediction of relative energies, in molecules capable of inter-molecular hydrogen bonding, introduces large errors in the prediction of conformationally averaged NMR properties in methods based on Boltzmann averaging such as DP4 or DP4+. In contrast CASE-3D conformational amplitudes are free parameters in the model. Here we show that the CASE-3D conformational model selection strategy, when combined with a larger energy cutoff in the molecular-modelling conformational exploration, was sufficient to correctly assign the relative configuration in five of seven cases. Introduction of more information, either by supplementing 1 H and 13 C data with a few J-couplings, or using a cutoff based on computed DFT energies for the definition of the conformational ensembles, allowed the safe assignment of configuration for all compounds.

Comparison of dynamic brain metabolism during antegrade cerebral perfusion versus deep hypothermic circulatory arrest using proton magnetic resonance spectroscopy

Comparison of dynamic brain metabolism during antegrade cerebral perfusion versus deep hypothermic circulatory arrest using proton magnetic resonance spectroscopy

Detection and Chiral Recognition of α‐Hydroxyl Acid through 1H and CEST NMR Spectroscopy Using a Ytterbium Macrocyclic Complex

AbstractChiral α‐hydroxyl acids are of great importance in chemical synthesis. Current methods for recognizing their chirality by 1H NMR are limited by their small chemical shift differences and intrinsic solubility problem in organic solvents. Herein, we developed three YbDO3A(ala)3 derivatives to recognize four different commercially available chiral α‐hydroxyl acids in aqueous solution through 1H NMR and chemical exchange saturation transfer (CEST) spectroscopy. The shift difference between chiral α‐hydroxyl acid observed by proton and CEST NMR ranged from 15–40 and 20–40 ppm, respectively. Our work demonstrates for first time, that even one chiral center on the side‐arm chain of cyclen could set the stage for rotation of the other two non‐chiral side chains into a preferred position. This is ascribed to the lower energy state of the structure. The results show that chiral YbDO3A‐like complexes can be used to discriminate chiral α‐hydroxyl acids with a distinct signal difference.

Detection and Chiral Recognition of α-Hydroxyl Acid through 1 H and CEST NMR Spectroscopy Using a Ytterbium Macrocyclic Complex.

Chiral α-hydroxyl acids are of great importance in chemical synthesis. Current methods for recognizing their chirality by 1 H NMR are limited by their small chemical shift differences and intrinsic solubility problem in organic solvents. Herein, we developed three YbDO3A(ala)3 derivatives to recognize four different commercially available chiral α-hydroxyl acids in aqueous solution through 1 H NMR and chemical exchange saturation transfer (CEST) spectroscopy. The shift difference between chiral α-hydroxyl acid observed by proton and CEST NMR ranged from 15-40 and 20-40 ppm, respectively. Our work demonstrates for first time, that even one chiral center on the side-arm chain of cyclen could set the stage for rotation of the other two non-chiral side chains into a preferred position. This is ascribed to the lower energy state of the structure. The results show that chiral YbDO3A-like complexes can be used to discriminate chiral α-hydroxyl acids with a distinct signal difference.

Rotamer Jumps, Proton Exchange, and Amine Inversion Dynamics of Dimethylated Lysine Residues in Proteins Resolved by pH-Dependent 1H and 13C NMR Relaxation Dispersion.

Post-translational methylation of lysine side chains is of great importance for protein regulation, including epigenetic control. Here, we present specific 13CHD2 labeling of dimethylated lysines as a sensitive probe of the structure, interactions, and dynamics of these groups, and outline a theoretical and experimental framework for analyzing their conformational dynamics using 1H and 13C CPMG relaxation dispersion experiments. Dimethylated lysine side chains in calcium-loaded calmodulin show a marked pH dependence of their Carr-Purcell-Meiboom-Gill (CPMG) dispersion profiles, indicating complex exchange behavior. Combined analysis of 1H and 13C CPMG relaxation dispersions requires consideration of 12-state correlated exchange of the two methyl groups due to circular three-state rotamer jumps around the Cε-Nζ axis combined with proton exchange and amine inversion. Taking into account a number of fundamental constraints, the exchange model can be reduced to include only three fitted parameters, namely, the geometric average of the rotamer-jump rate constants, the rate constant of deprotonation of Nζ, and the chemical shift difference between the trans and gauge positions of the 13C or 1H nuclei. The pH dependence indicates that protonation of the end group dramatically slows down rotamer exchange for some lysine residues, whereas deprotonation leads to rapid amine inversion coupled with rotamer scrambling. The observed variation among residues in their exchange behavior appears to depend on the structural environment of the side chain. Understanding this type of exchange process is critical to correctly interpreting NMR spectra of methylated lysine side chains. The exchange model presented here forms the basis for studying the structure and dynamics of epigenetically modified lysine side chains and perturbations caused by changes in pH or interactions with target proteins.

Repeatability and reproducibility of prospective motion- and shim corrected 2D glycoCEST MRI.

Repeated glycoCEST MRI measurements on the same subject should produce similar results under the same environmental and experimental conditions. However, fluctuations in the static B0 field, which may occur between and within measurements due to heating of the shim iron or subject motion, may alter results and affect reproducibility. Here we investigate the repeatability and reproducibility of glycoCEST measurements and examine the effectiveness of a real-time shim- and motion navigated chemical exchange saturation transfer (CEST) sequence to improve reproducibility. In nine subjects, double volumetric navigated (DvNav)-CEST acquisitions in the calf muscle were repeated five times in each of two sessions-the first without correction, and the second with real-time shim- and motion correction applied. In both sessions a dynamically changing field was introduced by running a 5-minute gradient intensive diffusion sequence. We evaluated the effect of the introduced B0 inhomogeneity on the reproducibility of glycoCEST, where the small chemical shift difference between the hydroxyl and bulk water protons at 3 T makes CEST quantification extremely sensitive to magnetic field inhomogeneities. With real-time shim- and motion correction, glycoCEST results were relatively consistent with mean coefficient of variation (CoV) 2.7%±1.4% across all subjects, whereas without correction the results were less consistent with CoV 84%±71%. Our results demonstrate that real-time shim- and motion correction can mitigate effects of B0 field fluctuations and improve reproducibility of glycoCEST data. This is important when conducting longitudinal studies or when using glycoCEST MRI to assess treatment or physiological responses over time.

Site-selective 1H/2H labeling enables artifact-free 1H CPMG relaxation dispersion experiments in aromatic side chains

Aromatic side chains are often key residues in enzyme active sites and protein binding sites, making them attractive probes of protein dynamics on the millisecond timescale. Such dynamic processes can be studied by aromatic 13C or 1H CPMG relaxation dispersion experiments. Aromatic 1H CPMG relaxation dispersion experiments in phenylalanine, tyrosine and the six-ring moiety of tryptophan, however, are affected by 3J 1H–1H couplings which are causing anomalous relaxation dispersion profiles. Here we show that this problem can be addressed by site-selective 1H/2H labeling of the aromatic side chains and that artifact-free relaxation dispersion profiles can be acquired. The method has been further validated by measuring folding–unfolding kinetics of the small protein GB1. The determined rate constants and populations agree well with previous results from 13C CPMG relaxation dispersion experiments. Furthermore, the CPMG-derived chemical shift differences between the folded and unfolded states are in excellent agreement with those obtained directly from the spectra. In summary, site-selective 1H/2H labeling enables artifact-free aromatic 1H CPMG relaxation dispersion experiments in phenylalanine and the six-ring moiety of tryptophan, thereby extending the available methods for studying millisecond dynamics in aromatic protein side chains.

High pressure response of 1H NMR chemical shifts of purine nucleotides

The study of the pressure response by NMR spectroscopy provides information on the thermodynamics of conformational equilibria in proteins and nucleic acids. For obtaining a database for expected pressure effects on free nucleotides and nucleotides bound in macromolecular complexes, the pressure response of 1H chemical shifts and J-coupling constants of the purine 5′-ribonucleotides AMP, ADP, ATP, GMP, GDP, and GTP were studied in the absence and presence of Mg2+-ions. Experiments are supported by quantum-chemical calculations of populations and chemical shift differences in order to corroborate structural interpretations and to estimate missing data for AMP. The preference of the ribose S puckering obtained from the analysis of the experimental J-couplings is also confirmed by the calculations. In addition, the pressure response of the non-hydrolysable GTP analogues GppNHp, GppCH2p, and GTPγS was examined within a pressure range up to 200 MPa. As observed earlier for 31P NMR chemical shifts of these nucleotides the pressure dependence of chemical shifts is clearly non-linear in most cases. In di- and tri-phospho nucleosides, the resonances of the two protons bound to the ribose 5′ carbon are non-equivalent and can be observed separately. The gg-rotamer at C4′- C5′ bond is strongly preferred and the downfield shifted resonance can be assigned to the H5″ proton in the nucleotides. In contrast, in adenosine itself the frequencies of the two resonances are interchanged.

PE-SERF: A sensitivity-improved experiment to measure JHH in crowded spectra.

Aiming at facilitating the analysis of molecular structure, the gradient-encoded selective refocusing methods (G-SERF) and a great number of its variants for measuring proton-proton coupling constants have been proposed. However, the sensitivity is an issue in the 2D gradient-encoded experiments, because the signal intensity is determined by the slice thickness of the sample that depends on encoding gradient and the bandwidth of selective pulses which is limited by the smallest chemical shift difference of any two coupled protons. Here, we present a method dubbed PE-SERF (perfect echo selective refocusing) which can determine all JHH values involving a selected proton with improved sensitivity compared to original G-SERF experiment. The modules of perfect echo involving selective pulses and gradient-encoded selective refocusing are combined in the method, so that the unwanted J couplings arising from coupled spin pairs in the same sample slice would be nullified. In this way, instead of single proton, a pair of coupled protons is allowed to share a sample slice, and thus the slice thickness can be increased and the spectral sensitivity can be improved. The performance of the method is demonstrated by experiments on quinine and strychnine.

Axially Chiral Trifluoromethylbenzimidazolylbenzoic Acid: A Chiral Derivatizing Agent for α-Chiral Primary Amines and Secondary Alcohols To Determine the Absolute Configuration.

Racemic 2-(2-trifluoromethyl)-1H-benzo[d]imidazol-1-yl)benzoic acid (TBBA) was synthesized in three steps from 1-fluoro-2-nitrobenzene. Target (P)- and (M)-TBBA atropisomers were stable with a racemization barrier above 30 kcal/mol. As a chiral derivatizing agent, TBBA showed much higher differences in chemical shifts (ΔδPM) than the conventional Mosher's acid.

Is perfection ever attainable? Strong coupling effects in the Perfect Echo

The Perfect Echo sequence, originally proposed in the late 1980s, has recently been popularised with many applications in the field of small-molecule proton NMR spectroscopy. The Perfect Echo refocuses all homonuclear J-couplings for AX spin systems and refocuses magnetization in-phase for more complex weakly coupled spin systems, albeit with some intensity reduction. In contrast to suggestions in previous publications, spectra acquired in our laboratory showed that the Perfect Echo caused intensity distortions in strongly coupled systems where the chemical shift difference between the coupled spins was not large compared to the J-coupling. This paper reports experimental observations and theoretical analysis of strongly coupled spins to confirm the distortions are real and that they originate principally from transfer of magnetization caused by the final inversion pulse of the Perfect Echo. The intensity changes are not large, but because of them, identifications of coupling partners based on resonance intensities ("roofing") can no longer be relied on when the Perfect Echo is used. However, theory and experiment confirm that adding an orthogonal excitation pulse at the end of the Perfect Echo greatly reduces the distortions.