Simultaneously enhancing spectral resolution and sensitivity in heteronuclear correlation NMR spectroscopy

Broadband homonuclear decoupled HSQMBC methods

Long-range heteronuclear coupling constants convey invaluable information for stereochemical and conformational analysis of molecules from synthetic and natural origin. Here, we report a real-time Zangger-Sterk CPMG-HSQMBC method for the precise and direct measurement of multiple-bond heteronuclear couplings. It is demonstrated that the real-time acquisition strategy combined with multiple slice selective excitation can provide substantial improvement in sensitivity (or reduction of experimental time) as compared to other variants of broadband homonuclear decoupled HSQMBC methods published previously. Scope and limitations of the different strategies applied for decoupling are reviewed. Moreover, practical guidelines for the choice of the most appropriate method are also presented. Applications are given on a metal complex incorporating P-heterocycles and two diglycosyl-selenides for the extraction of ⁿ J(¹ H, ³¹ P) and ⁿ J(¹ H, ⁷⁷ Se), respectively.

Combining pure shift and J-edited spectroscopies: A strategy for extracting chemical shifts and scalar couplings from highly crowded proton spectra of oligomeric saccharides

We report the application of pure shift and J-edited nuclear magnetic resonance spectroscopies to the structural analysis of a protected maltotrioside synthetic intermediate whose crowded ¹ H spectrum displays highly crowded regions. The analytical strategy is based on the implementation of J-edited and TOCSY experiments whose resolution is optimized by the use of broadband homonuclear decoupling and selective refocusing techniques, to assign and measure chemical shifts and homonuclear scalar couplings with high accuracy. The resulting data show a high level of complementarity, providing a detailed insight into each subunit of this oligomeric saccharide, even for proton sites whose nuclear magnetic resonance signals strongly overlap. This approach allowed for fully assigning proton chemical shifts and extracting 80% of the ³ J_HH couplings that are in excellent agreement with those expected for D-gluco-pyranosyl units in ⁴ C₁ conformations.

Compressed NMR: Combining compressive sampling and pure shift NMR techniques

Historically, the resolution of multidimensional nuclear magnetic resonance (NMR) has been orders of magnitude lower than the intrinsic resolution that NMR spectrometers are capable of producing. The slowness of Nyquist sampling as well as the existence of signals as multiplets instead of singlets have been two of the main reasons for this underperformance. Fortunately, two compressive techniques have appeared that can overcome these limitations. Compressive sensing, also known as compressed sampling (CS), avoids the first limitation by exploiting the compressibility of typical NMR spectra, thus allowing sampling at sub-Nyquist rates, and pure shift techniques eliminate the second issue "compressing" multiplets into singlets. This paper explores the possibilities and challenges presented by this combination (compressed NMR). First, a description of the CS framework is given, followed by a description of the importance of combining it with the right pure shift experiment. Second, examples of compressed NMR spectra and how they can be combined with covariance methods will be shown.

Semi-real-time acquisition for fast pure shift NMR at maximum resolution

Homonuclear decoupling can give impressive improvements in spectral resolution when overlapping multiplet structures limit the interpretability of NMR spectra. General homodecoupling methods ("pure shift NMR") typically use either interferogram-style acquisition, which is slow but gives optimum resolution, or real-time acquisition, which is fast but gives broader lines. Here we describe an alternative data acquisition strategy that allows rapid acquisition without resolution loss, illustrating the method with band-selective homodecoupling experiments and demonstrating its advantages over competing methods.

Rapid elucidation of chemical shift correlations in complex NMR spectra of organic molecules: Two-dimensional Hadamard pure shift NMR spectroscopy

Novel two dimensional Hadamard encoding/decoding based pure shift NMR acquisition techniques (TOCSY and HSQC) have been developed, which provide chemical shift information at ultra high resolution in very short spectrometer times. The efficacy of these methods for rapid assignment of chemical shifts in complex NMR spectra of organic molecules/natural products has been demonstrated. This would be of great help for rapid analysis of samples during separation of complex mixtures.

PSYCHE Pure Shift NMR Spectroscopy

Broadband homodecoupling techniques in NMR, also known as "pure shift" methods, aim to enhance spectral resolution by suppressing the effects of homonuclear coupling interactions to turn multiplet signals into singlets. Such techniques typically work by selecting a subset of "active" nuclear spins to observe, and selectively inverting the remaining, "passive", spins to reverse the effects of coupling. Pure Shift Yielded by Chirp Excitation (PSYCHE) is one such method; it is relatively recent, but has already been successfully implemented in a range of different NMR experiments. Paradoxically, PSYCHE is one of the trickiest of pure shift NMR techniques to understand but one of the easiest to use. Here we offer some insights into theoretical and practical aspects of the method, and into the effects and importance of the experimental parameters. Some recent improvements that enhance the spectral purity of PSYCHE spectra will be presented, and some experimental frameworks, including examples in 1D and 2D NMR spectroscopy, for the implementation of PSYCHE will be introduced.

Perfecting band selective homo-decoupling for decoupling two signals coupled within the same band

Recently, pure shift NMR methods have delivered ultrahigh resolution broadband proton NMR spectra that display singlet peak per chemical site. BASH/HOBS (band selective homo-decoupling/homonuclear band selective) decoupling is the only method that provides singlet only spectrum for a selected signal or a group of signals with a sensitivity higher than regular proton NMR, and an order of magnitude higher than broadband pure shift techniques. It is this aspect that makes this technique important. In the present work, we show that perfect echo (PE) when combined with band selective homo-decoupling (BASHD) permits increasing the bandwidth of the BASH/HOBS decoupling resulting in reduced experimental time for this class of experiments. Using new PE-BASHD technique two closely resonating coupled signals could be decoupled in a single experiment which otherwise demands two separate BASHD experiments. In polypeptides, it also allows decoupling of Hα and HN signals simultaneously from each other as well as from the side chain protons reducing experimental time. Further, pseudo 2D based PE-BASHD scheme provides spectrum superior to the real-time BASHD spectrum when applied to closely resonating coupled signals. Numerical simulation as well as experimental results indicate that the PE-BASHD sequence display good quality singlet only spectrum even in the presence of moderate strong coupling.

Isotropic/Anisotropic NMR Editing by Resolution-Enhanced NMR Spectroscopy

Modern resolution-enhanced NMR techniques can monitor the in situ discrimination of co-existing isotropic and anisotropic contributions of small molecules dissolved in weakly aligning PMMA/CDCl3 media. The simultaneous sign-sensitive determination of accurate Δδ^iso-aniso (¹ H), Δδ^iso-aniso (¹³ C) and/or isotropic ¹ J_CH and anisotropic ¹ T_CH coupling constants (and consequently ¹ H-¹³ C residual dipolar couplings and ¹ H/¹³ C residual chemical shift anisotropies) can be performed from spectral-aliased heteronuclear single-quantum correlation spectra.

Current developments in homonuclear and heteronuclear J-resolved NMR experiments

Two-dimensional J-resolved (Jres) NMR experiments offer a simple, user-friendly spectral representation where the information of coupling constants and chemical shifts are separated into two orthogonal frequency axis. Since its initial proposal 40 years ago, Jres has been the focus of considerable interest both in improving the basic pulse sequence and in its successful application to a wide range of studies. Here, the latest developments in the design of novel Jres pulse schemes are reviewed, mainly focusing on obtaining pure absorption lineshapes, minimizing strong coupling artifacts, and also optimizing sensitivity and experimental measurements. A discussion of several Jres versions for the accurate measurement of a different number of homonuclear (J_HH ) and heteronuclear (J_CH ) coupling constants is presented, accompanied by some illustrative examples.

Sugar-Functional Vinyl Addition Poly(norbornene)-Photopatternable Poly(norbornenyl gluconamide) Compositions Developed with Water

The norbornenyl gluconamide (NBGA) monomer can be polymerized by a number of palladium catalysts to give water-soluble, vinyl addition poly(NBGA). Depending on the catalyst used, the reaction conditions, and the chain-transfer additives employed, high-molecular-weight polymers can be obtained. These polymers can be thermally cross-linked at ca. 190 °C or at ca. 150 °C when the difunctional glutaraldehyde is added. A photopatternable composition is formed by the addition of a water-soluble diazide when the poly(NBGA) molecular weight is sufficiently high. After image-wise exposure, negative-tone patterns are revealed by water development. A detailed analysis of NBGA monomer structures by high-resolution nuclear magnetic resonance techniques including pure shift is reported showing that two diastereomers of each endo- and exo-isomer are formed from the reaction of norbornene methylamine and δ-gluconolactone.

Boosting the NMR Assignment of Carbohydrates with Clean In-Phase Correlation Experiments

Novel CLIP-COSY based homo- and heteronuclear correlation experiments are reported for the rapid, semi-automated NMR assignment of small to medium-sized molecules. The homonuclear CLIP-COSY and corresponding relayed experiments employ the perfect-echo based mixing sequence for in-phase coherence transfer between directly and/or indirectly coupled proton spins. The combined analysis of the resulting CLIP-COSY and relayed spectra made it possible to easily track down, layer by layer, the proton-proton connectivity network. In larger molecules the narrow chemical shift range of protons may, however, compromise the efficacy of the homonuclear correlation based assignment strategy. To overcome this limitation, an HSQC variant of the CLIP-COSY experiment has now been devised. Combined treatment of HSQC-CLIP-COSY (relayed) and standard HSQC spectra facilitates simultaneous and semi-automatic assignment of ¹ H and ¹³ C resonances of medium-sized molecules, such as pentasaccharides. The recently introduced PSYCHE broadband homonuclear decoupling scheme has been also implemented into the devised homo- and heteronuclear CLIP-COSY based experiments, resulting in fully decoupled high-resolution pure-shift correlation spectra.

The FAQUIRE Approach: FAst, QUantitative, hIghly Resolved and sEnsitivity Enhanced 1H, 13C Data

The targeted analysis of metabolites in complex mixtures is a challenging issue. NMR is one of the major tools in this field, but there is a strong need for more sensitive, better-resolved, and faster quantitative methods. In this framework, we introduce the concept of FAst, QUantitative, hIghly Resolved and sEnsitivity enhanced (FAQUIRE) NMR to push forward the limits of metabolite NMR analysis. 2D ¹H, ¹³C 2D quantitative maps are promising alternatives for enhancing the spectral resolution but are highly time-consuming because of (i) the intrinsic nature of 2D, (ii) the longer recycling times required for quantitative conditions, and (iii) the higher number of scans needed to reduce the level of detection/quantification to access low concentrated metabolites. To reach this aim, speeding up the recently developed QUantItative Perfected and pUre shifted HSQC (QUIPU HSQC) is an interesting attempt to develop the FAQUIRE concept. Thanks to the combination of spectral aliasing, nonuniform sampling, and variable repetition time, the acquisition time of 2D quantitative maps is reduced by a factor 6 to 9, while conserving a high spectral resolution thanks to a pure shift approach. The analytical potential of the new Quick QUIPU HSQC (Q QUIPU HSQC) is evaluated on a model metabolite sample, and its potential is shown on breast-cell extracts embedding metabolites at millimolar to submillimolar concentrations.

A pure shift experiment with increased sensitivity and superior performance for strongly coupled systems

Motivated by the persisting need for enhanced resolution in solution state NMR spectra, pure shift techniques such as Zangger-Sterk decoupling have recently attracted widespread interest. These techniques for homonuclear decoupling offer enhanced resolution in one- and multidimensional proton detected experiments by simplifying multiplet structures. In this work, a modification to the popular Zangger-Sterk technique PEPSIE (Perfect Echo Pure Shift Improved Experiment) is presented, which decouples pairs of spins even if they share the same volume element. This in turn can drastically improve the sensitivity, as compared to classical Zangger-Sterk decoupling, as larger volume elements can be used to collect the detected signal. Most interestingly, even in the presence of moderate strong coupling, the PEPSIE experiment produces clean and widely artifact free spectra. In order to better understand this - to us initially - surprising behaviour we performed analyses using numerical simulations and derived an (approximate) analytical solution from density matrix formalism. We show that this experiment is particularly suitable to study samples with strong signal clustering, a situation which can render classic Zangger-Sterk decoupling inefficient.

Desktop NMR and Its Applications From Materials Science To Organic Chemistry

NMR spectroscopy is an indispensable method of analysis in chemistry, which until recently suffered from high demands for space, high costs for acquisition and maintenance, and operational complexity. This has changed with the introduction of compact NMR spectrometers suitable for small-molecule analysis on the chemical workbench. These spectrometers contain permanent magnets giving rise to proton NMR frequencies between 40 and 80 MHz. The enabling technology is to make small permanent magnets with homogeneous fields. Tabletop instruments with inhomogeneous fields have been in use for over 40 years for characterizing food and hydrogen-containing materials by relaxation and diffusion measurements. Related NMR instruments measure these parameters in the stray field outside the magnet. They are used to inspect the borehole walls of oil wells and to test objects nondestructively. The state-of-the-art of NMR spectroscopy, imaging and relaxometry with compact instruments is reviewed.

Analyses of Complex Mixtures by F1 Homo-Decoupled Diagonal Suppressed Total Correlation Spectroscopy

A diagonal suppressed F₁ decoupled total correlation spectroscopy(TOCSY) experiment is developed for analyses of complex mixtures. In 2D homonuclear correlation, assignment of the cross peaks is crucial for structure elucidation. However, when cross peaks are close to the diagonal peaks in overcrowded spectral regions, their assignment becomes tedious. In complex mixtures, the presence of multiple spectra along with broad and complex proton multiplets owing to homonuclear scalar couplings degrade the resolution to the extent that assignment of various cross peaks becomes tedious or impossible. Herein, a diagonal suppressed total correlation technique with F₁ decoupling is presented to improve the resolution of the cross peaks. The resolution of the cross peaks is improved by both diagonal suppression as well as the collapse of the multiplets to singlets. Application of the method to a few mixtures of organic compounds reveals better identification of the cross peaks relative to other TOCSY variants.

High Resolution NMR Spectroscopy as a Structural and Analytical Tool for Unsaturated Lipids in Solution

Mono- and polyunsaturated lipids are widely distributed in Nature, and are structurally and functionally a diverse class of molecules with a variety of physicochemical, biological, medicinal and nutritional properties. High resolution NMR spectroscopic techniques including 1H-, 13C- and 31P-NMR have been successfully employed as a structural and analytical tool for unsaturated lipids. The objective of this review article is to provide: (i) an overview of the critical 1H-, 13C- and 31P-NMR parameters for structural and analytical investigations; (ii) an overview of various 1D and 2D NMR techniques that have been used for resonance assignments; (iii) selected analytical and structural studies with emphasis in the identification of major and minor unsaturated fatty acids in complex lipid extracts without the need for the isolation of the individual components; (iv) selected investigations of oxidation products of lipids; (v) applications in the emerging field of lipidomics; (vi) studies of protein-lipid interactions at a molecular level; (vii) practical considerations and (viii) an overview of future developments in the field.

SHARPER Reaction Monitoring: Generation of a Narrow Linewidth NMR Singlet, without X-Pulses, in an Inhomogeneous Magnetic Field

We report a new pure-shift method, termed SHARPER (Sensitive, Homogeneous, And Resolved PEaks in Real time) designed for the analysis of reactions and equilibria by NMR. By focusing on a single selected signal, SHARPER removes all heteronuclear couplings of a selected nucleus without the need to pulse on X channels, thus overcoming hardware limitations of conventional spectrometers. A more versatile decoupling scheme, termed sel-SHARPER, removes all heteronuclear and homonuclear couplings of the selected signal. Both methods are characterized by a periodic inversion of the active spin during the real-time acquisition. In addition to decoupling, they also compensate for pulse imperfections and magnetic field inhomogeneity, generating an extremely narrow singlet with a linewidth approaching limits dictated by the spin-spin relaxation. The decoupling and line narrowing effected by (sel)-SHARPER provide significant increases in the signal-to-noise (S/N) ratio. Increases of 20-fold were routinely achieved for ¹⁹F detection. sel-SHARPER is also applicable to first- and higher-order ¹H spectra. The sensitivity gains are substantially greater for inhomogeneous magnetic fields, including dynamic inhomogeneity caused by gas sparging. The parameters of the pulse sequences have been analyzed in detail to provide guidelines for their most effective application. The considerable reduction in the detection threshold induced by (sel)-SHARPER make the technique particularly suited for in situ monitoring of reaction kinetics. The approach is illustrated by a ¹⁹F NMR study of the protodeboronation of an aryl boronic acid. Here, the high S/N allowed reliable determination of the net protodeoboronation kinetics, and the excess line broadening of ¹⁹F singlets was utilized to characterize the boronic acid/boronate equilibrium kinetics. Oxidation of diphenylphosphine, monitored by ³¹P NMR under optimized gas-flow conditions, demonstrated the high tolerance of SHARPER to dynamic inhomogeneity. The principles of the (sel)-SHARPER sequences are expected to find numerous applications in the design of new NMR experiments.

Poecillastrosides, Steroidal Saponins from the Mediterranean Deep-Sea Sponge Poecillastra compressa (Bowerbank, 1866)

The first chemical investigation of the Mediterranean deep-sea sponge Poecillastra compressa (Bowerbank, 1866) led to the identification of seven new steroidal saponins named poecillastrosides A–G (1–7). All saponins feature an oxidized methyl at C-18 into a primary alcohol or a carboxylic acid. While poecillastrosides A–D (1–4) all contain an exo double bond at C-24 of the side-chain and two osidic residues connected at O-2′, poecillastrosides E–G (5–7) are characterized by a cyclopropane on the side-chain and a connection at O-3′ between both sugar units. The chemical structures were elucidated through extensive spectroscopic analysis (High-Resolution Mass Spectrometry (HRESIMS), 1D and 2D NMR) and the absolute configurations of the sugar residues were assigned after acidic hydrolysis and cysteine derivatization followed by LC-HRMS analyses. Poecillastrosides D and E, bearing a carboxylic acid at C-18, were shown to exhibit antifungal activity against Aspergillus fumigatus.

Real time band selective F1 -decoupled proton NMR for the demixing of overlay spectra of chiral molecules

The small chemical shift dispersion and complex multiplicity pattern in proton NMR limit quantifications, for instance the determination of enantiomeric excess (ee) for an enantiomeric mixture. Herein, we present a simple proton-proton correlation experiment with band selective homonuclear (BASH) decoupling in both F₁ and F₂ dimensions, for the removal of scalar and residual dipolar couplings to provide collapsed singlet for each chemical site. The method has been demonstrated to separate the severely overlapped spectra of enantiomers using both chiral isotropic and anisotropic phases as well as a small biomolecule, particularly for the diastereotopic protons and also for the determination of ee. Copyright © 2016 John Wiley & Sons, Ltd.

Pure shift edited ultra high resolution NMR spectrum with complete eradication of axial peaks and unwanted couplings

Poor chemical shift dispersion and pairwise interaction among the entire coupled network of spins results in complex one dimensional ¹H NMR spectra, severely hampering the analysis and the accurate determination of ⁿJ_HH. Available two dimensional selective refocusing based techniques suffer from the evolution of undesirable couplings and intense axial peaks, creating ambiguity in the analysis and the extraction of coupling values. In this work, we report a novel two dimensional experiment for the complete elimination of axial peaks and unwanted couplings, while retaining only the couplings of the selected proton to its partners, with a blend of ultra-high resolution achieved by real time broad band homonuclear decoupling.

Perfect 1JCH-resolved HSQC: Efficient measurement of one-bond proton-carbon coupling constants along the indirect dimension

A versatile ¹J_CH-resolved HSQC pulse scheme for the speedy, accurate and automated determination of one-bond proton-carbon coupling constants is reported. The implementation of a perfectBIRD element allows a straightforward measurement from the clean doublets obtained along the highly resolved F1 dimension, even for each individual ¹J_CHa and ¹J_CHb in diastereotopic H_aCH_b methylene groups. Real-time homodecoupling during acquisition and other alternatives to minimize accidental signal overlapping in overcrowded spectra are also discussed.

Improving the performance of J-modulated ADEQUATE experiments through homonuclear decoupling and non-uniform sampling

Homonuclear ¹³ C-¹³ C couplings at natural abundance can be measured using the J-modulated Adequate DoublE QUAntumTransfer Experiment (ADEQUATE) experiment. To somewhat ameliorate F₁ digitization requirements, a scaling factor was incorporated into the original pulse sequence. Non-uniform sampling provides an obvious avenue to further facilitate the acquisition of ¹ J_CC and ⁿ J_CC homonuclear coupling constant data. We introduce homonuclear decoupling (HD) analogous to that described for the 1,1-HD-ADEQUATE and 1,n-HD-ADEQUATE experiments and evaluate the combination of non-uniform sampling and HD on the acquisition of both ¹ J_CC and ⁿ J_CC homonuclear ¹³ C-¹³ C coupling constants using ibuprofen as a model compound. Copyright © 2015 John Wiley & Sons, Ltd.

Complex mixtures by NMR and complex NMR for mixtures: experimental and publication challenges

Untargeted strategies have changed the rules of the game in complex mixture analysis, introducing an amazing potential for medical and biological applications that is just starting to be tapped. But with great power come great challenges; although untargeted mixture analysis opens the road for many exciting possibilities, the road is still full of perils. On the one hand, this article highlights some of the difficulties that need to be sorted for mixture analysis by NMR to fulfill its potential, along with insight on how they may be managed. Highlighted key points include the need for 'computer friendly' solutions for sharing data, experimental design and algorithm to facilitate the steady growth of knowledge and modeling ability in the field, and the need for large-scale studies to improve confidence in newly identified biomarkers. On the other hand, the second part of this article presents some breakthroughs in NMR experiments that, when combined, may modify the landscape of mixture analysis. Copyright © 2016 John Wiley & Sons, Ltd.

Efficient simulation of ultrafast magnetic resonance experiments

We present a convenient and powerful simulation formalism for ultrafast NMR spectroscopy. The formalism is based on the Fokker–Planck equation that supports systems with complicated combinations of classical spatial dynamics and quantum mechanical spin dynamics.

Pure shift 1 H NMR: what is next?

Currently, pure shift nuclear magnetic resonance (NMR) is an area of high interest. The aim of this contribution is to describe briefly how this technique has evolved, where it is now and what could be the next challenges in the amazing adventure of the development and application of pure shift NMR experiments. Copyright © 2016 John Wiley & Sons, Ltd.

Characterization and Synthesis of Eudistidine C, a Bioactive Marine Alkaloid with an Intriguing Molecular Scaffold

An extract of Eudistoma sp. provided eudistidine C (1), a heterocyclic alkaloid with a novel molecular framework. Eudistidine C (1) is a racemic natural product composed of a tetracyclic core structure further elaborated with a p-methoxyphenyl group and a phenol-substituted aminoimidazole moiety. This compound presented significant structure elucidation challenges due to the large number of heteroatoms and fully substituted carbons. These issues were mitigated by application of a new NMR pulse sequence (LR-HSQMBC) optimized to detect four- and five-bond heteronuclear correlations and the use of computer-assisted structure elucidation software. Synthesis of eudistidine C (1) was accomplished in high yield by treating eudistidine A (2) with 4(2-amino-1H-imidazol-5-yl)phenol (4) in DMSO. Synthesis of eudistidine C (1) confirmed the proposed structure and provided material for further biological characterization. Treatment of 2 with various nitrogen heterocycles and electron-rich arenes provided a series of analogues (5-10) of eudistidine C. Chiral-phase HPLC resolution of epimeric eudistidine C provided (+)-(R)-eudistidine C (1a) and (-)-(S)-eudistidine C (1b). The absolute configuration of these enantiomers was assigned by ECD analysis. (-)-(S)-Eudistidine C (1b) modestly inhibited interaction between the protein binding domains of HIF-1α and p300. Compounds 1, 2, and 6-10 exhibited significant antimalarial activity against Plasmodium falciparum.

Structural Basis for Selective Interaction between the ESCRT Regulator HD-PTP and UBAP1

Endosomal sorting complexes required for transport (ESCRTs) are essential for ubiquitin-dependent degradation of mitogenic receptors, a process often compromised in cancer pathologies. Sorting of ubiquinated receptors via ESCRTs is controlled by the tumor suppressor phosphatase HD-PTP. The specific interaction between HD-PTP and the ESCRT-I subunit UBAP1 is critical for degradation of growth factor receptors and integrins. Here, we present the structural characterization by X-ray crystallography and double electron-electron resonance spectroscopy of the coiled-coil domain of HD-PTP and its complex with UBAP1. The coiled-coil domain adopts an unexpected open and rigid conformation that contrasts with the closed and flexible coiled-coil domain of the related ESCRT regulator Alix. The HD-PTP:UBAP1 structure identifies the molecular determinants of the interaction and provides a molecular basis for the specific functional cooperation between HD-PTP and UBAP1. Our findings provide insights into the molecular mechanisms of regulation of ESCRT pathways that could be relevant to anticancer therapies.

Extraction of distance restraints from pure shift NOE experiments

NMR techniques incorporating pure shift methods to improve signal resolution have recently attracted much attention, owing to their potential use in studies of increasingly complex molecular systems. Extraction of frequencies from these simplified spectra enables easier structure determination, but only a few of the methods presented provide structural parameters derived from signal integral measurements. In particular, for quantification of the nuclear Overhauser effect (NOE) it is highly desirable to utilize pure shift techniques where signal overlap normally prevents accurate signal integration, to enable measurement of a larger number of interatomic distances. However, robust methods for the measurement of interatomic distances using the recently developed pure shift techniques have not been reported to date. In this work we discuss some of the factors determining the accuracy of measurements of signal integrals in interferogram-based Zangger-Sterk (ZS) pure shift NMR experiments. The ZS broadband homodecoupling technique is used in different experiments designed for quantitative NOE determination from pure shift spectra. It is shown that the techniques studied can be used for quantitative extraction of NOE-derived distance restraints, as exemplified for the test case of strychnine.

Access to experimentally infeasible spectra by pure-shift NMR covariance

Covariance processing is a versatile processing tool to generate synthetic NMR spectral representations without the need to acquire time-consuming experimental datasets. Here we show that even experimentally prohibited NMR spectra can be reconstructed by introducing key features of a reference 1D CHn-edited spectrum into standard 2D spectra. This general procedure is illustrated with the calculation of experimentally infeasible multiplicity-edited pure-shift NMR spectra of some very popular homonuclear (ME-psCOSY and ME-psTOCSY) and heteronuclear (ME-psHSQC-TOCSY and ME-psHMBC) experiments.

Determination of unresolved heteronuclear scalar coupling constants by J(up)-HSQMBC

Long-range heteronuclear scalar coupling constants provide important structural information, which is necessary for obtaining stereospecific assignment or dihedral angle information. The measurement of small proton-carbon splittings is particularly difficult due to the low natural abundance of carbon-13 and the presence of homonuclear couplings of similar size. Here we present a real-time J-upscaled HSQMBC, which allows the measurement of heteronuclear coupling constants even if they are hidden in the signal linewidth of a regular spectrum.

Exploring the use of Generalized Indirect Covariance to reconstruct pure shift NMR spectra: Current Pros and Cons

The current Pros and Cons of a processing protocol to generate pure chemical shift NMR spectra using Generalized Indirect Covariance are presented and discussed. The transformation of any standard 2D homonuclear and heteronuclear spectrum to its pure shift counterpart by using a reference DIAG spectrum is described. Reconstructed pure shift NMR spectra of NOESY, HSQC, HSQC-TOCSY and HSQMBC experiments are reported for the target molecule strychnine.

Real-time bilinear rotation decoupling in absorptive mode J-spectroscopy: Detecting low-intensity metabolite peak close to high-intensity metabolite peak with convenience

"Pure shift" NMR spectra display singlet peak per chemical site. Thus, high resolution is offered at the cost of valuable J-coupling information. In the present work, real-time BIRD (BIlinear Rotation Decoupling) is applied to the absorptive-mode 2D J-spectroscopy to provide pure shift spectrum in the direct dimension and J-coupling information in the indirect dimension. Quite often in metabolomics, proton NMR spectra from complex bio-fluids display tremendous signal overlap. Although conventional J-spectroscopy in principle overcomes this problem by separating the multiplet information from chemical shift information, however, only magnitude mode of the experiment is practical, sacrificing much of the potential high resolution that could be achieved. Few J-spectroscopy methods have been reported so far that produce high-resolution pure shift spectrum along with J-coupling information for crowded spectral regions. In the present work, high-quality J-resolved spectrum from important metabolomic mixture such as tissue extract from rat cortex is demonstrated. Many low-intensity metabolite peaks which are obscured by the broad dispersive tails from high-intensity metabolite peaks in regular magnitude mode J-spectrum can be clearly identified in real-time BIRD J-resolved spectrum. The general practice of removing such spectral overlap is tedious and time-consuming as it involves repeated sample preparation to change the pH of the tissue extract sample and subsequent spectra recording.

Sensitive, highly resolved, and quantitative (1)H-(13)C NMR data in one go for tracking metabolites in vegetal extracts

The quantification of metabolites is essential for understanding and improving biological systems. With the aim to quantify in one map a complex mixture composed of low concentrated metabolites, a new experiment called the (1)H-(13)C QUIPU HSQC allows improving of both resolution and sensitivity for investigation of vegetal extracts.

Band-selective excited ultrahigh resolution PSYCHE-TOCSY: fast screening of organic molecules and complex mixtures

Precise assignments of (1) H atomic sites and establishment of their through-bond COSY or TOCSY connectivity are crucial for molecular structural characterization by using (1) H NMR spectroscopy. However, this exercise is often hampered by signal overlap, primarily because of (1) H-(1) H scalar coupling multiplets, even at typical high magnetic fields. The recent developments in homodecoupling strategies for effectively suppressing the coupling multiplets into nice singlets (pure-shift), particularly, Morris's advanced broadband pure-shift yielded by chirp excitation (PSYCHE) decoupling and ultrahigh resolution PSYCHE-TOCSY schemes, have shown new possibilities for unambiguous structural elucidation of complex organic molecules. The superior broadband PSYCHE-TOCSY exhibits enhanced performance over the earlier TOCSY methods, which however warrants prolonged experimental times due to the requirement of large number of dwell increments along the indirect dimension. Herein, we present fast and band-selective analog of the broadband PSYCHE-TOCSY, which is useful for analyzing complex organic molecules that exhibit characteristic yet crowded spectral regions. The simple pulse scheme relies on band-selective excitation (BSE) followed by PSYCHE homodecoupling in the indirect dimension. The BSE-PSYCHE-TOCSY has been exemplified for Estradiol and a complex carbohydrate mixture comprised of six constituents of closely comparable molecular weights. The experimental times are greatly reduced viz., ~20 fold for Estradiol and ~10 fold for carbohydrate mixture, with respect to the broadband PSYCHE-TOCSY. Furthermore, unlike the earlier homonuclear band-selective decoupling, the BSE-PSYCHE-decoupling provides fully decoupled pure-shift spectra for all the individual chemical sites within the excited band. The BSE-PSYCHE-TOCSY is expected to have significant potential for quick screening of complex organic molecules and mixtures at ultrahigh resolution. Copyright © 2015 John Wiley & Sons, Ltd.

Distinction of trans–cis photoisomers with comparable optical properties in multiple-state photochromic systems – examining a molecule with three azobenzenes via in situ irradiation NMR spectroscopy

In situ irradiation NMR spectroscopy allows for the distinction and quantification of different photoisomers.

HD-2D: routine high-dispersion two-dimensional NMR spectra at no extra cost

The synergic use of pure shift NMR techniques and compressive sensing (compressed NMR) allows the recording of two-dimensional NMR spectra with much higher dispersion (effective resolution) than can be achieved using conventional techniques.

Measurement of 1H NMR relaxation times in complex organic chemical systems: application of PSYCHE

In complex organic molecules, relaxation times measured from the PSYCHE homonuclear broadband decoupling methods provide a wealth of information on intramolecular dynamics and intermolecular interactions.

Real-time broadband proton-homodecoupled CLIP/CLAP-HSQC for automated measurement of heteronuclear one-bond coupling constants

A new method is proposed that allows broadband homonuclear decoupled CLIP/CLAP-HSQC NMR spectra to be acquired at virtually no extra cost in measurement time.