Simultaneously enhancing spectral resolution and sensitivity in heteronuclear correlation NMR spectroscopy

Making 1H-1H Couplings More Accessible and Accurate with Selective 2DJ NMR Experiments Aided by 13C Satellites

1H-1H coupling constants are one of the primary sources of information for nuclear magnetic resonance (NMR) structural analysis. Several selective 2DJ experiments have been proposed that allow for their individual measurement at pure shift resolution. However, all of these experiments fail in the not uncommon case when coupled protons have very close chemical shifts. First, the coupling between protons with overlapping multiplets is inaccessible due to the inability of a frequency-selective pulse to invert just one of them. Second, the strong coupling condition affects the accuracy of coupling measurements involving third spins. These shortcomings impose a limit on the effectiveness of state-of-the-art experiments, such as G-SERF or PSYCHEDELIC. Here, we introduce two new and complementary selective 2DJ experiments that we coin SERFBIRD and SATASERF. These experiments overcome the aforementioned issues by utilizing the 13C satellite signals at natural isotope abundance, which resolves the chemical shift degeneracy. We demonstrate the utility of these experiments on the tetrasaccharide stachyose and the challenging case of norcamphor, for the latter achieving measurement of all JHH couplings, while only a few were accessible with PSYCHEDELIC. The new experiments are applicable to any organic compound and will prove valuable for configurational and conformational analyses.

Pure shift edited NMR methodologies for the extraction of Homo- and heteronuclear couplings with ultra-high resolution

The scalar couplings that result in the splitting of the signals in the NMR spectrum arise due to the interaction of the nuclear spins, whereby the spin polarization is transmitted through chemical bonds. The interaction strengths depend inter alia on the number of consecutive chemical bonds intervening between the two interacting spins and on the molecular conformation. The pairwise interaction of many spins in a molecule resulting in a complex spectrum poses a severe challenge to analyse the spectrum and hence the determination of magnitudes and signs of homo- and heteronuclear couplings. The problem is more severe in the analysis of 1H spectra than the spectra of most of the other nuclei due to the often very small chemical shift dispersion. As a consequence, the straightforward analysis and the accurate extraction of the coupling constants from the 1H spectrum of a complex spin system continues to remain a challenge, and often may be a formidable task. Over the years, the several pure shift-based one-dimensional and two-dimensional methodologies have been developed by workers in the field, which provide broadband homonuclear decoupling of proton spectra, removing the complexity but at the cost of the very informative scalar couplings. To circumvent this problem, several one-dimensional and two-dimensional NMR experiments have been developed for the determination of homonuclear and heteronuclear couplings (nJHX, where n = 1,2,3) while retaining the high resolution obtained by implementing pure shift strategies. This review attempts to summarize the extensive work reported by a large number of researchers over the years for the accurate determination of homo- and heteronuclear scalar couplings.

Metabolic labeling of hyaluronan: Biosynthesis and quantitative analysis of 13C,15N-enriched hyaluronan by NMR and MS-based methods

Hyaluronan (HA), a member of the GAG family of glycans, has many diverse biological functions that vary a lot depending on the length of the HA chain and its concentration. A better understanding of the structure of different-sized HA at the atomic level is therefore crucial to decipher these biological functions. NMR is a method of choice for conformational studies of biomolecules, but there are limitations due to the low natural abundance of the NMR active nuclei ¹³C and ¹⁵N. We describe here the metabolic labeling of HA using the bacterium Streptococcus equi subsp. Zooepidemicus and the subsequent analysis by NMR and mass spectrometry. The level of ¹³C and ¹⁵N isotope enrichment at each position was determined quantitatively by NMR spectroscopy and was further confirmed by high-resolution mass spectrometry analysis. This study provides a valid methodological approach that can be applied to the quantitative assessment of isotopically labeled glycans and will help improve detection capabilities and facilitate future structure-function relationship analysis of complex glycans.

Benchtop NMR-Based Metabolomics: First Steps for Biomedical Application

Nuclear magnetic resonance (NMR)-based metabolomics is a valuable tool for identifying biomarkers and understanding the underlying metabolic changes associated with various diseases. However, the translation of metabolomics analysis to clinical practice has been limited by the high cost and large size of traditional high-resolution NMR spectrometers. Benchtop NMR, a compact and low-cost alternative, offers the potential to overcome these limitations and facilitate the wider use of NMR-based metabolomics in clinical settings. This review summarizes the current state of benchtop NMR for clinical applications where benchtop NMR has demonstrated the ability to reproducibly detect changes in metabolite levels associated with diseases such as type 2 diabetes and tuberculosis. Benchtop NMR has been used to identify metabolic biomarkers in a range of biofluids, including urine, blood plasma and saliva. However, further research is needed to optimize the use of benchtop NMR for clinical applications and to identify additional biomarkers that can be used to monitor and manage a range of diseases. Overall, benchtop NMR has the potential to revolutionize the way metabolomics is used in clinical practice, providing a more accessible and cost-effective way to study metabolism and identify biomarkers for disease diagnosis, prognosis, and treatment.

Resolving the complexity in human milk oligosaccharides using pure shift NMR methods and CASPER

Human milk oligosaccharides belong to an important class of bioactive molecules with diverse effects on the development of infants. NMR is capable of providing vital structural information about oligosaccharides which can aid in determining structure-function relationships. However, this information is often concealed by signal overlap in ¹H spectra, due to the narrow chemical shift range and signal multiplicity. Signal overlap in oligosaccharide spectra can be greatly reduced, and resolution improved, by utilising pure shift methods. Here the benefits of combining pure shift methods with the CASPER computational approach to resonance assignment in oligosaccharides are demonstrated.

Primary Structure of Glycans by NMR Spectroscopy

Glycans, carbohydrate molecules in the realm of biology, are present as biomedically important glycoconjugates and a characteristic aspect is that their structures in many instances are branched. In determining the primary structure of a glycan, the sugar components including the absolute configuration and ring form, anomeric configuration, linkage(s), sequence, and substituents should be elucidated. Solution state NMR spectroscopy offers a unique opportunity to resolve all these aspects at atomic resolution. During the last two decades, advancement of both NMR experiments and spectrometer hardware have made it possible to unravel carbohydrate structure more efficiently. These developments applicable to glycans include, inter alia, NMR experiments that reduce spectral overlap, use selective excitations, record tilted projections of multidimensional spectra, acquire spectra by multiple receivers, utilize polarization by fast-pulsing techniques, concatenate pulse-sequence modules to acquire several spectra in a single measurement, acquire pure shift correlated spectra devoid of scalar couplings, employ stable isotope labeling to efficiently obtain homo- and/or heteronuclear correlations, as well as those that rely on dipolar cross-correlated interactions for sequential information. Refined computer programs for NMR spin simulation and chemical shift prediction aid the structural elucidation of glycans, which are notorious for their limited spectral dispersion. Hardware developments include cryogenically cold probes and dynamic nuclear polarization techniques, both resulting in enhanced sensitivity as well as ultrahigh field NMR spectrometers with a ¹H NMR resonance frequency higher than 1 GHz, thus improving resolution of resonances. Taken together, the developments have made and will in the future make it possible to elucidate carbohydrate structure in great detail, thereby forming the basis for understanding of how glycans interact with other molecules.

Metabolomics and NMR

The purpose of this manuscript will be to convince the reader to dive deeper into NMR spectroscopy and prevent the technique from being just another "black-box" in the lab. We will try to concisely highlight interesting topics and supply additional references for further exploration at each stage. The advantages of delving into the technique will be shown. The secondary objective, i.e., avoiding common problems before starting, will hopefully then become clear. Lastly, we will emphasize the spectrometer information needed for manuscript reporting to allow reproduction of results and confirm findings.

NMR methods for the analysis of mixtures

NMR spectroscopy is a powerful approach for the analysis of mixtures. Its usefulness arises in large part from the vast landscape of methods, and corresponding pulse sequences, that have been and are being designed to tackle the specific properties of mixtures of small molecules. This feature article describes a selection of methods that aim to address the complexity, the low concentrations, and the changing nature that mixtures can display. These notably include pure-shift and diffusion NMR methods, hyperpolarisation methods, and fast 2D NMR methods such as ultrafast 2D NMR and non-uniform sampling. Examples or applications are also described, in fields such as reaction monitoring and metabolomics, to illustrate the relevance and limitations of different methods.

Pure shift amide detection in conventional and TROSY-type experiments of 13C,15N-labeled proteins

Large coupling networks in uniformly ¹³C,¹⁵N-labeled biomolecules induce broad multiplets that even in flexible proteins are frequently not recognized as such. The reason is that given multiplets typically consist of a large number of individual resonances that result in a single broad line, in which individual components are no longer resolved. We here introduce a real-time pure shift acquisition scheme for the detection of amide protons which is based on ¹³C-BIRD^r,X. As a result the full homo- and heteronuclear coupling network can be suppressed at low power leading to real singlets at substantially improved resolution and uncompromised sensitivity. The method is tested on a small globular and an intrinsically disordered protein (IDP) where the average spectral resolution is increased by a factor of ~ 2 and higher. Equally important, the approach works without saturation of water magnetization for solvent suppression and exchanging amide protons are not affected by saturation transfer.

Slice selective absorption-mode J-resolved NMR spectroscopy

Limited chemical shift dispersion and broad multiplet patterns limit resolution in ¹H NMR spectra. J-Resolved spectroscopy overcomes this problem to a great extent. However, the phase-twist line shape in J-Resolved spectroscopy allows only the magnitude mode of the experiment to be practical, which degrades resolution. Recently, various pure shift or broadband homonuclear decoupling approaches have been integrated with J-Resolved spectroscopy to eliminate the broad dispersive contribution. In the present work, we demonstrate a broadband ¹H-¹H J-Resolved spectrum with a greatly reduced dispersive contribution using the concept of slice selection. We show that slice selective excitation, t₁ encoding, storage, and detection of the in-phase absorptive signals can be executed, while a gradient-based suppression of the dispersive antiphase signals can be performed during the storage period. In more than two spin systems, a small part of the doubly antiphase absorptive signal may also contribute to the spectrum in addition to the inphase absorptive signals. The overall effect is a reduced multiplet pattern similar to a regular J-Resolved case as the passive spins remain unflipped due to slice selective pulses. However, the effect is broadband for a fraction of the spins when all slices are considered analogous to Zangger-Sterk (ZS) broadband homo-decoupling. Further, the fresh magnetization from neighboring slices can be accessed in different scans by frequency shifting of the slice selective pulses without a recycle delay-an elegant aspect of the ZS pulse element. This allows faster signal averaging, improving sensitivity which depends on the T₁ relaxation time of the signals. This method displays sensitivity up to 4-20 percent of the regular J-RES ¹H signals.

Boosting the resolution of multidimensional NMR spectra by complete removal of proton spin multiplicities

Over decades multidimensional NMR spectroscopy has become an indispensable tool for structure elucidation of natural products, peptides and medium sized to large proteins. Heteronuclear single quantum coherence (HSQC) spectroscopy is one of the work horses in that field often used to map structural connectivity between protons and carbons or other hetero nuclei. In overcrowded HSQC spectra, proton multiplet structures of cross peaks set a limit to the power of resolution and make a straightforward assignment difficult. In this work, we provide a solution to improve these penalties by completely removing the proton spin multiplet structure of HSQC cross peaks. Previously reported sideband artefacts are diminished leading to HSQC spectra with singlet responses for all types of proton multiplicities. For sideband suppression, the idea of restricted random delay (RRD) in chunk interrupted data acquisition is introduced and exemplified. The problem of irreducible residual doublet splitting of diastereotopic CH₂ groups is simply solved by using a phase sensitive JRES approach in conjunction with echo processing and real time broadband homodecoupling (BBHD) HSQC, applied as a 3D experiment. Advantages and limitations of the method is presented and discussed.

Longitudinal Spin Order Labeling on Multiple Quantum Coherences Enables NMR Analysis of Intrinsically Disordered Proteins at Ultrahigh Resolution

Intrinsically disordered proteins (IDPs) play an important role in cell signaling, and NMR is well-suited to study conformational ensembles and dynamics of IDPs. However, the intrinsic flexibility of IDPs often results in severe spectral overlap, which hampers accurate NMR data analysis. By labeling the longitudinal spin order of an α proton (i.e., H^α_z) on multiple quantum coherences of backbone nuclei (e.g., N_yC'_xC^α_y), we were able to apply pre-homonuclear decoupling (PHD) to transverse relaxation-optimized spectroscopy (TROSY). The proposed scheme provides ultrahigh resolution in both amide proton and nitrogen dimensions, as illustrated in the analysis of Tau and alpha-synuclein (α-Syn) proteins. The PHD-TROSY readout enabled complete backbone resonance assignment of α-Syn using a single 3D HNCA experiment performed on a 600 MHz NMR spectrometer.

Broadband measurement of true transverse relaxation rates in systems with coupled protons: application to the study of conformational exchange

Accurate measurement of transverse relaxation rates in coupled spin systems is important in the study of molecular dynamics, but is severely complicated by the signal modulations caused by scalar couplings in spin echo experiments. The most widely used experiments for measuring transverse relaxation in coupled systems, CPMG and PROJECT, can suppress such modulations, but they also both suppress some relaxation contributions, and average relaxation rates between coupled spins. Here we introduce a new experiment which for the first time allows accurate broadband measurement of transverse relaxation rates of coupled protons, and hence the determination of exchange rate constants in slow exchange from relaxation measurements. The problems encountered with existing methods are illustrated, and the use of the new method is demonstrated for the classic case of hindered amide rotation and for the more challenging problem of exchange between helical enantiomers of a gold(i) complex.

Multiplexing experiments in NMR and multi-nuclear MRI

Multiplexing NMR experiments by direct detection of multiple free induction decays (FIDs) in a single experiment offers a dramatic increase in the spectral information content and often yields significant improvement in sensitivity per unit time. Experiments with multi-FID detection have been designed with both homonuclear and multinuclear acquisition, and the advent of multiple receivers on commercial spectrometers opens up new possibilities for recording spectra from different nuclear species in parallel. Here we provide an extensive overview of such techniques, designed for applications in liquid- and solid-state NMR as well as in hyperpolarized samples. A brief overview of multinuclear MRI is also provided, to stimulate cross fertilization of ideas between the two areas of research (NMR and MRI). It is shown how such techniques enable the design of experiments that allow structure elucidation of small molecules from a single measurement. Likewise, in biomolecular NMR experiments multi-FID detection allows complete resonance assignment in proteins. Probes with multiple RF microcoils routed to multiple NMR receivers provide an alternative way of increasing the throughput of modern NMR systems, effectively reducing the cost of NMR analysis and increasing the information content at the same time. Solid-state NMR experiments have also benefited immensely from both parallel and sequential multi-FID detection in a variety of multi-dimensional pulse schemes. We are confident that multi-FID detection will become an essential component of future NMR methodologies, effectively increasing the sensitivity and information content of NMR measurements.

Gradient-based pulse sequences for benchtop NMR spectroscopy

Benchtop NMR spectroscopy has been on the rise for the last decade, by bringing high-resolution NMR in environments that are not easily compatible with high-field NMR. Benchtop spectrometers are accessible, low cost and show an impressive performance in terms of sensitivity with respect to the relatively low associated magnetic field (40-100 MHz). However, their application is limited by the strong and ubiquitous peak overlaps arising from the complex mixtures which are often targeted, often characterized by a great diversity of concentrations and by strong signals from non-deuterated solvents. Such limitations can be addressed by pulse sequences making clever use of magnetic field gradient pulses, capable of performing efficient coherence selection or encoding chemical shift or diffusion information. Gradients pulses are well-known ingredients of high-field pulse sequence recipes, but were only recently made available on benchtop spectrometers, thanks to the introduction of gradient coils in 2015. This article reviews the recent methodological advances making use of gradient pulses on benchtop spectrometers and the applications stemming from these developments. Particular focus is made on solvent suppression schemes, diffusion-encoded, and spatially-encoded experiments, while discussing both methodological advances and subsequent applications. We eventually discuss the exciting development and application perspectives that result from such advances.

Study of the Acetylation Pattern of Chitosan by Pure Shift NMR

Chitosan is a biodegradable, antibacterial, and nontoxic biopolymer used in a wide range of applications including biotechnology, pharmacy, and medicine. The physicochemical and biological properties of chitosan have been associated with parameters such as the degree of polymerization (DP) and the fraction of acetylation (F_A). New methods are being developed to yield chitosans of specific acetylation patterns, and, recently, a correlation between biological activity and the distribution of the acetylated units (P_A: pattern of acetylation) has been demonstrated. Although there are numerous well-established methods for the determination of DP and F_A values, this is not the case for P_A. The methods available are either not straightforward or not sensitive enough, limiting their use for routine analysis. In this study, we demonstrate that by applying HOmodecoupled Band-Selective (HOBS) decoupling NMR on signals assigned by multidimensional Pure Shift NMR methods, P_A can be easily and accurately determined on various chitosan samples. This novel methodology-easily implemented for routine analysis-could become a standard for chitosan P_A assessment. In addition, by applying Spectral Aliased Pure Shift HSQC, the analysis was enhanced with the determination of triads.

NPid: an Automatic Approach to Rapid Identification of Known Natural Products in the Crude Extract of Crabapple Based on 2D 1H-13C Heteronuclear Correlation Spectra of the Extract Mixture

An automatic approach to identification of natural products (NPid) in complex extracts by exploring pure shift HSQC (psHSQC) and H2BC spectra of the mixture is developed, which integrated information on chemical shifts (CS), adjacent relationships (AR) and peak intensities (PI) of ¹H-¹³C groups for identification of candidate natural product in a customized NMR database. A weighted comprehensive score is calculated for each candidate from the values of CS, AR and PI to rate the likelihood of its existence in the complex mixture. Using the crude extract of crabapple (Malus fusca) as an example, a customized NMR database of natural products from plants of the genus Malus was constructed. The performance of NPid was first evaluated using simulated data in four scenarios, that is, for identification of structurally similar natural products, identification of natural products with part of peaks missing in psHSQC due to low concentration, without available adjacent relationship information, or without useful peak intensity information. The false positive and false negative rates of the natural products identified by NPid were estimated by Monte Carlo simulation. It shows that AR and PI can effectively reduce the false positive rate of identification. Proof of concept of the proposed method was elucidated on a model mixture consisting of 10 known natural products. Application of this method was then demonstrated on an authentic sample of crude extract of crabapple and 19 known natural products were successfully identified and confirmed by standard spiking.

COLMAR Lipids Web Server and Ultrahigh-Resolution Methods for Two-Dimensional Nuclear Magnetic Resonance- and Mass Spectrometry-Based Lipidomics

Accurate identification of lipids in biological samples is a key step in lipidomics studies. Multidimensional nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool for this purpose as it provides comprehensive structural information on lipid composition at atomic resolution. However, the interpretation of NMR spectra of complex lipid mixtures is currently hampered by limited spectral resolution and the absence of a customized lipid NMR database along with user-friendly spectral analysis tools. We introduce a new two-dimensional (2D) NMR metabolite database "COLMAR Lipids" that was specifically curated for hydrophobic metabolites presently containing 501 compounds with accurate experimental 2D ¹³C-¹H heteronuclear single quantum coherence (HSQC) chemical shift data measured in CDCl₃. A new module in the public COLMAR suite of NMR web servers was developed for the (semi)automated analysis of complex lipidomics mixtures (http://spin.ccic.osu.edu/index.php/colmarm/index2). To obtain 2D HSQC spectra with the necessary high spectral resolution along both ¹³C and ¹H dimensions, nonuniform sampling in combination with pure shift spectroscopy was applied allowing the extraction of an abundance of unique cross-peaks belonging to hydrophobic compounds in complex lipidomics mixtures. As shown here, this information is critical for the unambiguous identification of underlying lipid molecules by means of the new COLMAR Lipids web server, also in combination with mass spectrometry, as is demonstrated for Caco-2 cell and lung tissue cell extracts.

Multinuclear NMR in polypeptide liquid crystals: Three fertile decades of methodological developments and analytical challenges

NMR spectroscopy of oriented samples makes accessible residual anisotropic intramolecular NMR interactions, such as chemical shift anisotropy (RCSA), dipolar coupling (RDC), and quadrupolar coupling (RQC), while preserving high spectral resolution. In addition, in a chiral aligned environment, enantiomers of chiral molecules or enantiopic elements of prochiral compounds adopt different average orientations on the NMR timescale, and hence produce distinct NMR spectra or signals. NMR spectroscopy in chiral aligned media is a powerful analytical tool, and notably provides unique information on (pro)chirality analysis, natural isotopic fractionation, stereochemistry, as well as molecular conformation and configuration. Significant progress has been made in this area over the three last decades, particularly using polypeptide-based chiral liquid crystals (CLCs) made of organic solutions of helically chiral polymers (as PBLG) in organic solvents. This review presents an overview of NMR in polymeric LCs. In particular, we describe the theoretical tools and the major NMR methods that have been developed and applied to study (pro)chiral molecules dissolved in such oriented solvents. We also discuss the representative applications illustrating the analytical potential of this original NMR tool. This overview article is dedicated to thirty years of original contributions to the development of NMR spectroscopy in polypeptide-based chiral liquid crystals.

Pure shift HMQC: Resolution and sensitivity enhancement by bilinear rotation decoupling in the indirect and direct dimensions

The heteronuclear multiple-quantum coherence in the indirect dimension of the two-dimensional HMQC experiment evolves under the passive ¹H-¹H J-couplings leading to multiplet structures in the F₁ dimension. Besides, ¹H-¹H J-multiplets appear in the direct dimension as well. Thus, multiplets along both dimensions lower the resolution and sensitivity of this technique, when high resolution is required along both dimensions. An efficient broadband homodecoupling scheme along the F₁ dimension of the HMQC experiment has not been realized to date. We have implemented broadband homonuclear decoupling using bilinear rotation decoupling (BIRD) by adding a ¹H SQ evolution period followed by BIRD before the ¹H-¹³C multiple-quantum evolution period in the HMQC. In the direct time domain, BIRD is implemented using a real-time or single-scan scheme, which further improves resolution and sensitivity of this technique. The resulting pure shift HMQC provides singlet peak per chemical site along F₁ as well as F₂ axes and, hence, better resolution and sensitivity than conventional HMQC spectrum for all peaks except diastereotopic methylene protons. Due to the incorporation of the BIRD, the indirect time domain becomes double in length compared to the conventional HMQC. However, slow relaxation of small molecules favors better sensitivity for ps-HMQC relative to conventional HMQC under all conditions. We also found that the sensitivity of ps-HMQC is only slightly less than ps-HSQC for small molecules.

All-in-one NMR spectroscopy of small organic molecules: complete chemical shift assignment from a single NMR experiment

A new class of NOAH NMR experiments (NOAH-AST and NOAH-AST_PS), with the abbreviations, A: 1,1-ADEQUATE, S: sensitivity improved version of multiplicity-edited (ME)-HSQC, T: TOCSY, and T_PS: pure shift TOCSY, are reported to obtain complete chemical shift assignments of small organic molecules from a single NMR experiment. While NOAH-AST provides ¹³C–¹³C, ¹H–¹³C, and ¹H–¹H connectivities for molecules with well resolved chemical shifts, NOAH-AST_PS experiments discern ¹H–¹H connectivities even in complex organic molecules such as steroids at ultra-high resolution. These methods are very flexible and allow to record data through non-uniform-sampling, which reduces the experimental time to a great extent. In order to make these methods friendly to non-NMR experts (especially organic chemists and natural product scientists), python scripts have been developed and they help researchers in using these methods.

All-in-one NOAH-AST/NOAH-AST_PS provides complete chemical shift information (¹³C–¹³C/¹H–¹³C/¹H–¹H) of small organic molecules from a single NMR experiment.

Production and characterization of Aspergillus niger GH29 family α-fucosidase and production of a novel non-reducing 1-fucosyllactose

Fucosylated oligosaccharides are interesting molecules due to their bioactive properties. In particular, their application as active ingredient in milk powders is attractive for dairy industries. The objective of this study was to characterize the glycosyl hydrolase family 29 α-fucosidase produced by Aspergillus niger and test its ability to transfucosylate lactose with a view towards potential industrial applications such as the valorization of the lactose side stream produced by dairy industry. In order to reduce costs and toxicity the use of free fucose instead of environmentally questionable fucose derivatives was studied. In contrast to earlier studies, a recombinantly produced A. niger α-fucosidase was utilized. Using pNP-fucose as substrate, the optimal pH for hydrolytic activity was determined to be 3.8. The optimal temperature for a 30-min reaction was 60 °C, and considering temperature stability, the optimal temperature for a 24-h reaction was defined as 45 °C For the same hydrolysis reaction, the kinetic values were calculated to be 0.385 mM for the K_M and 2.8 mmol/(mg*h) for the V_max. Transfucosylation of lactose occurred at high substrate concentrations when reaction time was elongated to several days. The structure of the product trisaccharide was defined as 1-fucosyllactose, where fucose is α-linked to the anomeric carbon of the β-glucose moiety of lactose. Furthermore, the enzyme was able to hydrolyze its own transfucosylation product and 2′-fucosyllactose but only poorly 3-fucosyllactose. As a conclusion, α-fucosidase from A. niger can transfucosylate lactose using free fucose as substrate producing a novel non-reducing 1-fucosyllactose.

Pre-Homonuclear Decoupling Enables High-Resolution NMR Analysis of Intrinsically Disordered Proteins in Solution

Probing the atomic details of intrinsically disordered proteins is crucial to understanding their biological function and relation to pathogenesis. Although amide-detected NMR experiments are widely employed in protein studies, ³J_HNHα couplings between amide (¹H^N) and alpha (¹H^α) protons impose an intrinsic limit on the achievable ¹H^N linewidth. Here, we present a homonuclear decoupling method that narrows the α-synuclein ¹H^N linewidths to 3-5 Hz. Tightly distributed ¹J_CαHα coupling values were employed to generate homogeneous antiphase coherences of 2H^α_zH^N_y and 4H^α(2)_zH^α(3)_zH^N_y for nonglycine and glycine residues, respectively, which were combined with their in-phase H^N_y counterparts to achieve homonuclear decoupling. By reducing the multiplet structure to a singlet, the width of the ¹H^N cross-peak was reduced by ∼3-fold in the 2D HSQC and 3D intra-HNCA spectra, and good spectral quality was achieved without the need for postprocessing.

NMR Spectroscopy for Metabolomics Research

Over the past two decades, nuclear magnetic resonance (NMR) has emerged as one of the three principal analytical techniques used in metabolomics (the other two being gas chromatography coupled to mass spectrometry (GC-MS) and liquid chromatography coupled with single-stage mass spectrometry (LC-MS)). The relative ease of sample preparation, the ability to quantify metabolite levels, the high level of experimental reproducibility, and the inherently nondestructive nature of NMR spectroscopy have made it the preferred platform for long-term or large-scale clinical metabolomic studies. These advantages, however, are often outweighed by the fact that most other analytical techniques, including both LC-MS and GC-MS, are inherently more sensitive than NMR, with lower limits of detection typically being 10 to 100 times better. This review is intended to introduce readers to the field of NMR-based metabolomics and to highlight both the advantages and disadvantages of NMR spectroscopy for metabolomic studies. It will also explore some of the unique strengths of NMR-based metabolomics, particularly with regard to isotope selection/detection, mixture deconvolution via 2D spectroscopy, automation, and the ability to noninvasively analyze native tissue specimens. Finally, this review will highlight a number of emerging NMR techniques and technologies that are being used to strengthen its utility and overcome its inherent limitations in metabolomic applications.

G-SERF Editing in Two-Dimensional Pure-Shift Total Correlation Spectroscopy: Scalar Coupling Measurements for a Group of Spins in Organic Molecules

A novel G-SERF-PSYCHE-TOCSY (gradient encoded selective refocusing in pure shift yielded by chirp excitation version of total correlation spectroscopy) NMR pulse scheme has been proposed, which produces TOCSY chemical shift correlations, on one hand, and scalar coupling values for the spins scalarly coupled to irradiated resonances, by showing them as doublets along the indirect dimension, on the other. Therefore, recording such an experiment, for a group of spins with overlapping chemical shifts, in organic molecules can adequately provide scalar coupling information in a G-SERF manner along the indirect dimensions, and they can be assigned to particular spin pairs. Such COSY chemical shift correlations (which appear as doublets for the scalarly coupled spins) can be readily discriminated from the TOCSY peaks (which do not show such splitting) in the G-SERF-PSYCHE-TOCSY spectrum.

Real-time pure shift measurements for uniformly isotope-labeled molecules using X-selective BIRD homonuclear decoupling

We introduce a novel selective inversion element for chunked homonuclear decoupling that combines isotope selection via BIRD-filtering with band-selective inversion on the X-heteronucleus and allows efficient real-time decoupling of homonuclear and heteronuclear couplings. It is especially suitable for uniformly isotope-labeled compounds. We discuss in detail the inversion element based on band-selective refocusing on the X-nuclei (BASEREX), highlighting in particular the role of appropriate band-selective shaped refocusing pulses and the application of broadband X-pulses for an effective BIRD^d element during homodecoupling. The approach is experimentally verified and studied in detail using uniformly ¹³C-labeled glucose and a uniformly ¹⁵N,¹³C-labeled amino acid mixture.

Real-Time Pure Shift HSQC NMR for Untargeted Metabolomics

Sensitivity and resolution are key considerations for NMR applications in general and for metabolomics in particular, where complex mixtures containing hundreds of metabolites over a large range of concentrations are commonly encountered. There is a strong demand for advanced methods that can provide maximal information in the shortest possible time frame. Here, we present the optimization and application of the recently introduced 2D real-time BIRD 1H-13C HSQC experiment for NMR-based metabolomics of aqueous samples at 13C natural abundance. For mouse urine samples, it is demonstrated how this real-time pure shift sensitivity-improved heteronuclear single quantum correlation method provides broadband homonuclear decoupling along the proton detection dimension and thereby significantly improves spectral resolution in regions that are affected by spectral overlap. Moreover, the collapse of the scalar multiplet structure of cross-peaks leads to a sensitivity gain of about 40-50% over a traditional 2D HSQC-SI experiment. The experiment works well over a range of magnetic field strengths and is particularly useful when resonance overlap in crowded regions of the HSQC spectra hampers accurate metabolite identification and quantitation.

Broadband homodecoupled time-shared 1H-13C and 1H-15N HSQC experiments

The concepts of pure-shift NMR and time-shared NMR are merged in a single experiment. A ¹³C/¹⁵N time-shared version of the real-time BIRD-based broadband homodecoupled HSQC experiment is described. This time-efficient approach affords simultaneously ¹H-¹³C and ¹H-¹⁵N pure-shift HSQC spectra in a single acquisition, while achieving substantial gains in both sensitivity and spectral resolution. We also present a related ¹³C/¹⁵N-F2-coupled homodecoupled version of the CLIP-HSQC experiment for the simultaneous measurement of ¹J_CH and ¹J_NH from the simplified doublets observed along the direct dimension. Finally, a novel J-resolved HSQC experiment has been designed for the simple and automated determination of both ¹J_CH/¹J_NH from a 2D J-resolved spectrum.

Towards perfect NMR: Spin-echo versus perfect-echo building blocks

The development of new tools to improve the quality of nuclear magnetic resonance (NMR) spectra is a challenging task. The concept of "perfect NMR" includes the design of robust pulse sequences that allow an investigator to obtain undistorted pure in-phase signals, with pure absorption lineshapes that are free of phase anomalies derived from undesired J modulations. Here, alternative NMR building blocks to the spin-echo that are based on a general double SE module, known as a perfect-echo, are reviewed. Several implementations to minimize/remove unwanted dispersive contributions in homonuclear and heteronuclear NMR experiments are described and illustrated with some examples of broad interest for small molecules.

Interleaved Dual NMR Acquisition of Equivalent Transfer Pathways in TOCSY and HSQC Experiments

A dual NMR data acquisition strategy to handle and detect two active equivalent transfer pathways is presented and discussed. We illustrate the power of this time-efficient approach by collecting two different 2D spectra simultaneously in a single experiment: i) TOCSY or HSQC-TOCSY spectra with different mixing times, ii) F2-¹³ C-coupled and decoupled HSQC spectra, iii) conventional and pure-shift HSQC spectra, or iv) complementary HSQC and HSQC-TOCSY spectra.

High-resolution methods for the measurement of scalar coupling constants

Scalar couplings provide important information regarding molecular structure and dynamics. The measurement of scalar coupling constants constitutes a topic of interest and significance in NMR spectroscopy. However, the measurement of J values is often not straightforward because of complex signal splitting patterns and signal overlap. Many methods have been proposed for the measurement of scalar coupling constants, both for homonuclear and heteronuclear cases. Different approaches to the measurement of scalar coupling constants are reviewed here with several applications presented. The accurate measurement of scalar coupling constants can greatly facilitate molecular structure elucidation and the study of molecule dynamics.

NMR supersequences with real-time homonuclear broadband decoupling: Sequential acquisition of protein and small molecule spectra in a single experiment

NOAH (NMR byOrderedAcquisition using ¹H-detection) type of pure shift NMR pulse scheme has been designed for the efficient utilization of magnetization that presents in a spin-system under consideration. The proposed strategy, PROSMASH-HSQC² (PROtein-HSQC and SMAll molecule-HSQC Signals with Homodecoupling) uses the real-time BIRD pure shift NMR strategy and two HSQC spectra (¹³C-HSQC for small molecules and ¹⁵N-HSQC for ¹⁵N-isotopic labelled proteins) can be recorded in a single NMR experiment. Thus, this method permits precise determination of drug-protein interactions at atomic levels by monitoring the chemical shift perturbations, and will have potential applications in drug discovery programs.

High Resolution for Chemical Shifts and Scalar Coupling Constants: The 2D Real-Time J-Upscaled PSYCHE-DIAG

The facile determination of chemical shift and scalar coupling constants in NMR spectra is often prevented by spectral overlap and limited resolution. Here, we present a high-resolution NMR experiment for the simultaneous detection of both resonance frequencies and coupling patterns even with small J-values. A PSYCHE-decoupled DIAG (Pure Shift Yielded by Chirp Excitation- DIAGonal) experiment, which resolves chemical shift in the indirect dimension of a 2D experiment is combined with real-time J-upscaling in order to visualize small coupling constants that would otherwise be hidden in the linewidth of a regular proton or DIAG spectrum.

Real-time J-upscaling in two-dimensional pure shift diagonal NMR: Simultaneous resolution enhancement in chemical shifts and scalar couplings

A two-dimensional real-time J-upscaled pure shift diagonal pulse scheme, JS-PSYCHE-DIAG has been developed. This method enhances the resolution in scalar coupling multiplets by real-time J-upscaling during the direct acquisition, and these J-upscaled multiplets resolve on the diagonal at the respective pure shift positions, which results in resolution enhancement in chemical shifts. Thus, both chemical shifts and scalar couplings get better resolved simultaneously in the same NMR experiment. The efficacy of the present method has been demonstrated, (i) on hesperidin for resolving the J-upscaled multiplets belonging to diastereomers and (ii) on a natural product, strychnine, to measure small scalar couplings including the long range values.

Orchestrated approaches using pure shift NMR: Extraction of spectral parameters, ultra-high resolution, and sensitivity enhancement

The limited chemical shift range of protons and pairwise interaction among all the abundant nuclear spins of a molecule makes ¹ H spectrum too complicated. As a consequence, the straightforward analysis and the accurate extraction of their interaction strengths from the ¹ H spectrum of a complex spin system are formidably difficult or often impossible. This problem persists in the determination of scalar couplings be it between two abundant homonuclear spins or between ¹ H and an abundant heteronuclear spin (viz., ¹⁹ F and ³¹ P). Such problems are encountered in many situations where the determination of homonuclear and heteronuclear couplings is challenging. The several pure shift based one-dimensional and two-dimensional NMR strategies recently developed in our laboratory for the straightforward extraction of homonuclear and heteronuclear interaction parameters in diverse situations are discussed. Initially, the unique application of pure shift technique that paves the way for easy and straightforward extraction of magnitudes of heteronuclear couplings, namely, ⁿ J_HX (where X stands for ¹⁹ F, ³¹ P, etc.), is discussed. Subsequently, several pure shift edited one-dimensional and two-dimensional NMR strategies that are developed for the direct extraction of homonuclear and heteronuclear couplings and for achieving ultra-high-resolved ¹ H spectra with complete eradication of zero frequency peaks and the evolution of unwanted couplings. The enhancement in the sensitivity has also been achieved in the slice-selective pure shift experiments by the rapid acquisition of proton spectrum where the polarization from the adjacent protons is transferred to the selectively excited proton.

Practical aspects of real-time pure shift HSQC experiments

Pure shift NMR spectroscopy has become an efficient tool for improving resolution in proton NMR spectra by removing the effect of homonuclear couplings. The introduction of real-time acquisition methods has allowed the main drawback of pure shift NMR, the long experiment times needed, to be circumvented. Real-time methods use periodic application of J-refocusing pulse sequence elements, acquiring a single free induction decay, in contrast to previous methods that construct a pure shift interferogram by concatenating excerpts from multiple free induction decays. In the important heteronuclear single-quantum correlation experiment, implementing real-time pure shift data acquisition typically leads to the simultaneous improvement of both resolution and sensitivity. The current limitations of and problems with real-time pure shift acquisition methods are discussed here in the context of heteronuclear single-quantum correlation experiments. We aim to provide a detailed account of the technical challenges, together with a practical guide to exploiting the full potential of such methods.

Fast and simultaneous determination of 1 H-1 H and 1 H-19 F scalar couplings in complex spin systems: Application of PSYCHE homonuclear broadband decoupling

The present manuscript focuses on fast and simultaneous determination of ¹ H-¹ H and ¹ H-¹⁹ F scalar couplings in fluorinated complex steroid molecules. Incorporation of broadband PSYCHE homonuclear decoupling in the indirect dimension of zero-quantum filtered diagonal experiments (F1-PSYCHE-DIAG) suppresses ¹ H-¹ H scalar couplings; however, it retains ¹ H-¹⁹ F scalar couplings (along F1 dimension) for the ¹⁹ F coupled protons while preserving the pure-shift nature for ¹ H resonances uncoupled to ¹⁹ F. In such cases, along the direct dimensions, ¹ H-¹ H scalar coupling multiplets deconvolute and they appear as duplicated multiplets for the ¹⁹ F coupled protons, which facilitates unambiguous discrimination of ¹⁹ F coupled ¹ H chemical sites from the others. Further, as an added advantage, data acquisition has been accelerated by invoking the known ideas of spectral aliasing in the F1-PSYCHE-DIAG scheme and experiments demand only ~10 min of spectrometer times.

Homonuclear decoupling by projection reconstruction

Similar to J-resolved spectroscopy, also, heteronuclear multiple bond correlation (HMBC), heteronuclear single bond correlation (HSBC), and heteronuclear multiple quantum coherence (HMQC) types of correlation experiments result in homonuclear tilted multiplet patterns. On the example of the high-resolution heteronuclear single bond correlation (HR-HSBC) pulse sequence, it is shown how the tilt angle can be varied within a wide range of positive and negative values. Projection along the tilt angles in all cases results in homonuclear decoupling. Using well-known projection reconstruction techniques, the different tilt angles can be used to reconstruct a homonuclear decoupled two-dimensional correlation spectrum. The concept is proven and further refined by segmental projection reconstruction and the use of a clean in-phase heteronuclear single quantum correlation (CLIP-HSQC) spectrum with an effective zero tilt angle for further filtering. The proof of principle, its application to one-bond coupling measurement, as well as a basic HMBC, and a detailed discussion with comparison to other homodecoupling techniques are given.

Extracting unresolved coupling constants from complex multiplets by a real-time J-upscaled SERF experiment

The measurement of small homonuclear coupling constants is often prevented by either their small size and/or overlap with other signal splittings. Here, we present a real-time method to extract such couplings without interference from other splittings, with a resolution that is beyond conventional NMR spectra. In this real-time J-upscaled SERF experiment, homonuclear coupling is removed by slice-selective pure shift NMR, whereas scalar coupling to only one selected signal is reintroduced by selective refocusing. The remaining couplings are enhanced by real-time J-upscaling during interruptions of the FID data acquisition. The resulting spectrum is not only simplified by the restriction of the scalar coupling but also its resolution enhanced. This improved resolution results from a reduction of signal broadening due to magnetic field inhomogeneities from 2 different sources: slice-selective excitation and the spin-echo type J-upscaling element.

Insight into old and new pure shift nuclear magnetic resonance methods for enantiodiscrimination

Enantiodiscrimination and their quantification using nuclear magnetic resonance (NMR) spectroscopy has always been a subject of great interest. Proton is the nucleus of choice for enantiodiscrimination due to its high sensitivity and ubiquitous presence in nature. Despite its advantages, enantiodiscrimination suffers from extensive signal splitting by the proton-proton scalar couplings, which give complex multiplets that spread over a frequency range of some tens of hertz. These multiplets often overlap, further complicating interpretation of the spectra and quantifications. In the present review, we discuss some of the recent developments in the pure shift ¹ H NMR based methods for enantiomer resolution and enantiodiscrimination. We also compare various pure shift methods used for enantiodiscrimination and measurement of enantiomeric excess, considering the fact that conventional ¹ H NMR fails to provide any detailed insight.

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.