An NMR technique for tracing out the carbon skeleton of an organic molecule

Clean PDI-1 SQ: Suppression of HSQC artifacts in 2D proton-detected INADEQUATE spectra by pulse sequence redesign

Proton-detected INADEQUATE NMR experiments are widely used for structure elucidation of small molecules, in particular the implementations that display 13C single-quantum rather than double-quantum frequencies in the indirect dimension of 2D spectra. But unfortunately, such spectra in addition to the desired 1H-13C two-bond correlations also contain HSQC artifacts of comparable magnitude. The redesigned versatile experiment presented in this paper requires no compromise based on different 13C multiplicities and suppresses the HSQC artifacts that are a source of possible spectral misinterpretation. Demonstration of the new method is shown by applications to typical small molecules of different complexity.

The Global Characterisation of a Drug-Dendrimer Conjugate - PEGylated poly-lysine Dendrimer

The recent emergence of drug-dendrimer conjugates within pharmaceutical industry research and development introduces a range of challenges for analytical and measurement science. These molecules are very high molecular weight (100-200kDa) with a significant degree of structural complexity. The characteristics and quality attributes that require understanding and definition, and impact efficacy and safety, are diverse. They relate to the intact conjugate, the various building blocks of these complex systems and the level of the free and bound active pharmaceutical ingredient (API). From an analytical and measurement science perspective, this necessitates the measurement of the molecular weight, impurity characterisation, the quantitation of the number of conjugated versus free API molecules, the determination of the impurity profiles of the building blocks, primary structure and both particle size and morphology. Here we report the first example of a global characterisation of a drug-dendrimer conjugate - PEGylated poly-lysine dendrimer currently under development (AZD0466). The impact of the wide variety of analytical and measurement techniques on the overall understanding of this complex molecular entity is discussed, with the relative capabilities of the various approaches compared. The results of this study are an essential platform for the research and development of the future generations of related dendrimer-based medicines.

Solid-State NMR: Methods for Biological Solids

In the last two decades, solid-state nuclear magnetic resonance (ssNMR) spectroscopy has transformed from a spectroscopic technique investigating small molecules and industrial polymers to a potent tool decrypting structure and underlying dynamics of complex biological systems, such as membrane proteins, fibrils, and assemblies, in near-physiological environments and temperatures. This transformation can be ascribed to improvements in hardware design, sample preparation, pulsed methods, isotope labeling strategies, resolution, and sensitivity. The fundamental engagement between nuclear spins and radio-frequency pulses in the presence of a strong static magnetic field is identical between solution and ssNMR, but the experimental procedures vastly differ because of the absence of molecular tumbling in solids. This review discusses routinely employed state-of-the-art static and MAS pulsed NMR methods relevant for biological samples with rotational correlation times exceeding 100's of nanoseconds. Recent developments in signal filtering approaches, proton methodologies, and multiple acquisition techniques to boost sensitivity and speed up data acquisition at fast MAS are also discussed. Several examples of protein structures (globular, membrane, fibrils, and assemblies) solved with ssNMR spectroscopy have been considered. We also discuss integrated approaches to structurally characterize challenging biological systems and some newly emanating subdisciplines in ssNMR spectroscopy.

More than ADEQUATE: doubling the sensitivity of 13CH–13CH correlations in double-quantum NMR experiments

Tracing the connectivity of CH–CHx moieties of organic compounds in an overnight cryoprobe NMR experiment using milligrams of material.

Bayesian probabilistic assignment of chemical shifts in organic solids

A prerequisite for NMR studies of organic materials is assigning each experimental chemical shift to a set of geometrically equivalent nuclei. Obtaining the assignment experimentally can be challenging and typically requires time-consuming multidimensional correlation experiments. An alternative solution for determining the assignment involves statistical analysis of experimental chemical shift databases, but no such database exists for molecular solids. Here, by combining the Cambridge Structural Database with a machine learning model of chemical shifts, we construct a statistical basis for probabilistic chemical shift assignment of organic crystals by calculating shifts for more than 200,000 compounds, enabling the probabilistic assignment of organic crystals directly from their two-dimensional chemical structure. The approach is demonstrated with the ¹³C and ¹H assignment of 11 molecular solids with experimental shifts and benchmarked on 100 crystals using predicted shifts. The correct assignment was found among the two most probable assignments in more than 80% of cases.

Application of INADEQUATE NMR techniques for directly tracing out the carbon skeleton of a natural product

INTRODUCTION

Nuclear magnetic resonance (NMR) measurement of ¹ J_CC coupling by two-dimensional (2D) INADEQUATE (incredible natural abundance double quantum transfer experiment), which is a special case of double-quantum (DQ) spectroscopy that offers unambiguous determination of ¹³ C-¹³ C spin-spin connectivities through the DQ transitions of the spin system, is especially suited to solving structures rich in quaternary carbons and poor in hydrogen content (Crews rule).

OBJECTIVE

To review published literature on the application of NMR methods to determine structure in the liquid-state, which specifically considers the interaction of a pair of carbon-13 (¹³ C) nuclei adjacent to one another, to allow direct tracing out of contiguous carbon connectivity using 2D INADEQUATE.

METHODOLOGY

A comprehensive literature search was implemented with various databases: Web of Knowledge, PubMed and SciFinder, and other relevant published materials including published monographs. The keywords used, in various combinations, with INADEQUATE being present in all combinations, in the search were 2D NMR, ¹ J_CC coupling, natural product, structure elucidation, ¹³ C-¹³ C connectivity, cryoprobe and CASE (computer-assisted structure elucidation)/PANACEA (protons and nitrogen and carbon et alia).

RESULTS

The 2D INADEQUATE continues to solve "intractable" problems in natural product chemistry, and using milligram quantities with cryoprobe techniques combined with CASE/PANACEA experiments can increase machine time efficiency. The ¹³ C-¹³ C-based structural elucidation by dissolution single-scan dynamic nuclear polarisation NMR can overcome disadvantages of ¹³ C insensitivity at natural abundance. Selected examples have demonstrated the trajectory of INADEQUATE spectroscopy from structural determination to clarification of metabolomics analysis and use of DFT (density functional theory) and coupling constants to clarify the connectivity, hybridisation and stereochemistry within natural products.

CONCLUSIONS

Somewhat neglected over the years because of perceived lack of sensitivity, the 2D INADEQUATE NMR technique has re-emerged as a useful tool for solving natural products structures, which are rich in quaternary carbons and poor in hydrogen content.

Ascorbigen A-NMR identification

The connectivities of all atoms in ascorbigen A, an important metabolite, were determined unambiguously for the first time. The connectivity between carbon atoms was established by 2D INADEQUATE, and one-bond ¹³ C-¹³ C coupling constants were determined for all pairs of directly connected carbon atoms except for two strongly coupled carbon pairs. The ¹³ C-¹³ C coupling in one of the pairs was proved by a modification of standard INADEQUATE; however, the signals from the other pair were too weak to be observed. The connectivity within the two strongly coupled C-C pairs was confirmed by a combination of COSY and gHSQC; the latter experiment also identified all C-H bonds. The proton nuclear magnetic resonance (¹ H NMR) spectra in dry dimethyl sulfoxide allowed identification and assignment of the signals due to NH and OH protons. The derived structure, 3-((1H-indol-3-yl)methyl)-3,3a,6-trihydroxytetrahydrofuro[3,2-b]furan-2(5H)-one, agrees with the structure suggested for ascorbigen A in 1966. The density functional theory (DFT) calculations showed that among 16 possible stereoisomers, only two complied with the almost zero value of the measured ³ J(H6-H6a). Of the two stereoisomers, 3S,3aS,6S,6aR and 3R,3aR,6R,6aS, the latter was excluded on synthetic grounds. The nuclear Overhauser effect measurements unveiled close proximity between H2' proton of the indole and the H6a proton of the tetrahydrofuro[3,2-b]furan part. Detailed structural interpretation of the measured NMR parameters by means of DFT NMR was hampered by rotational flexibility of the indole and tetrahydrofuro[3,2-b]furan parts and inadequacy of Polarizable Continuum Model (PCM) solvent model.

Natural Isotopic Abundance 13C and 15N Multidimensional Solid-State NMR Enabled by Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) has made feasible solid-state NMR experiments that were previously thought impractical due to sensitivity limitations. One such class of experiments is the structural characterization of organic and biological samples at natural isotopic abundance (NA). Herein, we describe the many advantages of DNP-enabled ssNMR at NA, including the extraction of long-range distance constraints using dipolar recoupling pulse sequences without the deleterious effects of dipolar truncation. In addition to the theoretical underpinnings in the analysis of these types of experiments, numerous applications of DNP-enabled ssNMR at NA are discussed.

Practical Guidelines for 13C-Based NMR Metabolomics

We present an overview of ¹³C-based NMR metabolomics. At first glance, the low sensitivity of ¹³C relative to ¹H NMR might seem like too great an obstacle to use this approach. However, there are several advantages to ¹³C NMR, whether samples can be isotopically enriched or not. At natural abundance, peaks are sharp and largely resolved, and peak frequencies are more stable to pH and other sample conditions. Statistical approaches can be used to obtain C-C and C-H correlation maps, which greatly aid in compound identification. With ¹³C isotopic enrichment, other experiments are possible, including both ¹³C-J-RES and INADEQUATE, which can be used for de novo identification of metabolites not in databases.NMR instrumentation and software has significantly improved, and probes are now commercially available that can record useful natural abundance 1D ¹³C spectra from real metabolomics samples in 2 h or less. Probe technology continues to improve, and the next generation should be even better. Combined with new methods of simultaneous data acquisition, which allows for two or more 1D or 2D NMR experiments to be collected using multiple receivers, very rich datasets can be collected in a reasonable amount of time that should improve metabolomics data analysis and compound identification.

Observation of untoward 3 Jcc correlations in 1,1-ADEQUATE spectra of pyrimidine analogs: Avoiding potential interpretation pitfalls

Recently, it has been reported that large ⁿ J_CC correlations can sometimes be observed in 1,1-ADEQUATE spectra with significant intensity, which opens the possibility of structural misassignment. In this work, we have focused on pyrimidine-based compounds, which exhibit multiple bond correlations in the 1,1-ADEQUATE experiment as a consequence of ³ J_CC coupling constants greater than 10 Hz. Results are supported by both the experimental measurement of ³ J_CC coupling constants in question using J-modulated-ADEQUATE and density functional theory calculations.

Kövér K, Sørensen OW. Distinguishing between two- and three-bond correlations for all 13C multiplicities in heteronuclear NMR spectroscopy

Imposing heteronuclear multiple-quantum multiplets on a ¹³C single-quantum spectrum distinguishes between two- and three-bond correlations in an HMBC-type NMR spectrum.

Analysis of the Electronic Structure of the Special Pair of a Bacterial Photosynthetic Reaction Center by 13 C Photochemically Induced Dynamic Nuclear Polarization Magic-Angle Spinning NMR Using a Double-Quantum Axis

The origin of the functional symmetry break in bacterial photosynthesis challenges since several decades. Although structurally very similar, the two branches of cofactors in the reaction center (RC) protein complex act very differently. Upon photochemical excitation, an electron is transported along one branch, while the other remains inactive. Photochemically induced dynamic nuclear polarization (photo-CIDNP) magic-angle spinning (MAS) ¹³ C NMR revealed that the two bacteriochlorophyll cofactors forming the "Special Pair" donor dimer are already well distinguished in the electronic ground state. These previous studies are relying solely on ¹³ C-¹³ C correlation experiments as radio-frequency-driven recoupling (RFDR) and dipolar-assisted rotational resonance (DARR). Obviously, the chemical-shift assignment is difficult in a dimer of tetrapyrrole macrocycles, having eight pyrrole rings of similar chemical shifts. To overcome this problem, an INADEQUATE type of experiment using a POST C7 symmetry-based approach is applied to selectively isotope-labeled bacterial RC of Rhodobacter (R.) sphaeroides wild type (WT). We, therefore, were able to distinguish unresolved sites of the macromolecular dimer. The obtained chemical-shift pattern is in-line with a concentric assembly of negative charge within the common center of the Special Pair supermolecule in the electronic ground state.

Atomic-Level Structure Characterization of Biomass Pre- and Post-Lignin Treatment by Dynamic Nuclear Polarization-Enhanced Solid-State NMR

Lignocellulosic biomass is a promising sustainable feedstock for the production of biofuels, biomaterials, and biospecialty chemicals. However, efficient utilization of biomass has been limited by our poor understanding of its molecular structure. Here, we report a dynamic nuclear polarization (DNP)-enhanced solid-state (SS)NMR study of the molecular structure of biomass, both pre- and postcatalytic treatment. This technique enables the measurement of 2D homonuclear ¹³C-¹³C correlation SSNMR spectra under natural abundance, yielding, for the first time, an atomic-level picture of the structure of raw and catalytically treated biomass samples. We foresee that further such experiments could be used to determine structure-function relationships and facilitate the development of more efficient, and chemically targeted, biomass-conversion technologies.

Efficient 2D double-quantum solid-state NMR spectroscopy with large spectral widths

STiC phase shifts enable the use of supercycled recoupling sequences for recording 2D DQ–SQ correlation spectra with arbitrary spectral widths.

Additional pitfalls of using 1,1-ADEQUATE for structure elucidation

1,1-ADEQUATE is a powerful and robust NMR experiment to establish carbon-carbon connectivities using modest sample quantities when cryogenic probe technology is available. Yet potential pitfalls of applying this method are not widely appreciated, such as weak or missing ¹ J_CC correlations in strongly coupled ¹³ C-¹³ C AB spin systems and unusually large multi-bond (ⁿ J_CC ) correlations associated with particular functional groups. These large ⁿ J_CC correlations observed in 1,1-ADEQUATE spectra could be mistaken for ¹ J_CC correlations. Copyright © 2016 John Wiley & Sons, Ltd.

Lone-Pair Orientation Effect of an α-Oxygen Atom on (1)JCC NMR Spin-Spin Coupling Constants in o-Substituted Phenols. Experimental and DFT Study

The well-known N lone-pair orientation effect on (1)JCC spin-spin coupling constants (SSCCs) in oximes and their derivatives was used to study how negative hyperconjugative interactions of type LP1(O) → σ*CC depend on ortho interactions involving the OH group. This study demanded the following analyses:  (i) a qualitative estimation of how (1)JCC SSCCs are affected by hyperconjugative interactions, (ii) a study of similar stereochemical effects to those in oximes, but in (1)JC1C2 and (1)JC1C6in a series of 2-substituted phenols, and (iii) a quantitative estimation, with the natural bond order approach, of some key electron delocalization interactions. A few unexpected results are quoted. LP1(O) → σ*CC interactions are affected by proximity interactions as follows:  (a) they are enhanced by hydrogen bonds transferring charge into the (O-H)* antibonding orbital; (b) they are enhanced by proximity interactions of type LP1(O)···H-C; (c) they are inhibited by interactions of type LP(O1)···H-O. Consequences of these observations are discussed.

Facing and Overcoming Sensitivity Challenges in Biomolecular NMR Spectroscopy

In the Spring of 2013, NMR spectroscopists convened at the Weizmann Institute in Israel to brainstorm on approaches to improve the sensitivity of NMR experiments, particularly when applied in biomolecular settings. This multi-author interdisciplinary Review presents a state-of-the-art description of the primary approaches that were considered. Topics discussed included the future of ultrahigh-field NMR systems, emerging NMR detection technologies, new approaches to nuclear hyperpolarization, and progress in sample preparation. All of these are orthogonal efforts, whose gains could multiply and thereby enhance the sensitivity of solid- and liquid-state experiments. While substantial advances have been made in all these areas, numerous challenges remain in the quest of endowing NMR spectroscopy with the sensitivity that has characterized forms of spectroscopies based on electrical or optical measurements. These challenges, and the ways by which scientists and engineers are striving to solve them, are also addressed.

(13)C NMR Metabolomics: INADEQUATE Network Analysis

The many advantages of (13)C NMR are often overshadowed by its intrinsically low sensitivity. Given that carbon makes up the backbone of most biologically relevant molecules, (13)C NMR offers a straightforward measurement of these compounds. Two-dimensional (13)C-(13)C correlation experiments like INADEQUATE (incredible natural abundance double quantum transfer experiment) are ideal for the structural elucidation of natural products and have great but untapped potential for metabolomics analysis. We demonstrate a new and semiautomated approach called INETA (INADEQUATE network analysis) for the untargeted analysis of INADEQUATE data sets using an in silico INADEQUATE database. We demonstrate this approach using isotopically labeled Caenorhabditis elegans mixtures.

ADEQUATE CR: 13C connectivity mapping in indirect detection mode with composite refocusing

We report a novel rare spin correlation experiment termed ADEQUATE with composite refocusing (CR), which is the (1)H-detected version of 2D INADEQUATE CR. ADEQUATE CR begins with a polarization transfer from protons to the attached carbon, followed by (13)C-(13)C double-quantum (DQ) preparation. Unlike the ADEQUATE class of experiments, (13)C DQ coherence is converted after evolution to single-quantum single transitions (SQ-STs) by CR. (13)C SQ-ST is then transferred back to the coupled protons by a coherence order selective reconversion. The present sequence produces partial transition selectivity in the (1)H dimension as does (1)H Indirect detected (13)C Low-Abundance Single-transition correlation Spectroscopy (HICLASS), thereby mitigating the reduction in sensitivity enhancement because of the presence of homonuclear proton couplings. However, unlike HICLASS (which is an experiment that involves SQ-TS evolution), no homonuclear zero quantum mixing is required on the (13)C channel in the present experiment. Experimental results are demonstrated on a variety of samples, establishing the efficiency of the proposed method.

Theoretical study of homonuclear J coupling between quadrupolar spins: single-crystal, DOR, and J-resolved NMR

The theory describing homonuclear indirect nuclear spin-spin coupling (J) interactions between pairs of quadrupolar nuclei is outlined and supported by numerical calculations. The expected first-order multiplets for pairs of magnetically equivalent (A2), chemically equivalent (AA'), and non-equivalent (AX) quadrupolar nuclei are given. The various spectral changeovers from one first-order multiplet to another are investigated with numerical simulations using the SIMPSON program and the various thresholds defining each situation are given. The effects of chemical equivalence, as well as quadrupolar coupling, chemical shift differences, and dipolar coupling on double-rotation (DOR) and J-resolved NMR experiments for measuring homonuclear J coupling constants are investigated. The simulated J coupling multiplets under DOR conditions largely resemble the ideal multiplets predicted for single crystals, and a characteristic multiplet is expected for each of the A2, AA', and AX cases. The simulations demonstrate that it should be straightforward to distinguish between magnetic inequivalence and equivalence using J-resolved NMR, as was speculated previously. Additionally, it is shown that the second-order quadrupolar-dipolar cross-term does not affect the splittings in J-resolved experiments. Overall, the homonuclear J-resolved experiment for half-integer quadrupolar nuclei is demonstrated to be robust with respect to the effects of first- and second-order quadrupolar coupling, dipolar coupling, and chemical shift differences.

Boron–boron J coupling constants are unique probes of electronic structure: a solid-state NMR and molecular orbital study

J couplings measured between ¹¹B spin pairs in solid diboron compounds provide insight into electronic structure and crystallographic symmetry.

Development of a ¹³C-optimized 1.5-mm high temperature superconducting NMR probe

We report a 1.5-mm NMR probe based on high temperature superconductors operating at 14.1T optimized for (13)C detection. The probe has a total sample volume of about 35 microliters (μL) with an active volume of 20 μL and provides exceptional mass sensitivity for (13)C detection. The probe also has excellent (1)H sensitivity and employs a (2)H lock; (15)N irradiation capability can be added in the future. The coils are cooled to about 20K using a standard Agilent cryogenic refrigeration system, and the sample temperature is regulated near room temperature. The coil design considerations are discussed in detail. This probe is ideal for directly detected (13)C NMR experiments for natural products chemistry and metabolomics applications, for which 35 μL is an optimal sample volume. The outstanding (13)C sensitivity of this probe allowed us to directly determine the (13)C connectivity on 1.1mg of natural abundance histidine using an INADEQUATE experiment. We demonstrated the utility of this probe for (13)C-based metabolomics using a synthetic mixture of common natural abundance metabolites whose concentrations ranged from 1 to 5mM (40-200 nmol).

Symmetry-amplified J splittings for quadrupolar spin pairs: a solid-state NMR probe of homoatomic covalent bonds

Chemically informative J couplings between pairs of quadrupolar nuclei in dimetallic and dimetalloid coordination motifs are measured using J-resolved solid-state NMR experiments. It is shown that the application of a double-quantum filter is necessary to observe the J splittings and that, under these conditions, only a simple doublet is expected. Interestingly, the splitting is amplified if the spins are magnetically equivalent, making it possible to measure highly precise J couplings and unambiguously probe the symmetry of the molecule. This is demonstrated experimentally by chemically breaking the symmetry about a pair of boron spins by reaction with an N-heterocyclic carbene to form a β-borylation reagent. The results show that the J coupling is a sensitive probe of bonding in diboron compounds and that the J values quantify the weakening of the B-B bond which occurs when forming an sp(2)-sp(3) diboron compound, which is relevant to their reactivity. Due to the prevalence of quadrupolar nuclei among transition metals, this work also provides a new approach to probe metal-metal bonding; results for Mn2(CO)10 are provided as an example.

Unambiguous structure elucidation of heterocyclic products from condensation-cyclisation reactions of enaminones by 2D INADEQUATE and 15N NMR

The reaction of enaminones with 3-amino-2-cyanopent-2-enedinitrile can lead to an array of 12 possible products, depending on the reaction pathway and tautomerization. The use of 2D INADEQUATE and (15)N NMR for the unambiguous structure elucidation of the reaction products is discussed in this manuscript.

1J(CC)-edited HSQC-1,n-ADEQUATE: a new paradigm for simultaneous direct and long-range carbon-carbon correlation

Establishing the carbon skeleton of a molecule greatly facilitates the process of structure elucidation, leaving only heteroatoms to be inserted, heterocyclic rings to be closed, and stereochemical features to be defined. INADEQUATE, and more recently PANACEA, have been the only means of coming close to the goal of totally defining the carbon skeleton of a molecule. Unfortunately, the extremely low sensitivity and prodigious sample requirements of these experiments and the multiple receiver requirement for the latter experiment have severely restricted the usage of these experiments. Proton-detected ADEQUATE experiments, in contrast, have considerably higher sensitivity and more modest sample requirements. By combining experiments such as 1,1-ADEQUATE and 1,n-ADEQUATE with higher sensitivity experiments such as GHSQC through covariance processing, sample requirements can be further reduced with a commensurate improvement in the s/n ratio and F(1) resolution of the covariance processed spectrum. We now wish to report the covariance processing of an inverted (1)J(CC) 1,n-ADEQUATE experiment with a non-edited GHSQC spectrum to afford a spectrum that can trace the carbon skeleton of a molecule with the exception of correlations between quaternary carbons.

Technical and practical aspects of (19) F NMR-based screening: toward sensitive high-throughput screening with rapid deconvolution

The technical and practical aspects of (19) F NMR-based screening against a macromolecular target are analyzed in detail. A novel method utilizing the relaxation of (19) F homonuclear double quantum coherence is proposed for performing NMR-based binding assays in a direct- or competition-mode format. A combined strategy based on (19) F NMR chemical shift prediction, 2D (19) F NMR DOSY, and 2D (19) F-(1) H NMR long-range COSY experiments is presented for the deconvolution of complex mixtures of fluorinated molecules generated by either addition of single compounds or by chemical synthesis. The approaches presented here allow the screening of complex mixtures, even in the case where the exact composition is not known, and the rapid identification of the binders contained in the mixtures.

Efficiency of heteronuclear dipolar decoupling schemes in solid-state NMR: investigation of effective transverse relaxation times

We here compare the T(2)(') values of various heteronuclear dipolar decoupling schemes commonly used in solid-state nuclear magnetic resonance experiments. Swept-frequency two-pulse phase modulation scheme is shown to give longer T(2)(') values for the majority of the magic-angle-spinning frequencies and radiofrequency amplitudes considered here. The longer T(2)(') values obtained are shown to yield spectra with higher resolution in experiments, such as INADEQUATE, which incorporate spin-echo blocks. Such blocks normally constitute the indirect dimension of a multidimensional experiment during which heteronuclear dipolar decoupling is applied, thereby making the relevance of T(2)(') manifest clearly. Experimental results are shown on samples of glycine, alanine, and Aβ(42).

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Parallel receivers and sparse sampling in multidimensional NMR

The recent introduction of NMR spectrometers with multiple receivers permits spectra from several different nuclear species to be recorded in parallel, and several standard pulse sequences to be combined into a single entity. It is shown how these improvements in the flow and quality of spectral information can be significantly augmented by compressive sensing techniques--controlled aliasing, Hadamard spectroscopy, single-point evaluation of evolution space (SPEED), random sampling, projection-reconstruction, and hyperdimensional NMR. Future developments of these techniques are confidently expected to mitigate one of the most serious limitations in multidimensional NMR--the excessive duration of the measurements.

HSQC-1,1-ADEQUATE and HSQC-1,n-ADEQUATE: enhanced methods for establishing adjacent and long-range 13C-13C connectivity networks

1H-13C GHSQC and GHMBC spectra are irrefutably among the most valuable 2D NMR experiments for the establishment of unknown chemical structures. However, the indeterminate nature of the length of the long-range coupling(s) observed via the (n)J(CH)-optimized delay of the GHMBC experiment can complicate the interpretation of the data when dealing with novel chemical structures. A priori there is no way to differentiate 2J(CH) from (n)J(CH) correlations, where n ≥ 3. Access to high-field spectrometers with cryogenic NMR probes brings 1,1- and 1,n-ADEQUATE experiments into range for modest samples. Subjecting ADEQUATE spectra to covariance processing with high sensitivity experiments such as multiplicity-edited GHSQC affords a diagonally symmetric 13C-13C correlation spectrum in which correlation data are observed with the apparent sensitivity of the GHSQC spectrum. HSQC-1,1-ADEQUATE covariance spectra derived by co-processing of GHSQC and 1,1-ADEQUATE spectra allow the carbon skeleton of molecules to be conveniently traced. HSQC-1,n-ADEQUATE spectra provide enhanced access to correlations equivalent to 4J(CH) correlations in a GHMBC spectrum. When these data are used to supplement GHMBC data, a powerfully synergistic set of heteronuclear correlations are available. The methods discussed are illustrated using retrorsine (1) as a model compound.

1H Indirect detected 13C Low-Abundance Single-transition correlation Spectroscopy (HICLASS)-13C homonuclear correlation at natural abundance

A novel proton-detected (13)C homonuclear correlation experiment is reported at natural abundance, viz., (1)H Indirect detected (13)C Low-Abundance Single-transition correlation Spectroscopy (HICLASS). HICLASS is based on the evolution of (13)C single-quantum single transitions, followed by their mixing, and (1)H detection subsequent to heteronuclear transfer. Reduced relaxation losses during the evolution time and partial selectivity in the (1)H multiplet structure result in enhanced sensitivity of HICLASS. The superior performance of HICLASS is demonstrated for (1)H-detected (13)C correlation work.

Assignment of phycocyanobilin in HMPT using triple resonance experiments

A complete assignment of all resonances of a small organic molecule is a prerequisite for a structure determination using NMR spectroscopy. This is conventionally obtained using a well-established strategy based on COSY, HMQC and HMBC spectra. In case of phycocyanobilin (PCB) in HMPT this strategy was unsuccessful due to the symmetry of the molecule and extreme signal overlap. Since (13)C and (15)N labeled material was available, an alternative strategy for resonance assignment was used. Triple resonance experiments derived from experiments conventionally performed for proteins are sensitive and easy to analyze. Their application led to a complete and unambiguous assignment using three types of experiments.

Geminal (2)J((29)Si-O-(29)Si) couplings in oligosiloxanes and their relation to direct (1)J((29) Si-(13)C) couplings

Absolute values of (79) geminal (2)J((29) Si-O-(29)Si) couplings were measured in an extensive series of (55) unstrained siloxanes dissolved in chloroform-d. Signs of (2)J((29)Si-O-(29)Si) in some (9) silicon hydrides were determined relative to (1)J((29)Si-(1)H) which are known to be negative. It is supposed that positive sign of the (2)J((29)Si-O-(29)Si) coupling found in all studied hydrides is common to all siloxanes. Theoretical calculations for simple model compounds failed to reproduce this sign and so their predictions of bond length and angle dependences cannot be taken as reliable. Useful empirical correlations were found between the (2)J((29)Si-O-(29)Si) couplings on one side and the total number m of oxygen atoms bonded to the silicon atoms, sum of (29)Si chemical shifts or product of (1)J((29)Si-(13)C) couplings on the other side. The significance of these correlations is briefly discussed.

Use of continuous optimization methods to find carbon links in 2D INADEQUATE spectra

The 2-D INADEQUATE experiment is a useful experiment for determining carbon structures of organic molecules, which is known for having low signal-to-noise ratios. A non-linear optimization method for solving low-signal spectra resulting from this experiment is introduced to compensate. The method relies on the peak locations defined by the INADEQUATE experiment to create boxes around these areas and measure the signal in each. By measuring pairs of these boxes and applying penalty functions that represent a priori information, we are able to quickly and reliably solve spectra with an acquisition time approximately a quarter of that required by traditional methods. Examples are shown using the spectrum of sucrose.