Ultrahigh-Resolution NMR with Water Signal Suppression for a Deeper Understanding of the Action of Antimetabolic Drugs on Diffuse Large B-Cell Lymphoma

NMR Pure Shift Spectroscopy and Its Potential Applications in the Pharmaceutical Industry

1H nuclear magnetic resonance (NMR) spectroscopy plays an important role in the pharmaceutical industry, but for complex substances, spectral analysis is challenging due to the narrow chemical shift range and signal splitting caused by scalar coupling. Pure shift techniques can suppress scalar coupling, improving spectral resolution. This article provides a review of pure shift techniques, including the main homonuclear broadband decoupling experiments and the methods for obtaining optimal pure shift spectra with the assistance of deep learning. Furthermore, it explores the potential application directions of pure shift techniques in the pharmaceutical industry, supported by relevant scientific examples. By summarizing recent advances and application opportunities, this article aims to promote the development and practical implementation of pure shift NMR techniques in the pharmaceutical industry.

Pure Shift NMR with Solvent Suppression: A Robust and General Method for Determining Quantitative Metabolic Profiles in Biofluids

Ultrahigh-resolution pure shift NMR has recently been shown as a promising approach for providing quantitative metabolic profiles that can be used to study the metabolic footprint left by cancer cells in their aqueous growth medium. In this approach, a library of reference 1H pure shift spectra with water suppression was implemented to determine metabolite concentrations from the NOESY-presat-PSYCHE-SAPPHIRE spectrum recorded on the extracellular medium. This achievement clearly called for a generalization of a quantification method relying on ultrahigh-resolution data to other biological samples of interest (urine, plasma, tissue extracts, etc.), which requires evaluating the robustness of the analytical workflow. We have first addressed the influence of sample preparation on the quality of metabolite quantification. The quantification performed on a model mixture of metabolites prepared under different conditions shows good linearity, trueness, and precision, which highlights the high reproducibility of the proposed analytical protocol regardless of the physicochemical conditions in the sample. Second, we have successfully implemented this quantification protocol to determine metabolite levels in real urine and plasma samples, thereby paving the way for the use of the library of pure shift reference spectra for accurate and quantitative metabolic profiling of a broad range of aqueous samples.

Glutamine: A key player in human metabolism as revealed by hyperpolarized magnetic resonance

In recent years, there has been remarkable progress in the field of dissolution dynamic nuclear polarization (D-DNP). This method has shown significant potential for enhancing nuclear polarization by over 10,000 times, resulting in a substantial increase in sensitivity. The unprecedented signal enhancements achieved with D-DNP have opened new possibilities for in vitro analysis. This method enables the monitoring of structural and enzymatic kinetics with excellent time resolution at low concentrations. Furthermore, these advances can be straightforwardly translated to in vivo magnetic resonance imaging and magnetic resonance spectroscopy (MRI and MRS) experiments. D-DNP studies have used a range of 13C labeled molecules to gain deeper insights into the cellular metabolic pathways and disease hallmarks. Over the last 15 years, D-DNP has been used to analyze glutamine, a key player in the cellular metabolism, involved in many diseases including cancer. Glutamine is the most abundant amino acid in blood plasma and the major carrier of nitrogen, and it is converted to glutamate inside the cell, where the latter is the most abundant amino acid. It has been shown that increased glutamine consumption by cells is a hallmark of tumor cancer metabolism. In this review, we first highlight the significance of glutamine in metabolism, providing an in-depth description of its use at the cellular level as well as its specific roles in various organs. Next, we present a comprehensive overview of the principles of D-DNP. Finally, we review the state of the art in D-DNP glutamine analysis and its application in oncology, neurology, and perfusion marker studies.

New analytical methods focusing on polar metabolite analysis in mass spectrometry and NMR-based metabolomics

Following in the footsteps of genomics and proteomics, metabolomics has revolutionised the way we investigate and understand biological systems. Rapid development in the last 25 years has been driven largely by technical innovations in mass spectrometry and nuclear magnetic resonance spectroscopy. However, despite the modest size of metabolomes relative to proteomes and genomes, methodological capabilities for robust, comprehensive metabolite analysis remain a major challenge. Therefore, development of new methods and techniques remains vital for progress in the field. Here, we review developments in LC-MS, GC-MS and NMR methods in the last few years that have enhanced quantitative and comprehensive metabolome coverage, highlighting the techniques involved, their technical capabilities, relative performance, and potential impact.

Ultra-clean pure shift NMR with optimal water suppression for analysis of aqueous pharmaceutical samples

Pure shift NMR experiments greatly enhance spectral resolution by collapsing multiplet structures into singlets and, with water suppression, can be used for aqueous samples. Here, we combine ultra-clean pure-shift NMR (SAPPHIRE) with two different internally encoded water suppression schemes to achieve optimal performance for small molecule and macrocyclic peptide pharmaceuticals in water and acetonitrile-water mixtures.

Solvent Suppression in Pure Shift NMR

Intense solvent signals in 1H solution-state NMR experiments typically cause severe distortion of spectra and mask nearby solute signals. It is often infeasible or undesirable to replace a solvent with its perdeuterated form, for example, when analyzing formulations in situ, when exchangeable protons are present, or for practical reasons. Solvent signal suppression techniques are therefore required. WATERGATE methods are well-known to provide good solvent suppression while enabling retention of signals undergoing chemical exchange with the solvent signal. Spectra of mixtures, such as pharmaceutical formulations, are often complicated by signal overlap, high dynamic range, the narrow spectral width of 1H NMR, and signal multiplicity. Here, we show that by combining WATERGATE solvent suppression with pure shift NMR, ultrahigh-resolution 1H NMR spectra can be acquired while suppressing intense solvent signals and retaining exchangeable 1H signals. The new method is demonstrated in the analysis of cyanocobalamin, a vitamin B12 supplement, and of an eye-drop formulation of atropine.

Implementation of pure shift 1 H NMR in metabolic phenotyping for structural information recovery of biofluid metabolites with complex spin systems

NMR spectroscopy is a mainstay of metabolic profiling approaches to investigation of physiological and pathological processes. The one-dimensional proton pulse sequences typically used in phenotyping large numbers of samples generate spectra that are rich in information but where metabolite identification is often compromised by peak overlap. Recently developed pure shift (PS) NMR spectroscopy, where all J-coupling multiplicities are removed from the spectra, has the potential to simplify the complex proton NMR spectra that arise from biosamples and hence to aid metabolite identification. Here we have evaluated two complementary approaches to spectral simplification: the HOBS (band-selective with real-time acquisition) and the PSYCHE (broadband with pseudo-2D interferogram acquisition) pulse sequences. We compare their relative sensitivities and robustness for deconvolving both urine and serum matrices. Both methods improve resolution of resonances ranging from doublets, triplets and quartets to more complex signals such as doublets of doublets and multiplets in highly overcrowded spectral regions. HOBS is the more sensitive method and takes less time to acquire in comparison with PSYCHE, but can introduce unavoidable artefacts from metabolites with strong couplings, whereas PSYCHE is more adaptable to these types of spin system, although at the expense of sensitivity. Both methods are robust and easy to implement. We also demonstrate that strong coupling artefacts contain latent connectivity information that can be used to enhance metabolite identification. Metabolite identification is a bottleneck in metabolic profiling studies. In the case of NMR, PS experiments can be included in metabolite identification workflows, providing additional capability for biomarker discovery.

Present and future of pure shift NMR in metabolomics

NMR is one of the most powerful techniques for the analysis of biological samples in the field of metabolomics. However, the high complexity of fluids, tissues, or other biological materials taken from living organisms is still a challenge for state-of-the-art pulse sequences, thereby limiting the detection, the identification, and the quantification of metabolites. In this context, the resolution enhancement provided by broadband homonuclear decoupling methods, which allows for simplifying 1 H multiplet patterns into singlets, has placed this so-called pure shift technique as a promising approach to perform metabolic profiling with unparalleled level of detail. In recent years, the many advances achieved in the design of pure shift experiments has paved the way to the analysis of a wide range of biological samples with ultra-high resolution. This review leads the reader from the early days of the main pure shift methods that have been successfully developed over the last decades to address complex samples, to the most recent and promising applications of pure shift NMR to the field of NMR-based metabolomics.

A Toolbox for Glutamine Use in Dissolution Dynamic Nuclear Polarization: from Enzymatic Reaction Monitoring to the Study of Cellular Metabolic Pathways and Imaging

Glutamine is under scrutiny regarding its metabolic deregulation linked to energetic reprogramming in cancer cells. Many analytical techniques have been used to better understand the impact of the metabolism of amino acids on biological processes, however only a few are suited to work with complex samples. Here, we report the use of a general dissolution dynamic nuclear polarization (D-DNP) formulation using an unexpensive radical as a multipurpose tool to study glutamine, with insights from enzymatic modelling to complex metabolic networks and fast imaging. First, hyperpolarized [5-13 C] glutamine is used as molecular probe to study the kinetic action of two enzymes: L-asparaginase that has been used as an anti-metabolic treatment for cancer, and glutaminase. These results are also compared with those acquired with another hyperpolarized amino acid, [1,4-13 C] asparagine. Second, we explored the use of hyperpolarized (HP) substrates to probe metabolic pathways by monitoring metabolic profiles arising from hyperpolarized glutamine in E. coli extracts. Finally, a highly concentrated sample formulation is proposed for the purpose of fast imaging applications. We think that this approach can be extended to formulate other amino acids as well as other metabolites and provide complementary insights into the analysis of metabolic networks.

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.

Kövér K. Ultrahigh-Resolution Homo- and Heterodecoupled 1H and TOCSY NMR Experiments

The original homonuclear decoupled (pure shift) experiments provide ultrahigh-resolution ¹H spectra of compounds containing NMR-active heteronuclei of low natural isotopic abundance (e.g., ¹³C or ¹⁵N). In contrast, molecules containing highly abundant heteronuclei (like ³¹P or ¹⁹F) give doublets or a multiple of doublets in their homonuclear decoupled spectra, depending on the number of heteronuclear coupling partners and the magnitude of the respective coupling constants. In these cases, the complex and frequently overlapping signals may hamper the unambiguous assignment of resonances. Here, we present new heteronuclear decoupled (HD) PSYCHE ¹H and TOCSY experiments, which result in simplified spectra with significantly increased resolution, allowing the reliable assessment of individual resonances. The utility of the experiments has been demonstrated on a challenging stereoisomeric mixture of a platinum-phosphine complex, where ultrahigh resolution of the obtained HD PSYCHE spectra made the structure elucidation of the chiral products feasible. HD PSYCHE methods can be potentially applied to other important ³¹P- or ¹⁹F-containing compounds in medicinal chemistry and metabolomics.

Absolute Metabolite Quantification Using Pure Shift NMR: Toward Quantitative Metabolic Profiling of Aqueous Biological Samples

Accurate quantification of metabolites by nuclear magnetic resonance (NMR) is of prime importance in the field of health sciences for understanding the metabolic pathways of the investigated system, to address the mechanisms of action of diseases, and improving their diagnosis, treatment, and prognosis. Unfortunately, the absolute quantitative analysis of complex samples is still limited by sensitivity and resolution issues that are intrinsic to this technique. Ultrahigh-resolution pure shift methods have especially shown to be suitable for interpreting mixtures of metabolites in biological samples. Here, we introduce a robust analytical protocol based on the use of a pure shift library of calibration reference spectra to fit the fingerprint of each metabolite of interest and determine its concentration. The approach based on the SAPPHIRE pulse sequence enhanced with a block for solvent suppression has been validated through the results of a series of model mixtures, exhibiting excellent trueness (slope values in the range of 0.93-1.02) and linearity (R² > 0.996) in a total time (a few hours) that is fully compatible with metabolomics studies. Furthermore, we have successfully applied our method to determine the absolute metabolite concentrations in a lymphoma extracellular medium, which improves metabolomic protocols reported to date by providing a quantitative and highly resolved vision of metabolic processes at play.