Rates of Chemical Reactions Embedded in a Metabolic Network by Dissolution Dynamic Nuclear Polarisation NMR

Simultaneous Observation of the Anomerization and Reaction Rates of Enzymatic Dehydrogenation of Glucose-6-Phosphate by Dissolution DNP

The hyperpolarization of biological samples using dissolution dynamic nuclear polarization (dDNP) has become an attractive method for the monitoring of fast chemical and enzymatic reactions using NMR by taking advantage of a large signal increase. This approach is actively developing but still needs key methodological breakthroughs to be used as an analytical method for the monitoring of complex networks of simultaneous metabolic pathways. In this article, we use the deceptively simple example of glucose-6-phosphate (G6P) oxidation reaction by the enzyme G6P dehydrogenase (G6PDH) to discuss some important methodological aspects of dDNP kinetic experiments, such as its robustness and its ability to provide repeatable results as well as the capacity of this time-resolved methodology to test kinetic models and hypotheses and to provide reliable parameter estimates. To illustrate the potential of our approach, we report the first direct and quantitative evidence of selectivity of G6PDH toward the β anomer of G6P.

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.

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.

Multiple Stroboscopic Detection of Long-Lived Nuclear Magnetization for Glutathione Oxidation Kinetics

Imaging the molecular kinetics of antioxidants by magnetic resonance can contribute to the mechanistic understanding of therapeutic approaches. Magnetic resonance detection of the response to flashes of oxidative stress requires sequential spectroscopy on the same time scale on which reactive oxygen species are generated. To this effect, we propose a single-polarization multiple-detection stroboscopic experiment. We demonstrate this experiment for the follow-up of glutathione oxidation kinetics. On-the-fly stroboscopic detection minimizes the durations necessary for single acquisitions yet necessitates sustaining of magnetization lifetimes. Long-lived proton spin states (LLS) in the cysteine and glycine residues of glutathione with T_LLS up to 16 s are reached. Based on ¹H LLS, we followed fast oxidation kinetics in the glutathione redox pair GSH/GSSG. This new detection method allows sampling of long-lived spin order multiple times via small flip-angle excitations. This establishes the ground for the follow-up of redox processes detecting GSH/GSSG kinetics as magnetic-resonance biomarker of FLASH oxidative processes on time scales of tens of seconds.

Frémy's Salt as a Low-Persistence Hyperpolarization Agent: Efficient Dynamic Nuclear Polarization Plus Rapid Radical Scavenging

Nuclear magnetic resonance (NMR) spectroscopy is a key technique for molecular structure determination in solution. However, due to its low sensitivity, many efforts have been made to improve signal strengths and reduce the required substrate amounts. In this regard, dissolution dynamic nuclear polarization (DDNP) is a versatile approach as signal enhancements of over 10 000-fold are achievable. Samples are signal-enhanced ex situ by transferring electronic polarization from radicals to nuclear spins before dissolving and shuttling the boosted sample to an NMR spectrometer for detection. However, the applicability of DDNP suffers from one major drawback, namely, paramagnetic relaxation enhancements (PREs) that critically reduce relaxation times due to the codissolved radicals. PREs are the primary source of polarization losses canceling the signal improvements obtained by DNP. We solve this problem by using potassium nitrosodisulfonate (Frémy's salt) as polarization agent (PA), which provides high nuclear spin polarization and allows for rapid scavenging under mild reducing conditions. We demonstrate the potential of Frémy's salt, (i) showing that both 1H and 13C polarization of ∼30% can be achieved and (ii) describing a hybrid sample shuttling system (HySSS) that can be used with any DDNP/NMR combination to remove the PA before NMR detection. This gadget mixes the hyperpolarized solution with a radical scavenger and injects it into an NMR tube, providing, within a few seconds, quantitatively radical-free, highly polarized solutions. The cost efficiency and broad availability of Frémy's salt might facilitate the use of DDNP in many fields of research.

A novel sample handling system for dissolution dynamic nuclear polarization experiments

We present a system for facilitated sample vitrification, melting, and transfer in dissolution dynamic nuclear polarization (DDNP) experiments. In DDNP, a sample is typically hyperpolarized at cryogenic temperatures before dissolution with hot solvent and transfer to a nuclear magnetic resonance (NMR) spectrometer for detection in the liquid state. The resulting signal enhancements can exceed 4 orders of magnitude. However, the sudden temperature jump from cryogenic temperatures close to 1 K to ambient conditions imposes a particular challenge. It is necessary to rapidly melt the sample to avoid a prohibitively fast decay of hyperpolarization. Here, we demonstrate a sample dissolution method that facilitates the temperature jump by eliminating the need to open the cryostat used to cool the sample. This is achieved by inserting the sample through an airlock in combination with a dedicated dissolution system that is inserted through the same airlock shortly before the melting event. The advantages are threefold: (1) the cryostat can be operated continuously at low temperatures. (2) The melting process is rapid as no pressurization steps of the cryostat are required. (3) Blockages of the dissolution system due to freezing of solvents during melting and transfer are minimized.

Observation of glucose-6-phosphate anomeric exchange in real-time using dDNP hyperpolarised NMR

A hyperpolarised-NMR acquisition approach that is sensitive to the process of glucose-6-phosphate anomerization is presented. Using selective depolarisation of one of the anomer's signals, it is possible to observe the replenishing of this signal due to the fast anomeric exchange of this compound. The forward to reverse reaction rate constants ratio was ca. 1.6.

Assessing the Onset of Calcium Phosphate Nucleation by Hyperpolarized Real-Time NMR

We report an experimental approach for high-resolution real-time monitoring of transiently formed species occurring during the onset of precipitation of ionic solids from solution. This is made possible by real-time nuclear magnetic resonance (NMR) monitoring using dissolution dynamic nuclear polarization (D-DNP) to amplify signals of functional intermediates and is supported by turbidimetry, cryogenic electron microscopy, and solid-state NMR measurements. D-DNP can provide drastic signal improvements in NMR signal amplitudes, permitting dramatic reductions in acquisition times and thereby enabling us to probe fast interaction kinetics such as those underlying formation of prenucleation species (PNS) that precede solid–liquid phase separation. This experimental strategy allows for investigation of the formation of calcium phosphate (CaP)-based minerals by ³¹P NMR—a process of substantial industrial, geological, and biological interest. Thus far, many aspects of the mechanisms of CaP nucleation remain unclear due to the absence of experimental methods capable of accessing such processes on sufficiently short time scales. The approach reported here aims to address this by an improved characterization of the initial steps of CaP precipitation, permitting detection of PNS by NMR and determination of their formation rates, exchange dynamics, and sizes. Using D-DNP monitoring, we find that under our conditions (i) in the first 2 s after preparation of oversaturated calcium phosphate solutions, PNS with a hydrodynamic radius of R_h ≈ 1 nm is formed and (ii) following this rapid initial formation, the entire crystallization processes proceed on considerably longer time scales, requiring >20 s to form the final crystal phase.

The Effect of Gadolinium Doping in [13 C6 ,2 H7 ]Glucose Formulations on 13 C Dynamic Nuclear Polarization at 3.35 T

The promise of hyperpolarized glucose as a non-radioactive imaging agent capable of reporting on multiple metabolic routes has led to recent advances in its dissolution-DNP (dDNP) driven polarization using UV-light induced radicals and trityl radicals at high field (6.7 T) and 1.1 K. However, most preclinical dDNP polarizers operate at the field of 3.35 T and 1.4-1.5 K. Minute amounts of Gd³⁺ complexes have shown large improvements in solid-state polarization, which can be translated to improved hyperpolarization in solution. However, this Gd³⁺ effect seems to depend on magnetic field strength, metal ion concentration, and sample formulation. The effect of varying Gd³⁺ concentrations at 3.35 T has been described for ¹³ C-labeled pyruvic acid and acetate. However, it has not been studied for other compounds at this field. The results presented here suggest that Gd³⁺ doping can lead to various concentration and temperature dependent effects on the polarization of [¹³ C₆ ,² H₇ ]glucose, not necessarily similar to the effects observed in pyruvic acid or acetate in size or direction. The maximal polarization for [¹³ C₆ ,² H₇ ]glucose appears to be at a Gd³⁺ concentration of 2 mM, when irradiating for more than 2 h at the negative maximum of the DNP intensity profile. Surprisingly, for shorter irradiation times, higher polarization levels were determined at 1.50 K compared to 1.45 K, at a [Gd³⁺ ]=1.3 mM. This was explained by the build-up time constant and maximum at these temperatures.

Application and methodology of dissolution dynamic nuclear polarization in physical, chemical and biological contexts

Dissolution dynamic nuclear polarization (d-DNP) is a versatile method to enhance nuclear magnetic resonance (NMR) spectroscopy. It boosts signal intensities by four to five orders of magnitude thereby providing the potential to improve and enable a plethora of applications ranging from the real-time monitoring of chemical or biological processes to metabolomics and in-cell investigations. This perspectives article highlights possible avenues for developments and applications of d-DNP in biochemical and physicochemical studies. It outlines how chemists, biologists and physicists with various fields of interest can transform and employ d-DNP as a powerful characterization method for their research.

Transport of hyperpolarized samples in dissolution-DNP experiments

The magnetic field strength during sample transfer in dissolution dynamic nuclear polarization influences the resulting spectra.