Geminal parahydrogen-induced polarization: accumulating long-lived singlet order on methylene proton pairs

Hyperpolarized long-lived spin state of methylene protons of 2-bromoethanol obtained from ethylene with non-equilibrium nuclear spin order

In this work we achieve a significant overpopulation (PLLS≈1%) of the long-lived spin state (LLS) of methylene protons in 2-bromoethan(2H)ol (BrEtOD) obtained in a reaction between ethylene with non-equilibrium nuclear spin order and bromine water. Given all protons in ethylene are magnetically equivalent, its nuclear states are classified into nuclear spin isomers (NSIM) with total spin I = 2,1,0. Addition of parahydrogen to acetylene produces ethylene with a population of only those NSIMs with I = 1,0. As a result of the reaction with bromine water the non-equilibrium spin order of ethylene is partly transferred to the singlet LLS involving the two methylene groups of BrEtOD. The 1H NMR signal enhancement (SE≈200) obtained as a result of the LLS readout is approximately equal to the SE of the hyperpolarized BrEtOD obtained with a single π/4 pulse. The LLS relaxation time (TLLS) was shown to be approximately 40 s (≈8T1) in the argon-bubbled sample.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Deuteron-Decoupled Singlet NMR in Low Magnetic Fields: Application to the Hyperpolarization of Succinic Acid

The reaction of unsaturated substrates with hydrogen gas enriched in the para spin isomer leads to products with a high degree of nuclear singlet spin order. This leads to greatly enhanced NMR signals, with important potential applications such as magnetic resonance imaging (MRI) of metabolic processes. Although parahydrogen-induced polarization has the advantage of being cheap, compact, and mobile, especially when performed in ultralow magnetic fields, efficiency is lost when more than a few protons are involved. This strongly restricts the range of compatible substances. We show that these difficulties may be overcome by a combination of deuteration with the application of a sinusoidally modulated longitudinal field as a well as a transverse rotating magnetic field. We demonstrate a six-fold enhancement in the ¹³ C hyperpolarization of [1-¹³ C, 2,3-d₂ ]-succinic acid, as compared with standard hyperpolarization methods, applied in the same ultralow field regime.

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolarisation technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting the chemical reactions of parahydrogen (para-H2), the spin-0 isomer of H2. These reactions break the molecular symmetry of para-H2 in a way that can produce dramatically enhanced NMR signals for reaction products, and are usually catalysed by a transition metal complex. In this review, we discuss recent advances in novel homogeneous catalysts that can produce hyperpolarised products upon reaction with para-H2. We also discuss hyperpolarisation attained in reversible reactions (termed signal amplification by reversible exchange, SABRE) and focus on catalyst developments in recent years that have allowed hyperpolarisation of a wider range of target molecules. In particular, recent examples of novel ruthenium catalysts for trans and geminal hydrogenation, metal-free catalysts, iridium sulfoxide-containing SABRE systems, and cobalt complexes for PHIP and SABRE are reviewed. Advances in this catalysis have expanded the types of molecules amenable to hyperpolarisation using PHIP and SABRE, and their applications in NMR reaction monitoring, mechanistic elucidation, biomedical imaging, and many other areas, are increasing.

Direct Production of a Hyperpolarized Metabolite on a Microfluidic Chip

Microfluidic systems hold great potential for the study of live microscopic cultures of cells, tissue samples, and small organisms. Integration of hyperpolarization would enable quantitative studies of metabolism in such volume limited systems by high-resolution NMR spectroscopy. We demonstrate, for the first time, the integrated generation and detection of a hyperpolarized metabolite on a microfluidic chip. The metabolite [1-¹³C]fumarate is produced in a nuclear hyperpolarized form by (i) introducing para-enriched hydrogen into the solution by diffusion through a polymer membrane, (ii) reaction with a substrate in the presence of a ruthenium-based catalyst, and (iii) conversion of the singlet-polarized reaction product into a magnetized form by the application of a radiofrequency pulse sequence, all on the same microfluidic chip. The microfluidic device delivers a continuous flow of hyperpolarized material at the 2.5 μL/min scale, with a polarization level of 4%. We demonstrate two methods for mitigating singlet–triplet mixing effects which otherwise reduce the achieved polarization level.

Synergies between Hyperpolarized NMR and Microfluidics: A Review

Hyperpolarized nuclear magnetic resonance and lab-on-a-chip microfluidics are two dynamic, but until recently quite distinct, fields of research. Recent developments in both areas increased their synergistic overlap. By microfluidic integration, many complex experimental steps can be brought together onto a single platform. Microfluidic devices are therefore increasingly finding applications in medical diagnostics, forensic analysis, and biomedical research. In particular, they provide novel and powerful ways to culture cells, cell aggregates, and even functional models of entire organs. Nuclear magnetic resonance is a non-invasive, high-resolution spectroscopic technique which allows real-time process monitoring with chemical specificity. It is ideally suited for observing metabolic and other biological and chemical processes in microfluidic systems. However, its intrinsically low sensitivity has limited its application. Recent advances in nuclear hyperpolarization techniques may change this: under special circumstances, it is possible to enhance NMR signals by up to 5 orders of magnitude, which dramatically extends the utility of NMR in the context of microfluidic systems. Hyperpolarization requires complex chemical and/or physical manipulations, which in turn may benefit from microfluidic implementation. In fact, many hyperpolarization methodologies rely on processes that are more efficient at the micro-scale, such as molecular diffusion, penetration of electromagnetic radiation into a sample, or restricted molecular mobility on a surface. In this review we examine the confluence between the fields of hyperpolarization-enhanced NMR and microfluidics, and assess how these areas of research have mutually benefited one another, and will continue to do so.

Spatially Resolved Kinetic Model of Parahydrogen Induced Polarisation (PHIP) in a Microfluidic Chip

We report a spatially resolved kinetic finite element model of parahydrogen-induced polarisation (PHIP) in a microfluidic chip that was calibrated using on-chip and off-chip NMR data. NMR spectroscopy has great potential as a read-out technique for lab-on-a-chip (LoC) devices, but is often limited by sensitivity. By integrating PHIP with a LoC device, a continuous stream of hyperpolarised material can be produced, and mass sensitivities of pmol s have been achieved. However, the yield and polarisation levels have so far been quite low, and can still be optimised. To facilitate this, a kinetic model of the reaction has been developed, and its rate constants have been calibrated using macroscopic kinetic measurements. The kinetic model was then coupled with a finite element model of the microfluidic chip. The model predicts the concentration of species involved in the reaction as a function of flow rate and position in the device. The results are in quantitative agreement with published experimental data.

Nuclear singlet relaxation by chemical exchange

The population imbalance between nuclear singlet states and triplet states of strongly coupled spin-1/2 pairs, also known as nuclear singlet order, is well protected against several common relaxation mechanisms. We study the nuclear singlet relaxation of ¹³C pairs in aqueous solutions of 1,2-¹³C₂ squarate over a range of pH values. The ¹³C singlet order is accessed by introducing ¹⁸O nuclei in order to break the chemical equivalence. The squarate dianion is in chemical equilibrium with hydrogen-squarate (SqH^-) and squaric acid (SqH₂) characterized by the dissociation constants pK₁ = 1.5 and pK₂ = 3.4. Surprisingly, we observe a striking increase in the singlet decay time constants T_S when the pH of the solution exceeds ∼10, which is far above the acid-base equilibrium points. We derive general rate expressions for chemical-exchange-induced nuclear singlet relaxation and provide a qualitative explanation of the T_S behavior of the squarate dianion. We identify a kinetic contribution to the singlet relaxation rate constant, which explicitly depends on kinetic rate constants. Qualitative agreement is achieved between the theory and the experimental data. This study shows that infrequent chemical events may have a strong effect on the relaxation of nuclear singlet order.

Singlet-Contrast Magnetic Resonance Imaging: Unlocking Hyperpolarization with Metabolism*

Hyperpolarization-enhanced magnetic resonance imaging can be used to study biomolecular processes in the body, but typically requires nuclei such as 13 C, 15 N, or 129 Xe due to their long spin-polarization lifetimes and the absence of a proton-background signal from water and fat in the images. Here we present a novel type of 1 H imaging, in which hyperpolarized spin order is locked in a nonmagnetic long-lived correlated (singlet) state, and is only liberated for imaging by a specific biochemical reaction. In this work we produce hyperpolarized fumarate via chemical reaction of a precursor molecule with para-enriched hydrogen gas, and the proton singlet order in fumarate is released as antiphase NMR signals by enzymatic conversion to malate in D2 O. Using this model system we show two pulse sequences to rephase the NMR signals for imaging and suppress the background signals from water. The hyperpolarization-enhanced 1 H-imaging modality presented here can allow for hyperpolarized imaging without the need for low-abundance, low-sensitivity heteronuclei.

Markovian exchange phenomena in magnetic resonance and the Lindblad equation

Nuclear Magnetic Resonance (NMR) experiments involve coherent and incoherent dynamics. Incoherent dynamics give rise to nuclear spin relaxation typically accounted for by a differential equation known as a quantum master equation. Out of the existing master equations the Lindblad equation is particularly important. A central theorem of open quantum theory states that a relaxation process represents a quantum Markovian process if and only if it can be described by a Lindblad master equation. This in turn raises serious questions regarding the compatibility of conventional NMR relaxation theory and one of the most fundamental results of open quantum theory. The NMR treatment of important Markovian relaxation mechanisms such as Brownian motion or chemical exchange events do in general not result in a master equation in Lindblad form and would therefore be at odds with the mathematically rigorous theory of open quantum systems. We explore this apparent conflict for two important chemical exchange phenomena and show that their conventional treatment may be recast into a suitable Lindblad equation. Similarities between spatial diffusion and multi-site exchange events allow us to extend our results to nuclear spin relaxation due to translational diffusion. As a result we are able to show that the NMR treatment of intramolecular exchange and translational diffusion represent valid quantum Markovian processes compatible with the rigorous definitions of open quantum theory.