Simon B. Duckett

Reversible interactions with para-hydrogen enhance NMR sensitivity by polarization transfer.

The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.

Iridium N-heterocyclic carbene complexes as efficient catalysts for magnetization transfer from para-hydrogen.

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)2(IMes)(py)3]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H2. These reversible processes bring para-H2 and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in 1H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)2(η2-H2)(IMes)(py)2]+, an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach.

A theoretical basis for spontaneous polarization transfer in non-hydrogenative parahydrogen-induced polarization

When parahydrogen adds to a metal template containing a substrate of interest, the substrate and parahydrogen become coupled, and polarization is shared between the two without the incorporation of the parahydrogen into the substrate. A mechanism for this polarization transfer is presented in which the transfer is propagated through the scalar couplings. At zero field, polarization is transferred between two-, three-, and four-spin zero quantum states, but no single spin magnetization is created. The interplay between the chemical shift evolution and the evolution under scalar coupling at non-zero field generates additional longitudinal spin order and now includes single spin longitudinal z-magnetization. The additional chemical shift interaction introduces a field dependency to the nuclear spin states of the polarized substrate. The net effect of the polarization field strength on the resultant nuclear spin states is shown to be predictable but complex.

Spontaneous Transfer of Parahydrogen Derived Spin Order to Pyridine at Low Magnetic Field

The cationic iridium complex [Ir(COD)(PCy(3))(py)]BF(4) (1) is shown to react with dihydrogen in the presence of pyridine (py) to form the dihydride complex fac,cis-[Ir(PCy(3))(py)(3)(H)(2)]BF(4) (2). Complex 2 undergoes rapid exchange of the two bound pyridine ligands which are trans to hydride with free pyridine; the activation parameters for this process in methanol are DeltaH(double dagger) = 97.4 +/- 9 kJ mol(-1) and DeltaS(double dagger) = 84 +/- 31 J K(-1) mol(-1). When parahydrogen is employed as a source of nuclear spin polarization, spontaneous magnetization transfer proceeds in low magnetic field from the two nascent hydride ligands of 2 to its other NMR active nuclei. Upon interrogation by NMR spectroscopy in a second step, signal enhancements in excess of 100 fold are observed for the (1)H, (13)C and (15)N resonances of free pyridine after ligand exchange. The degree of signal enhancement in the free substrate is increased by employing electronically rich and sterically encumbered phosphine ligands such as PCy(3), PCy(2)Ph, or P(i)Pr(3) and by optimizing the strength of the magnetic field in which polarization transfer occurs.

Application of Parahydrogen Induced Polarization Techniques in NMR Spectroscopy and Imaging

Magnetic resonance provides a versatile platform that allows scientists to examine many different types of phenomena. However, the sensitivity of both NMR spectroscopy and MRI is low because the detected signal strength depends on the population difference that exists between the probed nuclear spin states in a magnetic field. This population difference increases with the strength of the interacting magnetic field and decreases with measurement temperature. In contrast, hyperpolarization methods that chemically introduce parahydrogen (a spin isomer of hydrogen with antiparallel spins that form a singlet) based on the traditional parahydrogen induced polarization (PHIP) approach tackle this sensitivity problem with dramatic results. In recent years, the potential of this method for MRI has been recognized, and its impact on medical diagnosis is starting to be realized. In this Account, we describe the use of parahydrogen to hyperpolarize a suitable substrate. This process normally involves the introduction of a molecule of parahydrogen into a target to create large population differences between nuclear spin states. The reaction of parahydrogen breaks the original magnetic symmetry and overcomes the selection rules that prevent both NMR observation and parahydrogen/orthohydrogen interconversion, yielding access to the normally invisible hyperpolarization associated with parahydrogen. Therefore the NMR or MRI measurement delivers a marked increase in the detected signal strength over the normal Boltzmann-population derived result. Consequently, measurements can be made which would otherwise be impossible. This approach was pioneered by Weitekamp, Bargon, and Eisenberg, in the late 1980s. Since 1993, we have used this technique in York to study reaction mechanisms and to characterize normally invisible inorganic species. We also describe signal amplification by reversible exchange (SABRE), an alternative route to sensitize molecules without directly incorporating a molecule of parahydrogen. This approach widens the applicability of PHIP methods and the range of materials that can be hyperpolarized. In this Account we describe our parahydrogen studies in York over the last 20 years and place them in a wider context. We describe the characterization of organometallic reaction intermediates including those involved in catalytic reactions, either with or without hydride ligands. The collection of spectroscopic and kinetic data with rapid inverse detection methods has proved to be particularly informative. We can see enhanced signals for the organic products of catalytic reactions that are linked directly to the catalytic intermediates that form them. This method can therefore prove unequivocally that a specific metal complex is involved in a catalytic cycle, thus pinpointing the true route to catalysis. Studies where a pure nuclear spin state is detected show that it is possible to detect all of the analyte molecules present in a sample using NMR. In addition, we describe methods that achieve the selective detection of these enhanced signals, when set against a strong NMR background such as that of water.

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.

Optimization of SABRE for polarization of the tuberculosis drugs pyrazinamide and isoniazid

Hyperpolarization produces nuclear spin polarization that is several orders of magnitude larger than that achieved at thermal equilibrium thus providing extraordinary contrast and sensitivity. As a parahydrogen induced polarization (PHIP) technique that does not require chemical modification of the substrate to polarize, Signal Amplification by Reversible Exchange (SABRE) has attracted a lot of attention. Using a prototype parahydrogen polarizer, we polarize two drugs used in the treatment of tuberculosis, namely pyrazinamide and isoniazid. We examine this approach in four solvents, methanol-d4, methanol, ethanol and DMSO and optimize the polarization transfer magnetic field strength, the temperature as well as intensity and duration of hydrogen bubbling to achieve the best overall signal enhancement and hence hyperpolarization level.

Toward Biocompatible Nuclear Hyperpolarization Using Signal Amplification by Reversible Exchange: Quantitative in Situ Spectroscopy and High-Field Imaging

Signal amplification by reversible exchange (SABRE) of a substrate and parahydrogen at a catalytic center promises to overcome the inherent insensitivity of magnetic resonance. In order to apply the new approach to biomedical applications, there is a need to develop experimental equipment, in situ quantification methods, and a biocompatible solvent. We present results detailing a low-field SABRE polarizer which provides well-controlled experimental conditions, defined spins manipulations, and which allows in situ detection of thermally polarized and hyperpolarized samples. We introduce a method for absolute quantification of hyperpolarization yield in situ by means of a thermally polarized reference. A maximum signal-to-noise ratio of ∼103 for 148 μmol of substance, a signal enhancement of 106 with respect to polarization transfer field of SABRE, or an absolute 1H-polarization level of ≈10–2 is achieved. In an important step toward biomedical application, we demonstrate 1H in situ NMR as well as 1H and 13C high-field MRI using hyperpolarized pyridine (d3) and 13C nicotinamide in pure and 11% ethanol in aqueous solution. Further increase of hyperpolarization yield, implications of in situ detection, and in vivo application are discussed.

Biocontrol of avocado dematophora root rot by antagonistic Pseudomonas fluorescens PCL1606 correlates with the production of 2-hexyl 5-propyl resorcinol

A collection of 905 bacterial isolates from the rhizospheres of healthy avocado trees was obtained and screened for antagonistic activity against Dematophora necatrix, the cause of avocado Dematophora root rot (also called white root rot). A set of eight strains was selected on the basis of growth inhibitory activity against D. necatrix and several other important soilborne phytopathogenic fungi. After typing of these strains, they were classified as belonging to Pseudomonas chlororaphis, Pseudomonas fluorescens, and Pseudomonas putida. The eight antagonistic Pseudomonas spp. were analyzed for their secretion of hydrogen cyanide, hydrolytic enzymes, and antifungal metabolites. P. chlororaphis strains produced the antibiotic phenazine-1-carboxylic acid and phenazine-1-carboxamide. Upon testing the biocontrol ability of these strains in a newly developed avocado-D. necatrix test system and in a tomato-F oxysporum test system, it became apparent that P. fluorescens PCL1606 exhibited the highest biocontrol ability. The major antifungal activity produced by strain P. fluorescens PCL1606 did not correspond to any of the major classes of antifungal antibiotics produced by Pseudomonas biocontrol strains. This compound was purified and subsequently identified as 2-hexyl 5-propyl resorcinol (HPR). To study the role of HPR in biocontrol activity, two Tn5 mutants of P. fluorescens PCL1606 impaired in antagonistic activity were selected. These mutants were shown to impair HRP production and showed a decrease in biocontrol activity. As far as we know, this is the first report of a Pseudomonas biocontrol strain that produces HPR in which the production of this compound correlates with its biocontrol activity.

A hyperpolarized equilibrium for magnetic resonance.

Nuclear magnetic resonance spectroscopy and imaging (MRI) play an indispensable role in science and healthcare but use only a tiny fraction of their potential. No more than ≈10 p.p.m. of all 1H nuclei are effectively detected in a 3-Tesla clinical MRI system. Thus, a vast array of new applications lays dormant, awaiting improved sensitivity. Here we demonstrate the continuous polarization of small molecules in solution to a level that cannot be achieved in a viable magnet. The magnetization does not decay and is effectively reinitialized within seconds after being measured. This effect depends on the long-lived, entangled spin-order of parahydrogen and an exchange reaction in a low magnetic field of 10−3 Tesla. We demonstrate the potential of this method by fast MRI and envision the catalysis of new applications such as cancer screening or indeed low-field MRI for routine use and remote application.