para-Hydrogenation of unsaturated moieties on poly(lysine) derived substrates for the development of novel hyperpolarized MRI contrast agents

Parahydrogen-Based Hyperpolarization for Biomedicine

Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Probing signal amplification by reversible exchange using an NMR flow system

Hyperpolarization methods are used in NMR to overcome its inherent sensitivity problem. Herein, the biologically relevant target nicotinamide is polarized by the hyperpolarization technique signal amplification by reversible exchange. We illustrate how the polarization transfer field, and the concentrations of parahydrogen, the polarization-transfer-catalyst and substrate can be used to maximize signal amplification by reversible exchange effectiveness by reference to the first-order spin system of this target. The catalyst is shown to be crucial in this process, first by facilitating the transfer of hyperpolarization from parahydrogen to nicotinamide and then by depleting the resulting polarized states through further interaction. The 15 longitudinal one, two, three and four spin order terms produced are rigorously identified and quantified using an automated flow apparatus in conjunction with NMR pulse sequences based on the only parahydrogen spectroscopy protocol. The rates of build-up of these terms were shown to follow the order four~three > two > single spin; this order parallels their rates of relaxation. The result of these competing effects is that the less-efficiently formed single-spin order terms dominate at the point of measurement with the two-spin terms having amplitudes that are an order of magnitude lower. We also complete further measurements to demonstrate that (13)C NMR spectra can be readily collected where the long-lived quaternary (13)C signals appear with significant intensity. These are improved upon by using INEPT. In summary, we dissect the complexity of this method, highlighting its benefits to the NMR community and its applicability for high-sensitivity magnetic resonance imaging detection in the future.

Analysis of Nutation Patterns in Fourier-Transform NMR of Non-Thermally Polarized Multispin Systems

The complex spin order of hyperpolarized multispin systems giving rise to anomalous NMR spectral patterns that vary with the RF excitation angle is analyzed by decomposing its nutation behavior in a superposition of Fourier harmonics. The product operator formalism is applied to calculating the spectral contributions of the various mutual alignments of scalar coupled spins. Two cases are treated, namely systems exhibiting only differences in population of their spin states and systems showing in addition zero-quantum coherences between states, a situation often seen at hyperpolarization. After deriving the general solution a number of representative examples are discussed in detail. The theoretical treatment is applied to analyzing the spin order observed in a hyperpolarized two-spin system that is prepared in the singlet state by para-hydrogen induced polarization.

Transfer of Parahydrogen Induced Polarization in Scalar Coupled Systems at Variable Magnetic Field

Para-Hydrogen Induced Polarization (PHIP) experiments were performed in coupled multispin systems at variable magnetic fields. We studied the magnetic field dependence of PHIP in styrene, which is the product of hydrogenation of phenylacetylene. At low magnetic fields where the spins are coupled strongly by scalar interaction efficient polarization transfer among the interacting protons takes place. The experimentally observed spectra are in good agreement with the simulation, which takes into account eight coupled spins. We also demonstrate effects of nuclear spin level anti-crossings on the PHIP pattern. It is shown that rapid passage through the level anti-crossing enables highly efficient polarization transfer between specific spin orders. In addition, we studied PHIP transfer to 13 C and 19 F hetero-nuclei. It is shown that hetero-nuclei can be efficiently polarized in a wide field range; in particular, for polarizing them it is not necessary to go to ultra-low fields, which provide their strong coupling to protons. The resulting polarization is of the multiplet type and gives strong enhancements of the individual NMR lines. In general, variation of the magnetic field gives the opportunity for manipulating PHIP patterns and transferring polarization to target spins of choice.

Parahydrogen-induced polarization in heterogeneous catalytic processes

Parahydrogen-induced polarization of nuclear spins provides enhancements of NMR signals for various nuclei of up to four to five orders of magnitude in magnetic fields of modern NMR spectrometers and even higher enhancements in low and ultra-low magnetic fields. It is based on the use of parahydrogen in catalytic hydrogenation reactions which, upon pairwise addition of the two H atoms of parahydrogen, can strongly enhance the NMR signals of reaction intermediates and products in solution. A recent advance in this field is the demonstration that PHIP can be observed not only in homogeneous hydrogenations but also in heterogeneous catalytic reactions. The use of heterogeneous catalysts for generating PHIP provides a number of significant advantages over the homogeneous processes, including the possibility to produce hyperpolarized gases, better control over the hydrogenation process, and the ease of separation of hyperpolarized fluids from the catalyst. The latter advantage is of paramount importance in light of the recent tendency toward utilization of hyperpolarized substances in in vivo spectroscopic and imaging applications of NMR. In addition, PHIP demonstrates the potential to become a useful tool for studying mechanisms of heterogeneous catalytic processes and for in situ studies of operating catalytic reactors. Here, the known examples of PHIP observations in heterogeneous reactions over immobilized transition metal complexes, supported metals, and some other types of heterogeneous catalysts are discussed and the applications of the technique for hypersensitive NMR imaging studies are presented.

(13)C hyperpolarization of a barbituric acid derivative via parahydrogen induced polarization

Significant (13)C NMR signal enhancement by a factor of 5000 of a barbituric acid derivative (5-methyl-5-propenyl-barbituric acid) via parahydrogen induced polarization is presented. This hyperpolarization is achieved by hydrogenating 5-methyl-5-propargyl-barbituric acid with 98% enriched para-H(2) under elevated temperature and pressure and transferring the initially created (1)H hyperpolarization with an INEPT-derived pulse sequence to (13)C. The polarization can be selectively transferred to different carbons in the barbituric acid derivative by applying different pulse delays in the INEPT pulse sequence. These results demonstrate the potential of using hyperpolarized barbituric acid derivatives as "active" contrast agents in MRI and visualizing their pharmacokinetics in vivo.

Parahydrogen induced polarization of barbituric acid derivatives: 1H hyperpolarization studies

Homogeneous hydrogenation of barbituric acid derivatives with parahydrogen yields a substantial increase of the (1)H NMR signals of the reaction products. These physiologically relevant compounds were hydrogenated at both ambient and elevated temperatures and pressures using a standard cationic rhodium catalyst. The resulting nonthermal nuclear spin polarization (hyperpolarization) is limited by the spin-lattice relaxation time T(1) of the corresponding nuclei in the products, being shorter than the time constant of the hydrogenation. The signal-to-noise ratio of the NMR spectra could be further increased upon signal averaging the antiphase PHIP signals of 25 successive scans following 30 degrees pulse experiments and a delay of 10 s.