Inducing Dynamic Nuclear Polarization in Chemical Reactions

The experimental studies of the dynamic nuclear polarization In the chemical reactions(CINDP)[1-11] discover some characteristic peculiarities — the unusual behaviour of the spin multiplets, the appearance of nuclear polarization in the products of cage radical recombination, and finally, the dependence of the sign of polarization upon the type of reaction in which the radical participates. These pecularities are specific for CINDP and differentiate CINDP from the ordinary two-frequency dynamic nuclear polarization.     

High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR

Dynamic nuclear polarization (DNP) transfers electron spin-polarization to nuclear spins in close proximity, increasing sensitivity by two-to-three orders of magnitude. This enables nuclear magnetic resonance (NMR) experiments on samples with low concentrations of analyte. The requirement of using cryogenic temperatures in DNP-enhanced solid-state NMR (ssNMR) experiments may impair the resolution and hence limit its broad application to biological systems. In this work, we introduce a “High-Temperature DNP” approach, which aims at increasing spectral resolution by performing experiments at temperatures of around 180?K instead of?~100?K. By utilizing the extraordinary enhancements obtained on deuterated proteins, still sufficiently large DNP enhancements of 11–18 are obtained for proton and carbon, respectively. We recorded high sensitivity 2D ¹³C–¹³C spectra in?~9?min with higher resolution than at 100?K, which has similar resolution to the one obtained at room temperature for some favorable residues.     

Spin-trapped hydroxyl free radicals studied at low field by Field-Cycled Dynamic Nuclear Polarization

The technique of Field-Cycled Dynamic Nuclear Polarization (FC-DNP) involves the EPR irradiation of a free radical solution and the subsequent observation of the NMR signal, the experiment being carried out at a range of magnetic field strengths in order to measure the free radical’s EPR spectrum. In this work FC-DNP has been used to study the EPR spectrum of DMPO spin-trapped hydroxyl free radicals at magnetic field strengths between 0.5 mT and 13.0 mT (5–130 Gauss). The low-field EPR spectrum contains six separate EPR lines, in contrast to the well-known X-band spectrum where only four are seen. Knowledge of the spin-adduct’s EPR spectrum will be of use to workers involved in low-field EPR, especially those conducting biological or in-vivo spin-trapping experiments.     

Solid-Effect Rate for Dynamic Nuclear Polarization of Spin-l/2 and Spin-1 Nuclei

The solid-effect rate equations for the dynamic nuclear polarization of spin-1/2 (tritium or hydrogen) or spin-1 (deuterium) nuclei are derived in the limiting case of the electronspin-resonance line width being much smaller than the nuclear-magnetic-resonance frequency. The exact analytical solutions of the rate equations for spin-1/2 nuclei are given, while the rate equations for spin-1 nuclei, which are nonlinear, are solved in decoupling approximation. The nuclear polarization and the electronic polarization as well as the times to polarization are calculated in low temperature limit. It is found that there is an optimal radio frequency pumping rate for the nuclear polarizations, which is not considered until now.     

Dynamic Nuclear Polarization by Thermal Mixing Under Partial Saturation

We describe a low-temperature thermodynamic model for dynamic nuclear polarization (DNP) via continuous-wave partial saturation of electron spin resonance (ESR) lines that are both homogeneously and inhomogeneously broadened. It is a variant of a reasoning proposed by Borghini, which in turn used Redfield’s thermodynamic treatment of saturation. Our variant is furthermore based on Provotorov’s insight that under partial saturation of a coupled-spin system two distinct spin temperatures should appear in a thermodynamical theory. We apply our model to DNP results obtained at a temperature of 1.2?K and in magnetic fields of 3.35 and 5?T on 1-¹³C labeled sodium acetate dissolved in a frozen D₂O/ethanol-d₆ solution doped with the free radical TEMPO.     

A High-Conversion-Factor, Double-Resonance Structure for High-Field Dynamic Nuclear Polarization

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W^1/2 for the microwave conversion factor and 0.8 mT/W^1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state ¹H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about ?16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure.     

Low Field (10 mT) Pulsed Dynamic Nuclear Polarization

EPR irradiation by a train of inverting pulses has potential advantages over continuous-wave EPR irradiation in DNP applications; however, it has previously been used only at high field (5 T). This paper presents the design and testing of an apparatus for performing pulsed DNP experiments at 10 mT with large samples (17 ml). Experimental results using pulsed DNP with an aqueous solution of a narrow-linewidth paramagnetic probe are presented. A maximum DNP enhancement of about -36 with a train of inverting pulses (width 500 ns, repetition time 4 micros) was measured. A preliminary comparison showed that, when the same enhancement value is considered, the pulsed DNP technique requires an average power that is about three times higher than that required with the CW irradiation. However, for in vivo DNP applications it is very important to minimize the average power deposited in the sample. From the experimental results reported in this work, when considering the maximum enhancement, the pulsed technique requires only 2% of the average power necessary with the CW DNP technique. We believe that this reduction in the average power can be important for future DNP studies with large biological samples.     

The feasibility of assessing branched-chain amino acid metabolism in cellular models of prostate cancer with hyperpolarized [1-C]-ketoisocaproate

Recent advancements in the field of hyperpolarized ¹³C magnetic resonance spectroscopy (MRS) have yielded powerful techniques capable of real-time analysis of metabolic pathways. These non-invasive methods have increasingly shown application in impacting disease diagnosis and have further been employed in mechanistic studies of disease onset and progression. Our goals were to investigate branched-chain aminotransferase (BCAT) activity in prostate cancer with a novel molecular probe, hyperpolarized [1-¹³C]-2-ketoisocaproate ([1-¹³C]-KIC), and explore the potential of branched-chain amino acid (BCAA) metabolism to serve as a biomarker. Using traditional spectrophotometric assays, BCAT enzymatic activities were determined in vitro for various sources of prostate cancer (human, transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse and human cell lines). These preliminary studies indicated that low levels of BCAT activity were present in all models of prostate cancer but enzymatic levels are altered significantly in prostate cancer relative to healthy tissue. The MR spectroscopic studies were conducted with two cellular models (PC-3 and DU-145) that exhibited levels of BCAA metabolism comparable to the human disease state. Hyperpolarized [1-¹³C]-KIC was administered to prostate cancer cell lines, and the conversion of [1-¹³C]-KIC to the metabolic product, [1-¹³C]-leucine ([1-¹³C]-Leu), could be monitored via hyperpolarized ¹³C MRS.     

THz-waves channeling in a monolithic saddle-coil for Dynamic Nuclear Polarization enhanced NMR

A saddle coil manufactured by electric discharge machining (EDM) from a solid piece of copper has recently been realized at EPFL for Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance experiments (DNP-NMR) at 9.4 T. The corresponding electromagnetic behavior of radio-frequency (400 MHz) and THz (263 GHz) waves were studied by numerical simulation in various measurement configurations. Moreover, we present an experimental method by which the results of the THz-wave numerical modeling are validated. On the basis of the good agreement between numerical and experimental results, we conducted by numerical simulation a systematic analysis on the influence of the coil geometry and of the sample properties on the THz-wave field, which is crucial in view of the optimization of DNP-NMR in solids.     

Operational Characteristics of a 14-W 140-GHz Gyrotron for Dynamic Nuclear Polarization

The operating characteristics of a 140-GHz 14-W long pulse gyrotron are presented. The device is being used in dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) spectroscopy experiments. The gyrotron yields 14 W peak power at 139.65 GHz from the TE(0,3) operating mode using a 12.3-kV 25-mA electron beam. Additionally, up to 12 W peak has been observed in the TE(2,3) mode at 136.90 GHz. A series of mode converters transform the TE(0,3) operating mode to the TE(1,1) mode. Experimental results are compared with nonlinear simulations and show reasonable agreement. The millimeter-wave output beam was imaged in a single shot using a pyroelectric camera. The mode patterns matched reasonably well to theory for both the TE(0,1) mode and the TE(1,1) mode. Repeatable mode patterns were obtained at intervals ranging from 0.8 s apart to 11 min apart at the output of the final mode converter.