Experimental Verification of the Overhauser Nuclear Polarization Effect

Carver, Thomas R.; Slichter, Charles P.

doi:10.1103/physrev.102.975

Cited by 285 publications

(156 citation statements)

References 16 publications

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“…In these approaches, the sensitivity is enhanced by a factor of (γ I /γ S ) or about 4 for I= 1 H and S= 13 C and 10 when S= 15 N. Another, and in fact the original, example of a polarization transfer experiment was proposed by Overhauser [8] and involved transfer of conduction electron polarization to nuclear spins in metals. Carver and Slichter [9,10] verified Overhauser's hypothesis that such transfers and signal enhancements were possible with low field (3.03 mT) experiments performed on samples of Li metal and other materials with mobile electrons. During the 1970's, the analogous nuclear Overhauser effect (NOE) was used extensively to increase sensitivity in spectra of low-γ species, and it is currently employed to estimate internuclear distances for structure determination by solution state NMR.…”

Section: Introductionmentioning

confidence: 82%

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

Bajaj

Hornstein

Kreischer

et al. 2007

Journal of Magnetic Resonance

162

130

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In this paper, we describe a 250 GHz gyrotron oscillator, a critical component of an integrated system for magic angle spinning (MAS) dynamic nuclear polarization (DNP) experiments at 9T, corresponding to 380 MHz 1 H frequency. The 250 GHz gyrotron is the first gyro-device designed with the goal of seamless integration with an NMR spectrometer for routine DNP-enhanced NMR spectroscopy and has operated under computer control for periods of up to 21 days with a 100% duty cycle. Following a brief historical review of the field, we present studies of the membrane protein bacteriorhodopsin (bR) using DNP-enhanced multidimensional NMR. These results include assignment of active site resonances in [U-13 C, 15 N]-bR and demonstrate the utility of DNP for studies of membrane proteins. Next, we review the theory of gyro-devices from quantum mechanical and classical viewpoints and discuss the unique considerations that apply to gyrotron oscillators designed for DNP experiments. We then characterize the operation of the 250 GHz gyrotron in detail, including its long-term stability and controllability. We have measured the spectral purity of the gyrotron emission using both homodyne and heterodyne techniques. Radiation intensity patterns from the corrugated waveguide that delivers power to the NMR probe were measured using two new techniques to confirm pure mode content: a thermometric approach based on the temperaturedependent color of liquid crystalline media applied to a substrate and imaging with a pyroelectric camera. We next present a detailed study of the mode excitation characteristics of the gyrotron. Exploration of the operating characteristics of several fundamental modes reveals broadband continuous frequency tuning of up to 1.8 GHz as a function of the magnetic field alone, a feature that may be exploited in future tunable gyrotron designs. Oscillation of the 250 GHz gyrotron at the second harmonic of cyclotron resonance begins at extremely low beam currents (as low 12 mA) at frequencies between 320-365 GHz, suggesting an efficient route for the generation of even higher frequency radiation. The low starting currents were attributed to an elevated cavity Q, which is confirmed by cavity thermal load measurements. We conclude with an appendix containing a detailed description of the control system that safely automates all aspects of the gyrotron operation.

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Section: Introductionmentioning

confidence: 82%

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

Bajaj

Hornstein

Kreischer

et al. 2007

Journal of Magnetic Resonance

162

130

View full text Add to dashboard Cite

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“…DNP increases the sensitivity of NMR experiments (signal-to-noise ratio, S/N), [1][2][3] enabling the study of low concentrated systems which were not in the scope of NMR until now. NMR signal enhancement is achieved by transferring large electron polarization to surrounding nuclei via DNP.…”

Section: A Introductionmentioning

confidence: 99%

Dynamic nuclear polarization of spherical nanoparticles

Akbey

Altın

Linden

et al. 2013

Phys. Chem. Chem. Phys.

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Spherical silica nanoparticles of various particle sizes (B10 to 100 nm), produced by a modified Stoeber method employing amino acids as catalysts, are investigated using Dynamic Nuclear Polarization (DNP) enhanced Nuclear Magnetic Resonance (NMR) spectroscopy. This study includes ultra-sensitive detection of surface-bound amino acids and their supramolecular organization in trace amounts, exploiting the increase in NMR sensitivity of up to three orders of magnitude via DNP. Moreover, the nature of the silicon nuclei on the surface and the bulk silicon nuclei in the core (sub-surface) is characterized at atomic resolution. Thereby, we obtain unique insights into the surface chemistry of these nanoparticles, which might result in improving their rational design as required for promising applications, e.g. as catalysts or imaging contrast agents. The non-covalent binding of amino acids to surfaces was determined which shows that the amino acids not just function as catalysts but become incorporated into the nanoparticles during the formation process. As a result only three distinct Q-types of silica signals were observed from surface and core regions. We observed dramatic changes of DNP enhancements as a function of particle size, and very small particles (which suit in vivo applications better) were hyperpolarized with the best efficiency. Nearly one order of magnitude larger DNP enhancement was observed for nanoparticles with 13 nm size compared to particles with 100 nm size.We determined an approximate DNP penetration-depth (B4.2 or B5.7 nm) for the polarization transfer from electrons to the nuclei of the spherical nanoparticles. Faster DNP polarization buildup was observed for larger nanoparticles. Efficient hyperpolarization of such nanoparticles, as achieved in this work, can be utilized in applications such as magnetic resonance imaging (MRI). A IntroductionDNP increases the sensitivity of NMR experiments (signal-to-noise ratio, S/N), 1-3 enabling the study of low concentrated systems which were not in the scope of NMR until now. NMR signal enhancement is achieved by transferring large electron polarization to surrounding nuclei via DNP. In the last decade, there has been remarkable progress in solution and solid-state DNP-NMR as well as in imaging through the application of hyperpolarization methods. [4][5][6][7] As pointed out recently, this ''renaissance of DNP'' combined with high magnetic fields will further grow in the coming years, enabling demanding applications with better resolution. 8,9 Materials and biological systems such as membrane or microcrystalline proteins, 10-13 fibrils, 14 ligands bound to receptors, 15 polymers, [16][17][18] surface-bound species, 19-23 and other low concentrated samples were studied beneficially using DNP-NMR. 13,[24][25][26] Cryogenic temperatures are required to slow down nuclear and electron relaxation and to allow an efficient polarization transfer in solid-state DNP NMR experiments. In contrast to biological systems, 15,27-29 most technically relevant materials ...

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“…In applied fields of several tesla, for instance, a nuclear-spin system must be cooled to millikelvin temperatures for the thermal polarization P to be of order unity, and at such low temperatures, the spin-lattice interactions which restore spins to thermal equilibrium between transients become "frozen out," yielding impractically long relaxation times [1]. Nonequilibrium methods for hyperpolarizing nuclear spins have widely applied [2][3][4][5][6][7][8][9][10][11], but the problem of long relaxation times at low temperatures has prevented the use of refrigeration as a practical method of achieving high levels of polarization for NMR applications.…”

Section: Introductionmentioning

confidence: 99%

Polarization of nuclear spins by a cold nanoscale resonator

Butler

Weitekamp

2011

Phys. Rev. A

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A cold nanoscale resonator coupled to a system of nuclear spins can induce spin relaxation. In the lowtemperature limit where spin-lattice interactions are "frozen out," spontaneous emission by nuclear spins into a resonant mechanical mode can become the dominant mechanism for cooling the spins to thermal equilibrium with their environment. We provide a theoretical framework for the study of resonator-induced cooling of nuclear spins in this low-temperature regime. Relaxation equations are derived from first principles, in the limit where energy donated by the spins to the resonator is quickly dissipated into the cold bath that damps it. A physical interpretation of the processes contributing to spin polarization is given. For a system of spins that have identical couplings to the resonator, the interaction Hamiltonian conserves spin angular momentum, and the resonator cannot relax the spins to thermal equilibrium unless this symmetry is broken by the spin Hamiltonian. The mechanism by which such a spin system becomes "trapped" away from thermal equilibrium can be visualized using a semiclassical model, which shows how an indirect spin-spin interaction arises from the coupling of multiple spins to one resonator. The internal spin Hamiltonian can affect the polarization process in two ways: (1) By modifying the structure of the spin-spin correlations in the energy eigenstates, and (2) by splitting the degeneracy within a manifold of energy eigenstates, so that zero-frequency off-diagonal terms in the density matrix are converted to oscillating coherences. Shifting the frequencies of these coherences sufficiently far from zero suppresses the development of resonator-induced correlations within the manifold during polarization from a totally disordered state. Modification of the spin-spin correlations by means of either mechanism affects the strength of the fluctuating spin dipole that drives the resonator. In the case where product states can be chosen as energy eigenstates, spontaneous emission from eigenstate populations into the resonant mode can be interpreted as independent emission by individual spins, and the spins relax exponentially to thermal equilibrium if the development of resonator-induced correlations is suppressed. When the spin Hamiltonian includes a significant contribution from the homonuclear dipolar coupling, the energy eigenstates entail a correlation specific to the coupling network. Simulations of dipole-dipole coupled systems of up to five spins suggest that these systems contain weakly emitting eigenstates that can trap a fraction of the population for time periods 100/R 0 , where R 0 is the rate constant for resonator-enhanced spontaneous emission by a single spin 1/2. Much of the polarization, however, relaxes with rates comparable to R 0 . A distribution of characteristic high-field chemical shifts tends to increase the relaxation rates of weakly emitting states, enabling transitions to states that can quickly relax to thermal equilibrium. The theoretical framework presented in this pa...

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Experimental Verification of the Overhauser Nuclear Polarization Effect

Cited by 285 publications

References 16 publications

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

Dynamic nuclear polarization of spherical nanoparticles

Polarization of nuclear spins by a cold nanoscale resonator

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