Dynamic nuclear polarization (DNP) is a method that permits NMR signal intensities of solids and liquids to be enhanced significantly, and is therefore potentially an important tool in structural and mechanistic studies of biologically relevant molecules. During a DNP experiment, the large polarization of an exogeneous or endogeneous unpaired electron is transferred to the nuclei of interest (I) by microwave (μw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (γ e /γ l ), being ∼660 for protons. In the early 1950s, the DNP phenomenon was demonstrated experimentally, and intensively investigated in the following four decades, primarily at low magnetic fields. This review focuses on recent developments in the field of DNP with a special emphasis on work done at high magnetic fields (≥5 T), the regime where contemporary NMR experiments are performed. After a brief historical survey, we present a review of the classical continuous wave (cw) DNP mechanisms-the Overhauser effect, the solid effect, the cross effect, and thermal mixing. A special section is devoted to the theory of coherent polarization transfer mechanisms, since they are potentially more efficient at high fields than classical polarization schemes. The implementation of DNP at high magnetic fields has required the development and improvement of new and existing instrumentation. Therefore, we also review some recent developments in μw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.
Conspectus During the three decades 1980–2010, magic angle spinning (MAS) NMR developed into the method of choice to examine many chemical, physical and biological problems. In particular, a variety of dipolar recoupling methods to measure distances and torsion angles can now constrain molecular structures to high resolution. However, applications are often limited by the low sensitivity of the experiments, due in large part to the necessity of observing spectra of low-γ nuclei such as the I = ½ species 13C or 15N. The difficulty is still greater when quadrupolar nuclei, like 17O or 27Al, are involved. This problem has stimulated efforts to increase the sensitivity of MAS experiments. A particularly powerful approach is dynamic nuclear polarization (DNP) which takes advantage of the higher equilibrium polarization of electrons (which conventionally manifests in the great sensitivity advantage of EPR over NMR). In DNP, the sample is doped with a stable paramagnetic polarizing agent and irradiated with microwaves to transfer the high polarization in the electron spin reservoir to the nuclei of interest. The idea was first explored by Overhauser and Slichter in 1953. However, these experiments were carried out on static samples, at magnetic fields that are low by current standards. To be implemented in contemporary MAS NMR experiments, DNP requires microwave sources operating in the subterahertz regime — roughly 150–660 GHz — and cryogenic MAS probes. In addition, improvements were required in the polarizing agents, because the high concentrations of conventional radicals that are required to produce significant enhancements compromise spectral resolution. In the last two decades scientific and technical advances have addressed these problems and brought DNP to the point where it is achieving wide applicability. These advances include the development of high frequency gyrotron microwave sources operating in the subterahertz frequency range. In addition, low temperature MAS probes were developed that permit in-situ microwave irradiation of the samples. And, finally, biradical polarizing agents were developed that increased the efficiency of DNP experiments by factors of ~4 at considerably lower paramagnet concentrations. Collectively these developments have made it possible to apply DNP on a routine basis to a number of different scientific endeavors, most prominently in the biological and material sciences. This Account reviews these developments, including the primary mechanisms used to transfer polarization in high frequency DNP, and the current choice of microwave sources and biradical polarizing agents. In addition, we illustrate the utility of the technique with a description of applications to membrane and amyloid proteins that emphasizes the unique structural information that is available in these two cases.
Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz 1H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water–glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period.
DNP (dynamic nuclear polarization) experiments at 5 T are reported, in which a cyclotron resonance maser (gyrotron) is utilized as a 20 W, 140 6Hz microwave source to perform the polarization. MAS (magic angle spinning) NMR spectroscopy with DNP has been performed on samples of polystyrene doped with the free radical BDPA (a, y-bisdiphenylene-P-phenylallyl) at room temperature. Maximal DNP enhancements of -10 for 'H and -40 for ' C are observed and are considerably larger than expected. The DNP and spin relaxation mechanisms that lead to these enhancements at 5 T are discussed.PACS numbers: 76.70.Fz DNP (dynamic nuclear polarization) is a magnetic resonance technique utilized to enhance the polarization of nuclei in samples containing paramagnetic centers [1,2] through irradiation of the electron spins with microwaves in the neighborhood of their Larmor frequency. In studies on solids, this technique has been primarily employed to produce polarized targets for neutron scattering experiments, and to explore magnetic ordering at pK tempera-More recently, DNP has been utilized in high-resolution solid-state NMR (nuclear magnetic resonance) experiments. In MAS (magic angle spinning) NMR spectra, signal enhancements of greater than an order of magnitude have been regularly obtained at room temperature [3][4][5]. Moreover, with high eSciency microwave cavities and with small static samples, enhancements of 50-200 have been achieved [6]. Such enhancements clearly enable significant reductions in the amount of sample or the acquisition time needed to perform an NMR experiment and are therefore of great interest.The utility of these experiments has, however, been limited to date, since they have been performed only at a relatively low field -specifically 1.4 T, corresponding to Larmor frequencies for 'H and electron of coH/2tr= 60 MHz and to, /2tr=40 GHz, respectively. Consequently, the inherent sensitivity and spectral resolution routinely available in high field NMR has not been accessed in DNP-NMR experiments. The restriction to low fields arises mainly from the diSculty in procuring adequate high-power, high-frequency microwave sources. The primary millimeter wave sources, such as the EIO (extended interaction oscillator) or BWO (backward wave oscillator), rely on fragile slow-wave structures to generate microwave radiation, and thus at the high power levels required for DNP experiments have limited operating lifetimes.In the experiments described here, we have surmounted this problem by introducing a cyclotron resonance maser, or gyrotron, as the microwave source. Gyrotrons are presently employed extensively in plasma physics [7]. In the gyrotron, a continuous magnetic field replaces the slow wave structure, and the device is thus capable of generating high microwave powers over a lifetime that is considerably longer than that of an EIO or BWO [8].The microwave frequency of a gyrotron is determined by the applied magnetic field strength and accelerating voltage: gyrotron operation has been demonstrated from 8-600 GHz...
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