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 (microw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (gamma(e)gamma(l)), being approximately 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 (> or =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 microw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.

Dynamic nuclear polarization at high magnetic fields

Nitroxide free radicals are the most commonly used source for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) experiments and are also exclusively employed as spin labels for electron spin resonance (ESR) spectroscopy of diamagnetic molecules and materials. Nitroxide free radicals have been shown to have strong dipolar coupling to H1 in water, and thus result in large DNP enhancement of H1 NMR signal via the well known Overhauser effect. The fundamental parameter in a DNP experiment is the coupling factor, since it ultimately determines the maximum NMR signal enhancements which can be achieved. Despite their widespread use, measurements of the coupling factor of nitroxide free radicals have been inconsistent, and current models have failed to successfully explain our experimental data. We found that the inconsistency in determining the coupling factor arises from not taking into account the characteristics of the ESR transitions, which are split into three (or two) lines due t...

A new model for Overhauser enhanced nuclear magnetic resonance using nitroxide radicals.

The dynamic nuclear polarization (DNP) effect between the polarizers TEMPOL and trityl and the 1H protons of water solution was investigated at two different magnetic fields corresponding to electron pumping frequencies of 9 and 94 GHz. For TEMPOL, large DNP enhancements up to about −100 (9 GHz) and −20 (94 GHz) were observed by continuous microwave irradiation on one of the three nitroxide hyperfine lines. These enhancements are considerably larger than those ever reported for this system and also larger than the ones we obtained with the second proposed polarizer agent, the trityl radical, under the same conditions. 1H Overhauser coupling factors were extracted from NMRD relaxation dispersion experiments and used to estimate the degree of effective saturation of the TEMPOL EPR line. Our results show that nitroxide radicals are well-suited polarizers for DNP experiments in aqueous solutions at variable fields. The large enhancements open up attractive perspectives for the application potential of DNP in ...

Field dependent dynamic nuclear polarization with radicals in aqueous solution.

Dynamic nuclear polarization is emerging as a potential tool to increase the sensitivity of NMR aiming at the detection of macromolecules in liquid solution. One possibility for such an experimental design is to perform the polarization step between electrons and nuclei at low magnetic fields and then transfer the sample to a higher field for NMR detection. In this case, an independent optimization of the polarizer and detection set ups is required. In the present paper we describe the optimization of a polarizer set up at 15 MHz 1H NMR/9.7 GHz EPR frequencies based on commercial hardware. The sample consists of the nitroxide radical TEMPONE-D,15N in water, for which the dimensions were systematically decreased to fit the homogeneous B1 region of a dielectric ENDOR resonator. With an available B1 microwave field up to 13 G we observe a maximum DNP enhancement of −170 at room temperature by irradiating on either one of the EPR lines. The DNP enhancement was saturated at all polarizer concentrations. Pulsed ELDOR experiments revealed that the saturation level of the two hyperfine lines is such that the DNP enhancements are well consistent with the coupling factors derived from NMRD data. By raising the polarizing field and frequencies 10-fold, i.e. to 140 MHz 1H/94 GHz EPR, we reach an enhancement of −43 at microwave field strengths (B1 ≈ 5 G). The results are discussed in view of an application for a DNP spectrometer.

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Optimization of dynamic nuclear polarization experiments in aqueous solution at 15 MHz/9.7 GHz: a comparative study with DNP at 140 MHz/94 GHz.

Dynamic nuclear polarization properties of nitroxyl radicals used in Overhauser-enhanced MRI for simultaneous molecular imaging.

Dynamic nuclear polarization with nitroxides dissolved in biological fluids.

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Dynamic nuclear polarization with nitroxides dissolved in biological fluids.