Dynamic nuclear polarization at high magnetic fields

Abstract

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.

Overhauser effects in insulating solids

“…These approaches have resulted in large signal enhancements and have enabled many experiments that would otherwise be impossible. [19][20][21][22][23] by Overhauser 25 and confirmed by Carver and Slichter, 26 has not been identified or utilized during the course of this renaissance. Although the possibility of an OE in insulator was discussed by Abragam, 27 the conventional wisdom is that Overhauser DNP is important only in systems with mobile electrons such as conductors (metals and low dimensional conductors) or in liquid solution.…”

Exploiting level anti-crossings for efficient and selective transfer of hyperpolarization in coupled nuclear spin systems

“…Although hyperpolarization techniques made numerous new NMR applications possible, 6,[9][10][11][12][13][14][15][16][17] there are still many problems to be solved in this field. In particular, it is of importance to further develop strategies for optimizing the hyperpolarization process and the transfer of polarization from the primarily polarized spins to the target nuclei of choice.…”

An analysis of NMR sensitivity enhancements obtained using non-uniform weighted sampling, and the application to protein NMR

“…Since the inception of NMR spectroscopy, spectroscopists have sought to improve the sensitivity of this intrinsically insensitive technique, for example through pulse sequence design [1,2,3,4,5], the development of cryogenic probes [6], and the application of dynamic nuclear polarisation [7,8,9]. Of course, as technology has developed, so has the desire to study increasingly large, complex or dilute samples -as has the desire to make efficient use of valuable spectrometer time.…”