A method for obtaining strongly polarized nuclear spins in solution has
 been developed. The method uses low temperature, high magnetic field, and
 dynamic nuclear polarization (DNP) to strongly polarize nuclear spins in the
 solid state. The solid sample is subsequently dissolved rapidly in a suitable
 solvent to create a solution of molecules with hyperpolarized nuclear spins.
 The polarization is performed in a DNP polarizer, consisting of a
 super-conducting magnet (3.35 T) and a liquid-helium cooled sample space. The
 sample is irradiated with microwaves at ≈94 GHz. Subsequent to
 polarization, the sample is dissolved by an injection system inside the DNP
 magnet. The dissolution process effectively preserves the nuclear
 polarization. The resulting hyperpolarized liquid sample can be transferred to
 a high-resolution NMR spectrometer, where an enhanced NMR signal can be
 acquired, or it may be used as an agent for in vivo imaging or
 spectroscopy. In this article we describe the use of the method on aqueous
 solutions of [ 13 C]urea. Polarizations of 37% for 13 C and
 7.8% for 15 N, respectively, were obtained after the dissolution.
 These polarizations correspond to an enhancement of 44,400 for 13 C
 and 23,500 for 15 N, respectively, compared with thermal equilibrium
 at 9.4 T and room temperature. The method can be used generally for signal
 enhancement and reduction of measurement time in liquid-state NMR and opens up
 for a variety of in vitro and in vivo applications of
 DNP-enhanced NMR.

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Increase in signal-to-noise ratio of > 10,000 times in liquid-state NMR

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

Many of the functions and applications of advanced materials result from their interfacial structures and properties. However, the difficulty in characterizing the surface structure of these materials at an atomic level can often slow their further development Solid-state NMR can probe surface structure and complement established surface science techniques, but its low sensitivity often limits its application. Many materials have low surface areas and/or low concentrations of active/surface sites. Dynamic nuclear polarization (DNP) is one Intriguing method to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude. In a DNP experiment, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of similar to 658 for H-1 NMR. In this Account, we discuss the application of DPIP to enhance surface NMR signals, an approach known as DNP surface enhanced NMR spectroscopy (DNP SENS). Enabling DNP for these systems requires bringing an exogeneous radical solution into contact with surfaces without diluting the sample. We proposed the incipient wetness impregnation technique (IWI) a well-known method in materials science, to impregnate porous and particulate materials with just enough radical containing solution to fill the porous volume. IWI offers several advantages: it is extremely simple, provides a uniform wetting of the surface, and does not increase the sample volume or substantially reduce the concentration of the sample. This Account describes the basic principles behind DNP SENS through results obtained for mesoporous and nanoparticulate samples impregnated with radical solutions. We also discuss the quantification of the overall sensitivity enhancements obtained with DNP SENS and compare that with ordinary room temperature NMR spectroscopy. We then review the development of radicals and solvents that give the best possible enhancements today. With the best polarizing mixtures, DNP SENS enhances sensitivity by a factor of up to 100, which decreases acquisition time by five orders of magnitude. Such enhancement enables the detailed and expedient atomic level characterization of the surfaces of complex materials at natural isotopic abundance and opens new avenues for NMR. To illustrate these improvements, we describe the successful application of DNP SENS to characterize hybrid materials, organometallic surface species, and metal-organic frameworks.

https://works.bepress.com/aaron-rossini/17/download/

Dynamic nuclear polarization surface enhanced NMR spectroscopy.

Large dynamic nuclear polarization signal enhancements (up to a factor of 100) were obtained in the solid-state magic-angle spinning nuclear magnetic resonance (NMR) spectra of arginine and the protein T4 lysozyme in frozen glycerol-water solutions with the use of dynamic nuclear polarization. Polarization was transferred from the unpaired electrons of nitroxide free radicals to nuclear spins through microwave irradiation near the electron paramagnetic resonance frequency. This approach may be a generally applicable signal enhancement scheme for the high-resolution solid-state NMR spectroscopy of biomolecules.

https://science.sciencemag.org/content/sci/276/5314/930.full.pdf

C. van der Lugt

Papers

Applications of dynamic nuclear polarization in 13C NMR in solids

13C NMR spectroscopy in diamonds using dynamic nuclear polarization

The use of parametric spectral analysis in 13C NMR in solids