Mechanisms of dynamic nuclear polarization in insulating solids

Abstract

Dynamic nuclear polarization (DNP) is a technique used to enhance signal intensities in NMR experiments by transferring the high polarization of electrons to their surrounding nuclei. The past decade has witnessed a renaissance in the development of DNP, especially at high magnetic fields, and its application in several areas including biophysics, chemistry, structural biology and materials science. Recent technical and theoretical advances have expanded our understanding of established experiments: for example, the cross effect DNP in samples spinning at the magic angle. Furthermore, new experiments suggest that our understanding of the Overhauser effect and its applicability to insulating solids needs to be re-examined. In this article, we summarize important results of the past few years and provide quantum mechanical explanations underlying these results. We also discuss future directions of DNP and current limitations, including the problem of resolution in protein spectra recorded at 80–100 K.

High resolution observed in 800 MHz DNP spectra of extremely rigid type III secretion needles

“…In a previous study conducted at 600 MHz (Fricke et al 2014), this system exhibited one of the best resolutions ever achieved for a protein sample under DNP conditions (Can et al 2015;Su et al 2015). Increasing the field strength to 800 MHz and using the biradical AMUPol (Sauvee et al 2013), the observed line-widths at half height are about 22 % smaller, improving the overall resolution of the spectrum sizably.…”

Line-Broadening in Low-Temperature Solid-State NMR Spectra of Fibrils

“…With the application of dynamic nuclear polarization (DNP), one could even gain further in the signal-tonoise ratio, up to a factor of 660. (Can et al 2015) Unfortunately, starting at the freezing temperature of the solvent water, one typically observes a significant broadening of the protein resonances (Linden et al 2011;Siemer et al 2012). This effect compromises most benefits of an increased signal strength.…”

Time domain DNP with the NOVEL sequence

“…As the magnetic field increases, the efficiency of the SE decreases rapidly as predicted by theory and also observed in experiments. 4,17,19,53 Pulsed DNP, in general, and the NOVEL experiment, in particular, do not depend on the B 0 . An analogy is that cross polarization is operating at all magnetic fields in solid state NMR.…”

“…14,[17][18][19] Single crystals were harvested and polished to fit into a quartz capillary of 0.4 mm inner diameter for experiments at 5 T. For experiments at 0.35 T, a larger sample that fit into a 4 mm EPR sample tube was used. In order to suppress the 1 H NMR signal from trapped solvent, we used perdeuterated ethanol (Cambridge Isotope).…”

“…12 Initially, the primary application of DNP was preparation of polarized targets for neutron scattering experiments; 13 however, over the past decade, DNP has been used extensively to enhance the inherently low sensitivity of nuclear magnetic resonance (NMR) signals. [14][15][16][17][18][19] Enhancements on the order of 10 2 − 10 3 were made possible via the solid effect using narrow-line radicals 20-23 and cross effect using biradicals 24-27 as polarizing agents and high frequency gyrotrons as microwave sources. [28][29][30] The latter operate in the 140-560 GHz regime and enable DNP to be performed at magnet field strengths used in contemporary NMR experiments (5-20 T).…”