IAS Seminar: Future of Solid-State DNP NMR

This event consists of two seminars:

13:00-14:00   Professor Yoh Matsuki (Institute for Protein Research, Osaka University)

Seminar Title: DNP-Enhanced MAS NMR Spectroscopy at 16.4 T and 30 K –Instrumentation and Applications–

Precise information on 3D structure of bio-macromolecules is valuable for elucidating their functions. Magic-angle sample spinning solid-state NMR (MAS SSNMR) spectroscopy is suited to study atomic-resolution structure and dynamics of insoluble and/or non-crystalline molecular systems such as membrane proteins and amyloid fibrils, but suffers from its inherently low sensitivity.

Dynamic nuclear polarization (DNP) is a method that enhances the NMR sensitivity by transferring three order of magnitude larger electron polarization to the nuclei of interest through a high-power microwave irradiation. Recently, DNP-enhanced SSNMR is becoming increasingly popular at moderate external field conditions (B₀ < 9.4 T) and temperatures (T > 90 K), but less accepted at higher field conditions. High-field condition is crucial for gaining spectral resolution and basic NMR sensitivity, but decreases the efficiency of the Cross Effect-based DNP, progressively diminishing the merit of high field DNP.

In coping with this issue for biological applications, advanced hardware development should play a key role. Here, we describe two innovations we have recently made in instrumentation for DNP-enhanced SSNMR at B₀ = 16.4 T (700MHz for ¹H, 460 GHz for electron spin resonance):

1. Closed-cycle helium-cooling MAS probe system¹. The low sample temperature (~30K) improves both the DNP efficiency and the Boltzmann nuclear polarization, recovering the sensitivity gain at high fields. This system cools the compressed spinner gases with Gifford-McMahon cryo-coolers, spins the rotor while cooling, and re-compresses the return gas, for the first time, in a completely closed gas circulation path. Thus, the system does not consume any helium to sustain stable MAS (4-12 kHz) at cryogenic temperatures (35-120 K) for an extended period of time (e.g. >2 weeks) with low running cost (~$3/hr for mostly electricity expense). The long-term stability has enabled routine use of high-dimensional spectroscopy at cryogenic temperatures, strongly promoting application of high-field DNP to very complex and/or high-molecular weight chemical/ biological systems. With the present system, we have so far obtained the sensitivity gain of over 1000 from DNP and the temperature effect combined at 30 K. 

2. Double-frequency sub-millimeter wave (SMMW) irradiation system². Many exciting possibilities will open up for high-field DNP NMR if the spectrometer is able to handle two SMMWs in different frequencies. The system involves two 460 GHz frequency-tunable and frequency-agile gyrotrons as SMMW sources, quasi-optical transmission system, universal polarizer, and a custom designed beam combiner. Recently, we have confirmed in a simple experiment the benefit of using double-gyrotron irradiation.

In the presentation, we present a brief overview on the high-field DNP techniques (methods and instruments), as well as the latest experimental DNP data we obtained with the above setups.

14:00-15:00   Dr. Hiroki Takahashi (JEOL Resonance Inc.)

Seminar Title: Potential of Dynamic Nuclear Polarization in Solid-State NMR

Solid-state NMR can analyze samples in a wide variety of states (crystalline/amorphous solids, proteins in lipid bilayers, etc.) and thus is widely used for samples ranging from inorganic/organic materials to biological molecules. However, it is not yet used to its full potential due to the inherently low sensitivity of NMR. Sensitivity enhancement, therefore, is always a central concern in NMR spectroscopy. Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of NMR through the polarization transfer from electron spins to nuclear spins. Recently, this technique has emerged as a powerful tool in solid-state NMR thanks to the development of high-power, high-frequency microwave sources, low temperature magic angle spinning (MAS) probes, efficient polarizing agents, etc.

In this talk, recent applications of solid-state NMR enhanced by DNP under MAS conditions (MAS-DNP) will be reviewed. They include optimization of an absolute sensitivity ratio¹, rapid natural-abundance ¹³C–¹³C correlation experiments^2,3, and applications to bacterial cells⁴ and mesoporous alumina⁵ as well as the work done by other groups. The collaboration work on MAS-DNP between JEOL RESONANCE and Osaka University will also be briefly presented.

*If you're not an OIST member, please contact Kazuo Yamauchi(kazuo.yamauchi@oist.jp) for registration.*