Dynamic Nuclear Polarization (DNP) NMR

Introduction to DNP-NMR

Dynamic Nuclear Polarization (DNP) is a technique used to enhance the sensitivity of NMR experiments. It involves the transfer of polarization from highly polarized electron spins to the nuclear spins in a sample, resulting in a significant increase in NMR signal intensity.

The basic principle behind DNP is the utilization of unpaired electron spins, typically found in stable radicals or paramagnetic species, to transfer their polarization to nearby nuclei. This process occurs through microwave irradiation, in combination with a strong magnetic field and low temperatures.

The sample of interest is typically prepared by incorporating a radical and sample into a glassy solvent matrix. The radical’s unpaired electron is the source of polarization, 660× more polarized than ¹H nuclei at a given magnetic field and temperature. For DNP, the sample is typically cooled around 100 K in the NMR spectrometer. The low temperature boosts both the source polarization, and suppresses thermal spin relaxation processes, to preserve polarization as it is transferred to various nuclei of biomolecular and pharmaceutical interest (¹H, ¹³C, ¹⁵N, ³¹P, ¹⁹F, etc.), as well as many others common in materials applications.

Microwaves are then applied to the sample at a specific frequency (e.g. 263 GHz for a 400 MHz NMR spectrometer). This causes transitions between coupled electron-nuclear spin states and yields an accumulation of nuclear spin polarization. This is the fundamental process of DNP, which thus may be thought of as a coupled NMR / EPR (electron paramagnetic resonance) experiment. Various mechanisms are available to achieve the electron-to-nuclear polarization transfers, including the Overhauser Effect (OE), the Solid Effect (SE), or the Cross Effect (CE), depending on specifics of the electron spins available and on fine tuning of the magnetic field or the microwave frequency.

The result is NMR signal intensity up to 200-fold larger than in experiments at the same temperature but without DNP (even more than 200× in favorable cases, with an ideal of 660×). For sensitivity-limited applications, such signal gains correspond to an astounding 200² = 40,000-fold time savings compared to signal-averaging approaches.

Such dramatic gains enable investigation of previously inaccessible samples, ranging from in-cell elucidation of molecular interactions, to localization of drug components in complex nanoscale delivery systems, to the interrogation of buried interfaces in cutting edge battery materials.

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A schematic diagram of a Bruker DNP system with a gyrotron microwave source is shown in Figure 1, and with a klystron microwave source in Figure 2. A photo of an actual installation is shown in Figure 3.

Bruker‘s DNP-NMR Spectrometers

Bruker’s DNP spectrometers consist of the latest generation AVANCE NMR console and NMR magnet, a microwave source, a special DNP probe and a low-temperature MAS (LT-MAS) cooling system.

• The NMR magnet can be equipped with a sweep coil, enabling the user to change the B0 field within the range required to tune the DNP matching condition to the microwave frequency.
• For microwaves, Bruker provides gyrotron sources for 400 – 900 MHz NMR as shown in Figure 1; or, a klystron option at 400 MHz only, as shown in Figure 2. In all cases, a low-loss transmission line delivers microwaves to the base of the NMR probe.
• Bruker’s DNP NMR probes are equipped with a waveguide to transmit the microwaves to the NMR sample and are further optimized near the sample for high-efficiency microwave coupling, all leading to optimal polarization and very limited sample heating.
• Together, the probes and Bruker’s LT-MAS Control Cabinet provide extremely stable LT-MAS operation, with fully automated spin control, convenient sample insert/eject while the probe is in its cold state, and remote user access through Bruker’s excellent Topspin spectrometer control software.

Bruker‘s Wide-Bore DNP Probes

The upper region of a Bruker DNP-NMR probe for wide-bore (WM, 89 mm) magents is shown in Figure 4 and Figure 5. Efficient coupling of microwaves to the sample starts with the mirrors and the launcher adjacent to the MAS stator in which the rotor spins, which likewise facilitate DNP by management of beam focus and reflection. The innovative rotor insert/eject mechanism is also shown at top left in Figure 4. These optimizations are especially critical in probes for small rotor diameters, where a special design carefully matches the beam size to the rotor cross section, leading to the best DNP enhancements available in any commercial system. Additionally, DNP-NMR probes are capable of reaching and maintaining low temperatures typically at 95 – 100 K during the experiment, and within 5 - 10 K of that range even in the most-challenging cases of fastest spinning and full microwave power.

The range of LT-MAS DNP probes offered by Bruker for WB systems is tailored to needs across bio- and small-molecule and material applications. Corresponding rotor diameters and low-temperature spinning speeds are shown in the table below. The spinning provides good resolution, in spite of the natural reduction in spinning speeds for low-temperature compared to room-temperature MAS.

Temperature control of the gas streams is provided by Bruker’s LT-MAS cabinet, providing three cryogenically cooled gas channels for MAS (1) drive, (2) bearing and (3) VT (variable temperature). Bruker’s LT-MAS cabinet couples seamlessly with Bruker’s field-leading MAS control unit, for the full operational convenience and reliability of standard solids NMR. The controls of the LT-MAS cabinet are also fully integrated into the TopSpin NMR software, enabling remote access and other comfort features.

The NMR probes utilized in DNP-NMR systems are typically triple-resonance resonance (H/X/Y) in WB systems, where a variety of X/Y configurations may be set by the user to address broad ranges of nuclei. Special non-standard probe configurations are available on request, e.g. with the proton channel tunable to fluorine (i.e., H-F/X/Y), to address modern applications with ¹⁹F in pharma, bio and material compounds.  As another example for a special configuration, options may be applied to H/X (e.g., as H-F/X) probes, where the double resonance probe has an X channel targeted to low-gamma nuclei (i.e., with NMR frequency below that of ¹⁵N), a powerful tool in materials research.  Bruker also offers a variety of options on request for static (non-spinning) applications, which have been applied to research including aligned biomolecular crystals, polymers and battery materials.

Microwave Sources

Bruker microwave sources are a key enabling technology for DNP.  These provide exceptional frequency and power stability, as needed for reproducible DNP enhancements. The NMR and microwave frequencies are linked by the respective gyromagnetic ratios of proton and electron spins. The table below shows the correspondence across the Bruker DNP-NMR product line for gyrotrons (400 – 900 MHz) and the Klystron (263 GHz / 400 MHz only).

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The klystron is a versatile, reliable, and cost-effective option at 400 MHz. As illustrated in Figure 2, it has a much-smaller footprint than gyrotrons. A gyrotron requires its own superconducting magnet, through which acceleration of a vacuum electron beam provides microwave output whose frequency is a multiple of the cyclotron resonance of the electrons in the beam.

The klystron also utilizes a vacuum electron beam, but in contrast, produces microwaves via interaction of the beam with a ladder of cavities whose rungs are spaced at a set fraction of the wavelength of the desired microwave output. This fine internal structure limits the klystron’s power output to near 5 W at 263 GHz and, in practice, prevents its application to higher frequencies. Nonetheless, for 400 MHz NMR, DNP enhancements at 5 W power achieve on the order of > 70 % of the value accessible using full power from a gyrotron (> 10 W), and an even greater fraction for probes with rotor diameters < 3.2 mm. Figure 8 shows a Bruker 263 GHz klystron, compactly mounted to the underside of a magnet for 400 MHz NMR.

As noted, Bruker gyrotrons from 400 – 900 MHz offer the full power required for maximum DNP benefits. The noted reliance on electron / magnetic field interactions (cyclotron resonance) is readily applied to the full frequency range (263 – 593 GHz) of our product line. This requires the magnetic fields of the gyrotron magnets (4.5 – 10.9 T) rising linearly with frequency. Importantly, the gyrotron fields are approximately half the target NMR field. This is due to a cutting edge gyrotron design that utilizes 2^nd-harmonic microwave generation at roughly twice the cyclotron frequency. This enables much more compact magnets than otherwise required. Bruker’s worldclass cryogen-free gyrotron magnet designs employ a pulsed-tube cooler with a generous 8-year replacement interval, for worry-free operation and freedom from liquid cryogens.

Polarizing Agents for DNP-NMR

Just as Bruker continually improves and expands the instrument offering for DNP, the user community has also busily worked on radical chemistry to increase the gains available from polarizing agents.  Radicals provide the electron spin that interacts with the microwave and magnetic fields to transfer its strong spin polarization into the otherwise weakly polarized nuclear spins, thus enabling ultrasensitive NMR. However, the quality of that DNP process varies with both the chemical structure of the radicals and environmental conditions.

Early DNP methods relied on molecules with a single radical electron, suitable for OE or SE DNP. Later, stable and commercially available bi-radicals came into wide use for both aqueous (AMUpol) and organic (TEKpol) applications. The two spatially separated, but magnetically coupled electrons in a bi-radical molecule enabled a newer DNP mechanism, the Cross effect (CE), that led to greater sensitivity gains in many applications. Bi-nitroxides like AMUpol and TEKpol gave stunning enhancements for DNP at 400 MHz, but as shown in Figure 6, their performance tapers off for DNP with 600, 800 and 900 MHz NMR.

Chemical innovations led to a generation of radicals, including ‘mixed’ bi-radicals, like HyTEK2, cAsymPolPOK and TEMPtriPOL, that include one each of radicals with narrow and broad EPR spectra. These greatly improved DNP gains for 600 – 900 MHz NMR (Figure 9) and promise continued top performance for future DNP applications at higher fields.

Quality Assessments for Radical & Sample

All DNP labs can benefit from Bruker’s convenient and compact ESR5000 benchtop EPR spectrometer. This is an excellent companion for a DNP-NMR system, especially for the quantitative assessment of radical concentrations. The ‘SpinCount’ method is an automated procedure on the ESR5000 and is rapidly predictive of DNP performance after sample preparation. It is also valuable to assess radical degradation in older samples, or when used in reducing environments (e.g., in-cell DNP). SpinCount is applicable to both material and solvated samples (aqueous or organic), for either stock solutions or direct assessment of the final form of your sample: a packed and sealed LT-MAS rotor. Details are described here.

The ESR5000 is also a fully functional X-band spectrometer, replete with analysis and simulation tools to complement advanced spectral acquisition. Assessment of electron spin relaxation, inter-electron dipolar couplings, and power-dependent saturation of EPR transitions are all advanced predictors of DNP performance. Furthermore, low-temperature accessories enable measurements from 77 K to room temperature, covering the vast majority of conditions for DNP NMR.

Any DNP lab, from standard users to advanced developers of polarizing agents, can greatly benefit from the versatility, reliability and convenience of the ESR5000.

High Resolution and DNP Gains Enable Cutting-Edge Biomolecular Applications

Both advanced DNP equipment and newer polarizing agents are enabling cutting-edge applications, as in this case with Aβ_1-42, a protein fragment implicated in the onset of Alzheimer’s disease. The DNP-enhanced ¹³C-¹³C correlation spectrum reveals structural interactions within the protein molecule.

The upper right inset emphasizes the 22 fold DNP gain, without which the few hours needed to collect this 2D result would expand to a 500× longer experiment (22²), a wholy impractical duration. Note the exceptional resolution (highlighted in the central inset), in spite of many challenging factors, including a fully protonated protein.

DNP and Fast MAS Enable Exquisite Sensitivity and Resolution

Even beyond DNP, fast MAS (e.g., > 60 kHz spinning rate) is known for providing the highest spectral resolution in solids NMR and as the powerful enabler of ¹H-detected spectra. The latter offers substantial sensitivity gains over ¹³C.  Figure 12 shows an example with a DNP gain of 200 over the benefits already provided by ¹H detection. This allowed a sophisticated interrogation of 2D intranuclear correlations in spite of mere natural-abundance (1.1%) levels of ¹³C in the sample. This shows how DNP can be used for broad classes of experiments where isotropic enrichment is not practical, as in study of many pharmaceutical compounds and natural extracts.  

The example also demonstrates the key value of HyTEK2 as a polarizing agent with exceptional DNP performance at high field. Here, the result was obtained at 900 MHz ¹H frequency, currently the highest-field DNP system installed worldwide.

DNP and Materials Science

Buried interfaces, nanoscopic sub-structures and dilute species are typical targets of experiments to unlock mysterious origins of either function or failure in modern materials, not to mention uncovering improvements needed for next-generation products. DNP NMR opens a unique window by combining the physico-chemical resolution of solids NMR with needed sensitivity gains from DNP to reveal minute internal features, typically otherwise lost amidst a background of bulk material.

A key example is in the study of batteries. For example, dendritic structures can significantly impair the performance of a battery. Li-metal DNP NMR helps to better understand dendrite formation. Figure 13 is an example of Li-metal DNP spectra with and without DNP, achieving 60-fold enhancement for detection of ⁷Li NMR. This enables sophisticated NMR of various components of the solid-electrolyte interface between anode and cathode. DNP-enhanced Li signals give a great direct view of dendrites and other features in batteries. In addition, multinuclear experiments in which Li spin polarization is transferred to other nuclei (e.g., ¹H, ¹³C and ¹⁹F) have been used to light up small-molecule additives in the solid-electrolyte interface that can be used to prevent dendrite formation.