What is Dynamic Nuclear Polarization (DNP-NMR)?

Researchers can use dynamic nuclear polarization to boost signal intensities in nuclear magnetic resonance (NMR) experiments, a technique known as DNP-NMR. 

NMR spectroscopy is an important technique for the determination of structures in material science and structural biology. However, its major drawback is the inherent low sensitivity of the method due to the small magnetic moment of the nuclei under study (1H, 13C, 15N etc.). This directly results in a small polarization of the nuclear spin reservoir (difference of number of spins aligned parallel or anti-parallel to the external magnetic field). Therefore the signal intensities in NMR experiments are small and extensive signal averaging is required to obtain a sufficiently high signal-to-noise ratio.

Dynamic Nuclear Polarization

Figure 1: Temperature dependence of the electron and nuclear spin reservoir polarization at a given external field strength of 14 T, corresponding to a 1H nuclear Larmor frequency of 600 MHz.

Electron spin reservoir has much larger polarization

In contrast, at the same magnetic field and temperature, the polarization of the electron spin reservoir (e.g. free, stable paramagnetic polarizing agent) is significantly larger due to the much higher magnetic moment of the electron spin. This behavior is shown in Figure 1. For example, at a temperature of 90 K the polarization of the proton (1H) spin reservoir is 0.016%, while it is 10.541% for the electron spin reservoir, a factor 660 times larger. This difference in polarization is given by the ratio of the electron to proton gyromagnetic ratio. A large list of gyromagnetic ratios can be found on the web page of the National Institute of Standards and Technology.

DNP leverages polarization to enhance signal intensities for NMR

DNP-NMR makes it possible to transfer this large Boltzman polarization of the electron spin reservoir to the nuclear spin reservoir to provide a boost in NMR signal intensities by several orders of magnitude; thus increasing the signal intensity and data acquisition rate in a NMR experiment dramatically.

This is no new scientific area. First DNP-NMR experiments were performed in the early 1950s at low magnetic fields (1) but until recently the technique was of limited applicability because of the lack of high-frequency, high-power terahertz sources. Briefly, in DNP-NMR spectroscopy, the large electron polarization of a polarizing agent is transferred to surrounding nuclei (typically protons, 1H) by terahertz (microwave) irradiation near or at the electron paramagnetic resonance (EPR) transition. The electron spin system (polarizing agent) required for DNP spectroscopy can either be an endogenous or exogenous paramagnetic system.

For liquid-state NMR the only DNP mechanism currently known is the Overhauser Effect (2) while for solid-state different DNP mechanisms can employed such as the solid-effect, thermal-mixing or the cross-effect (3,4).

Dynamic Nuclear Polarization Process

Figure 2: Schematic representation of the DNP process. Without DNP the sample is in its thermal equilibrium state. The DNP process is initiated and driven by microwave irradiation of the sample. During this process the large thermal polarization of the electrons spin reservoir is transferred to the nuclear spin reservoir.

At high magnetic fields, the cross-effect is the mechanism that yields the largest signal enhancements in dynamic nuclear polarization solid-state NMR experiments. The cross-effect can be exploited, if the homogenous linewidth (δ) and the inhomogeneous breadth (Δ) of the EPR spectrum of the paramagnetic polarizing agent, is larger compared to nuclear Larmor frequency  (ω0I). The underlying mechanism is a two-step process involving two electrons with Larmor frequencies ω0S1 and ω0S2 separated by the ω0I (matching condition) [4,5]. The DNP-enhanced nuclear polarization then disperses throughout the bulk via spin diffusion (5). To date most polarizing agents for high-field DNP experiments are based on TEMPO moieties, which employ the cross-effect (CE) as the DNP mechanism. No sample-shuttling is necessary, which makes in-situ DNP a straightforward approach to combine with solid-state NMR spectroscopy. For high-field, DNP-enhanced solid-state NMR spectroscopy biradicals such as TOTAPOL or bTbk are very efficient (6,7). For liquid-state DNP experiments TEMPO can be simple added to the solution.

Further Reading 

References

  1. Abragam A, Goldman M. Principles of Dynamic Nuclear Polarization. Rep. on Prog. Phys. 1978;41(3):395-467.
  2. Overhauser A. W. Polarization of Nuclei in Metals. Phys. Rev. 1953;92(2):411-415.
  3. Maly T, Debelouchina G. T, Bajaj V. S, Hu K.-N, Joo C.-G, Mak-Jurkauskas M. L et al. Dynamic Nuclear Polarization at High Magnetic Fields. J. Chem. Phys 2008;128(5):052211-19.
  4. Barnes A. B, De Paepe G, Van der Wel P. C, Hu K. N, Joo C. G, Bajaj V. S et al. High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR. Apl. Magn. Reson. 2008;34(3):237-263.
  5. Bloembergen N. On the Interaction of Nuclear Spins in a Crystalline Latice. Physica 1949;15(3-4):386-426.
  6. Song C, Hu K, Joo C, Swager T, Griffin R. TOTAPOL: A Biradical Polarizing Agent for Dynamic Nuclear Polarization Experiments in Aqueous Media. JACS 2006;128(35):11385-11390.
  7. Matsuki Y, Maly T, Ouari O, Karoui H, Moigne Le F, Rizzato E et al. Dynamic Nuclear Polarization with a Rigid Biradical. Angew. Chem. Int. Ed. 2009;48:4996-5000.