Category Archives: cross-polarization

Maximizing nuclear hyperpolarization in pulse cooling under MAS #DNPNMR

Björgvinsdóttir, Snædís, Brennan J. Walder, Nicolas Matthey, and Lyndon Emsley. “Maximizing Nuclear Hyperpolarization in Pulse Cooling under MAS.” Journal of Magnetic Resonance 300 (March 2019): 142–48.

It has recently been shown how dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free solids. This is achieved by generating the polarization in a wetting phase, transferring it to nuclei near the surface and relaying it towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. Pulse cooling is a strategy to achieve this that uses a multiple contact cross-polarization sequence for bulk hyperpolarization. Here, we show how to maximize sensitivity using the pulse cooling method by experimentally optimizing pulse parameters and delays on a sample of powdered SnO2. To maximize sensitivity we introduce an approach where the magic angle spinning rate is modulated during the experiment: the CP contacts are carried out at a slow spin rate to benefit from faster spin diffusion, and the spin rate is then accelerated before detection to improve line narrowing. This method can improve the sensitivity of pulse cooling for 119Sn spectra of SnO2 by an additional factor of 3.5.

13C → 1H transfer of light-induced hyperpolarization allows for selective detection of protons in frozen photosynthetic reaction center

Bielytskyi, Pavlo, Daniel Gräsing, Kaustubh R. Mote, Karthick Babu Sai Sankar Gupta, Shimon Vega, P.K. Madhu, A. Alia, and Jörg Matysik. “13C → 1H Transfer of Light-Induced Hyperpolarization Allows for Selective Detection of Protons in Frozen Photosynthetic Reaction Center.” Journal of Magnetic Resonance 293 (August 2018): 82–91.

In the present study, we exploit the light-induced hyperpolarization occurring on 13C nuclei due to the solid-state photochemically induced dynamic nuclear polarization (photoCIDNP) effect to boost the NMR signal intensity of selected protons via inverse crosspolarization. Such hyperpolarization transfer is implemented into 1H-detected twodimensional 13C-1H correlation magic-angle-spinning (MAS) NMR experiment to study protons in frozen photosynthetic reaction centers (RCs). As a first trial, the performance of such an experiment is tested on selectively 13C labeled RCs from the purple bacteria of Rhodobacter sphaeroides. We observed response from the protons belonging to the photochemically active cofactors in their native protein environment. Such an approach is a potential heteronuclear spin-torch experiment which could be complementary to the classical heteronuclear correlation (HETCOR) experiments for mapping proton chemical shifts of photosynthetic cofactors and to understand the role of the proton pool around the electron donors in the electron transfer process occurring during photosynthesis.

Hyperpolarization of nitrogen-15 nuclei by cross polarization and dissolution dynamic nuclear polarization

Milani, J., et al., Hyperpolarization of nitrogen-15 nuclei by cross polarization and dissolution dynamic nuclear polarization. Review of Scientific Instruments, 2017. 88(1): p. 015109.

Dynamic Nuclear Polarization (DNP) is often achieved by the direct transfer of polarization from electrons to nuclei such as 13C, induced by microwavesaturation of the wings of narrow EPR lines of radicals like trityl. In the indirectapproach on the other hand, DNP is used to transfer the polarization from the electrons of radicals such as nitroxides that have broad EPR lines to nuclear spins I = 1H, followed by cross-polarization (CP) from I = 1H to S = 13C or other nuclei with low gyromagnetic ratios. This approach is particularly attractive for S = 15N, since direct DNP yields modest polarizations P(15N) < 4% with build-up times that can be as long as τDNP(15N) > 2 h. In this paper, we show that CP from 1H to 15N at 1.2 K can yield P(15N) = 25% with τCP-DNP(15N) = 10–15 min. After rapid dissolution and transfer to a solution-state NMR spectrometer, a polarizationP(15N) = 20% was observed at 300 K. The longitudinal relaxation times in solution can be as long as T1(15N) > 800 s in favorable cases.

Cross Polarization for Dissolution Dynamic Nuclear Polarization

Batel, M., et al., Cross Polarization for Dissolution Dynamic Nuclear Polarization. Phys. Chem. Chem. Phys., 2014.

Dynamic nuclear polarization (DNP) in combination with subsequent dissolution of the sample allows the detection of low-???? nuclei in solution state with a signal gain of tens of thousands compared to experiments starting from Boltzmann conditions. The long polarization build-up times of typically more than one hour are a drawback of this technique. The combination of dissolution DNP with crosspolarization (CP) in the solid state was shown to have the potential to overcome this disadvantage. In this article we discuss the cross-polarization step under dissolution DNP conditions in more detail. We show that adiabatic half-passage pulses allow us to enhance the CP efficiency in power-limited DNP probes. As a low-power alternative to Hartmann-Hahn CP we also demonstrate the applicability of frequency-swept de- and re-magnetization pulses for polarization transfer via dipolar order. We investigate implications and restrictions of the common solid-state DNP mechanisms to the DNP-CP technique and apply a spin-thermodynamic model based on the thermal-mixing mechanism. The model allows us to investigate the dynamics of the polarization levels in a system with two nuclear Zeeman reservoirs and explains the enhanced DNP efficiency upon solvent deuteration within a spinthermodynamic picture.

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