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1. Carbon-13 dynamic nuclear polarization in diamond via a microwave-free integrated cross effect

   https://doi.org/10.1073/pnas.1908780116  

   Henshaw, Jacob ; Pagliero, Daniela ; Zangara, Pablo R. ; Franzoni, María B. ; Ajoy, Ashok ; Acosta, Rodolfo H. ; Reimer, Jeffrey A. ; Pines, Alexander ; Meriles, Carlos A. ( September 2019 , Proceedings of the National Academy of Sciences)

   Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13 C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13 C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with themore »diamond crystal.« less
2. Enhanced dynamic nuclear polarization via swept microwave frequency combs

   https://doi.org/10.1073/pnas.1807125115  

   Ajoy, A. ; Nazaryan, R. ; Liu, K. ; Lv, X. ; Safvati, B. ; Wang, G. ; Druga, E. ; Reimer, J. A. ; Suter, D. ; Ramanathan, C. ; et al ( October 2018 , Proceedings of the National Academy of Sciences)

   Dynamic nuclear polarization (DNP) has enabled enormous gains in magnetic resonance signals and led to vastly accelerated NMR/MRI imaging and spectroscopy. Unlike conventionalcw-techniques, DNP methods that exploit the full electron spectrum are appealing since they allow direct participation of all electrons in the hyperpolarization process. Such methods typically entail sweeps of microwave radiation over the broad electron linewidth to excite DNP but are often inefficient because the sweeps, constrained by adiabaticity requirements, are slow. In this paper, we develop a technique to overcome the DNP bottlenecks set by the slow sweeps, using a swept microwave frequency comb that increases the effective number of polarization transfer events while respecting adiabaticity constraints. This allows a multiplicative gain in DNP enhancement, scaling with the number of comb frequencies and limited only by the hyperfine-mediated electron linewidth. We demonstrate the technique for the optical hyperpolarization of¹³C nuclei in powdered microdiamonds at low fields, increasing the DNP enhancement from 30 to 100 measured with respect to the thermal signal at 7T. For low concentrations of broad linewidth electron radicals [e.g., TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)], these multiplicative gains could exceed an order of magnitude.
3. Zero-field nuclear magnetic resonance of chemically exchanging systems

   https://doi.org/10.1038/s41467-019-10787-9  

   Barskiy, Danila A. ; Tayler, Michael C. D. ; Marco-Rius, Irene ; Kurhanewicz, John ; Vigneron, Daniel B. ; Cikrikci, Sevil ; Aydogdu, Ayca ; Reh, Moritz ; Pravdivtsev, Andrey N. ; Hövener, Jan-Bernd ; et al ( July 2019 , Nature Communications)

   Zero- to ultralow-field (ZULF) nuclear magnetic resonance (NMR) is an emerging tool for precision chemical analysis. In this work, we study dynamic processes and investigate the influence of chemical exchange on ZULF NMRJ-spectra. We develop a computational approach that allows quantitative calculation ofJ-spectra in the presence of chemical exchange and apply it to study aqueous solutions of [¹⁵N]ammonium (¹⁵N$${\mathrm{H}}_4^ +$$$H_4^{+}$) as a model system. We show that pH-dependent chemical exchange substantially affects theJ-spectra and, in some cases, can lead to degradation and complete disappearance of the spectral features. To demonstrate potential applications of ZULF NMR for chemistry and biomedicine, we show a ZULF NMR spectrum of [2-¹³C]pyruvic acid hyperpolarized via dissolution dynamic nuclear polarization (dDNP). We foresee applications of affordable and scalable ZULF NMR coupled with hyperpolarization to study chemical exchange phenomena in vivo and in situations where high-field NMR detection is not possible to implement.
4. PHIP hyperpolarized [1-13C]pyruvate and [1-13C]acetate esters via PH-INEPT polarization transfer monitored by 13C NMR and MRI

   https://doi.org/10.1038/s41598-021-85136-2  

   Svyatova, Alexandra ; Kozinenko, Vitaly P. ; Chukanov, Nikita V. ; Burueva, Dudari B. ; Chekmenev, Eduard Y. ; Chen, Yu-Wen ; Hwang, Dennis W. ; Kovtunov, Kirill V. ; Koptyug, Igor V. ( March 2021 , Scientific Reports)

   Parahydrogen-induced polarization of¹³C nuclei by side-arm hydrogenation (PHIP-SAH) for [1-¹³C]acetate and [1-¹³C]pyruvate esters with application of PH-INEPT-type pulse sequences for¹H to¹³C polarization transfer is reported, and its efficiency is compared with that of polarization transfer based on magnetic field cycling (MFC). The pulse-sequence transfer approach may have its merits in some applications because the entire hyperpolarization procedure is implemented directly in an NMR or MRI instrument, whereas MFC requires a controlled field variation at low magnetic fields. Optimization of the PH-INEPT-type transfer sequences resulted in¹³C polarization values of 0.66 ± 0.04% and 0.19 ± 0.02% for allyl [1-¹³C]pyruvate and ethyl [1-¹³C]acetate, respectively, which is lower than the corresponding polarization levels obtained with MFC for¹H to¹³C polarization transfer (3.95 ± 0.05% and 0.65 ± 0.05% for allyl [1-¹³C]pyruvate and ethyl [1-¹³C]acetate, respectively). Nevertheless, a significant¹³C NMR signal enhancement with respect to thermal polarization allowed us to perform¹³C MR imaging of both biologically relevant hyperpolarized molecules which can be used to produce useful contrast agents for the in vivo imaging applications.
5. High field para hydrogen induced polarization of succinate and phospholactate

   https://doi.org/10.1039/d0cp06281b  

   Berner, Stephan ; Schmidt, Andreas B. ; Ellermann, Frowin ; Korchak, Sergey ; Chekmenev, Eduard Y. ; Glöggler, Stefan ; von Elverfeldt, Dominik ; Hennig, Jürgen ; Hövener, Jan-Bernd ( January 2021 , Physical Chemistry Chemical Physics)

   The signal enhancement provided by the hyperpolarization of nuclear spins of metabolites is a promising technique for diagnostic magnetic resonance imaging (MRI). To date, most 13 C-contrast agents are hyperpolarized utilizing a complex or cost-intensive polarizer. Recently, the in situ para hydrogen-induced 13 C hyperpolarization was demonstrated. Hydrogenation, spin order transfer (SOT) by a pulsed NMR sequence, in vivo administration, and detection was achieved within the magnet bore of a 7 Tesla MRI system. So far, the hyperpolarization of the xenobiotic molecule 1- 13 C-hydroxyethylpropionate (HEP) and the biomolecule 1- 13 C-succinate (SUC) through the PH-INEPT+ sequence and a SOT scheme proposed by Goldman et al. , respectively, was shown. Here, we investigate further the hyperpolarization of SUC at 7 Tesla and study the performance of two additional SOT sequences. Moreover, we present first results of the hyperpolarization at high magnetic field of 1- 13 C-phospholactate (PLAC), a derivate to obtain the metabolite lactate, employing the PH-INEPT+ sequence. For SUC and PLAC, 13 C polarizations of about 1–2% were achieved within seconds and with minimal equipment. Effects that potentially may explain loss of 13 C polarization have been identified, i.e. low hydrogenation yield, fast T 1 / T 2 relaxationmore »and the rarely considered 13 C isotope labeling effect.« less