Friday, September 28, 2018

Boosting sensitivity and suppressing artifacts via multi-acquisition in direct polarization NMR experiments with small flip-angle pulses

This is not directly related to DNP spectroscopy but an interesting method to increase the sensitivity specifically in solid-state NMR experiments. This method is compatible with DNP and in combination can lead to even larger sensitivity gains than just DNP.


Fu, Riqiang, and Arturo J. Hernández-Maldonado. “Boosting Sensitivity and Suppressing Artifacts via Multi-Acquisition in Direct Polarization NMR Experiments with Small Flip-Angle Pulses.” Journal of Magnetic Resonance 293 (August 1, 2018): 34–40.


A small flip-angle pulse direct polarization is the simplest method commonly used to quantify various compositions in many materials applications. This method sacrifices the sensitivity per scan in exchange for rapid repeating of data acquisition for signal accumulation. In addition, the resulting spectrum often encounters artifacts from background signals from probe components and/or from acoustic rings leading to a distorted baseline, especially in low-γ nuclei and wideline NMR. In this work, a multi-acquisition scheme is proposed to boost the sensitivity per scan and at the same time effectively suppress these artifacts. Here, an adiabatic inversion pulse is first applied in order to bring the magnetization from the +z to −z axis and then a small flip-angle pulse excitation is used before the data acquisition. Right after the first acquisition, the adiabatic inversion pulse is applied again to flip the magnetization back to the +z axis. The second data acquisition takes place after another small flip-angle pulse excitation. The difference between the two consecutive acquisitions cancels out any artifacts, while the wanted signals are accumulated. This acquisition process can be repeated many times before going into next scan. Therefore, by acquiring the signals multiple times in a single scan the sensitivity is improved. A mixture sample of flufenamic acid and 3,5-difluorobenzoic acid and a titanium silicate sample have been used to demonstrate the advantages of this newly proposed method.

Wednesday, September 26, 2018

Continuous wave electron paramagnetic resonance of nitroxide biradicals in fluid solution #DNPNMR

Biradicals are very important polarizing agents used in DNP-enhanced NMR spectroscopy. Specifically in solid-state experiments they often out-perform monoradicals. Understanding the influence of all the different interactions present in a biradical is still ongoing research and only by using liquid and solid state EPR spectroscopy is it possible to characterize, understand and finally optimize polarizing agents for DNP.


Eaton, Sandra S., Lukas B. Woodcock, and Gareth R. Eaton. “Continuous Wave Electron Paramagnetic Resonance of Nitroxide Biradicals in Fluid Solution.” Concepts in Magnetic Resonance Part A, May 25, 2018, e21426.


Nitroxide biradicals have been prepared with electron-electron spin-spin exchange interaction, J, ranging from weak to very strong. EPR spectra of these biradicals in fluid solution depend on the ratio of J to the nitrogen hyperfine coupling, AN, and the rates of interconversion between conformations with different values of J. For relatively rigid biradicals EPR spectra can be simulated as the superposition of AB splitting patterns arising from different combinations of nitrogen nuclear spin states. For more flexible biradicals spectra can be simulated with a Liouville representation of the dynamics that interconvert conformations with different values of J on the EPR timescale. Analysis of spectra, factors that impact J, and examples of applications to chemical and biophysical problems are discussed.

Monday, September 24, 2018

Electronic control of DNA-based nanoswitches and nanodevices #DNPNMR

Ranallo, Simona, Alessia Amodio, Andrea Idili, Alessandro Porchetta, and Francesco Ricci. “Electronic Control of DNA-Based Nanoswitches and Nanodevices.” Chem. Sci. 7, no. 1 (2016): 66–71.


The exchange (J) interaction of organic biradicals is a crucial factor controlling their physiochemical properties and potential applications, and can be modulated by changing the nature of the linker. In the present work, we for the first time demonstrate the effect of chiral configurations of radical parts on the J values of trityl-nitroxide (TN) biradicals. Four diastereoisomers (TNT1, TNT2, TNL1 and TNL2) of TN biradicals were synthesized and purified by conjugation of a racemic (R/S) nitroxide with the racemic (M/P) trityl radical via L-proline. The absolute configurations of these diastereoisomers were assigned by comparing experimental and calculated electronic circular dichroism (ECD) spectra as (M, S, S) for TNT1, (P, S, S) for TNT2, (M, S, R) for TNL1 and (P, S, R) for TNL2. Electron paramagnetic resonance (EPR) results showed that the configuration of the nitroxide part instead of the trityl part is dominant in controlling the exchange interactions and the order of the J values at room temperature is TNT1 (252 G) > TNT2 (127 G) >> TNL2 (33 G) > TNL1 (14 G). Moreover, the J values of TNL1/ TNL2 with the S configuration in the nitroxide part vary with temperature and polarity of solvents due to their flexible linker, whereas the J values of TNT1/ TNT2 are almost insensitive to these two factors due to the rigidity of their linkers. The distinct exchange interactions between TNT1,2 and TNL1,2 in frozen state led to strongly different high-field dynamic nuclear polarization (DNP) enhancements with ε = 7 for TNT1,2 and 40 for TNL1,2 at 800 MHz DNP conditions.

Friday, September 21, 2018

Orientation-independent room temperature optical 13C hyperpolarization in powdered diamond #DNPNMR

Ajoy, Ashok, Kristina Liu, Raffi Nazaryan, Xudong Lv, Pablo R. Zangara, Benjamin Safvati, Guoqing Wang, et al. “Orientation-Independent Room Temperature Optical 13C Hyperpolarization in Powdered Diamond.” Science Advances 4, no. 5 (May 1, 2018):

Dynamic nuclear polarization via contact with electronic spins has emerged as an attractive route to enhance the sensitivity of nuclear magnetic resonance beyond the traditional limits imposed by magnetic field strength and temperature. Among the various alternative implementations, the use of nitrogen vacancy (NV) centers in diamond—a paramagnetic point defect whose spin can be optically polarized at room temperature—has attracted widespread attention, but applications have been hampered by the need to align the NV axis with the external magnetic field. We overcome this hurdle through the combined use of continuous optical illumination and a microwave sweep over a broad frequency range. As a proof of principle, we demonstrate our approach using powdered diamond with which we attain bulk 13C spin polarization in excess of 0.25% under ambient conditions. Remarkably, our technique acts efficiently on diamond crystals of all orientations and polarizes nuclear spins with a sign that depends exclusively on the direction of the microwave sweep. Our work paves the way toward the use of hyperpolarized diamond particles as imaging contrast agents for biosensing and, ultimately, for the hyperpolarization of nuclear spins in arbitrary liquids brought in contact with their surface.

Wednesday, September 19, 2018

Water Dynamics from the Surface to the Interior of a Supramolecular Nanostructure #DNPNMR #ODNP

Ortony, Julia H., Baofu Qiao, Christina J. Newcomb, Timothy J. Keller, Liam C. Palmer, Elad Deiss-Yehiely, Monica Olvera de la Cruz, Songi Han, and Samuel I. Stupp. “Water Dynamics from the Surface to the Interior of a Supramolecular Nanostructure.” Journal of the American Chemical Society 139, no. 26 (July 5, 2017): 8915–21.


Water within and surrounding the structure of a biological system adopts context-specific dynamics that mediate virtually all of the events involved in the inner workings of a cell. These events range from protein folding and molecular recognition to the formation of hierarchical structures. Water dynamics are mediated by the chemistry and geometry of interfaces where water and biomolecules meet. Here we investigate experimentally and computationally the translational dynamics of vicinal water molecules within the volume of a supramolecular peptide nanofiber measuring 6.7 nm in diameter. Using Overhauser dynamic nuclear polarization relaxometry, we show that drastic differences exist in water motion within a distance of about one nanometer from the surface, with rapid diffusion in the hydrophobic interior and immobilized water on the nanofiber surface. These results demonstrate that water associated with materials designed at the nanoscale is not simply a solvent, but rather an integral part of their structure and potential functions.

Monday, September 17, 2018

Baudin, Mathieu, Basile Vuichoud, Aurélien Bornet, Geoffrey Bodenhausen, and Sami Jannin. “A Cryogen-Consumption-Free System for Dynamic Nuclear Polarization at 9.4 T.” Journal of Magnetic Resonance 294 (September 1, 2018): 115–21.


A novel system for dissolution dynamic nuclear polarization based on a cost-effective “cryogen-free” magnet that can generate fields up to 9.4 T with a sample space that can reach temperatures below 1.4 K in a continuous and stable manner. Polarization levels up to P(1H) = 60 ± 5% can be reached with TEMPOL in about 20 min, and P(13C) = 50 ± 5% can be achieved using adiabatic cross polarization.

Friday, September 14, 2018

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

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 1, 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 (photo-CIDNP) effect to boost the NMR signal intensity of selected protons via inverse cross-polarization. Such hyperpolarization transfer is implemented into 1H-detected two-dimensional 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.

Wednesday, September 12, 2018

Generating para-water from para-hydrogen: A Gedankenexperiment #DNPNMR

Ivanov, Konstantin L., and Geoffrey Bodenhausen. “Generating Para-Water from Para-Hydrogen: A Gedankenexperiment.” Journal of Magnetic Resonance 292 (July 1, 2018): 48–52.


A novel conceptual approach is described that is based on the transfer of hyperpolarization from para-hydrogen in view of generating a population imbalance between the two spin isomers of H2O. The approach is analogous to SABRE (Signal Amplification By Reversible Exchange) and makes use of the transfer of spin order from para-hydrogen to H2O in a hypothetical organometallic complex. The spin order transfer is expected to be most efficient at avoided level crossings. The highest achievable enrichment levels of para- and ortho-water are discussed.

Monday, September 10, 2018

Continuous hyperpolarization with parahydrogen in a membrane reactor #DNPNMR

Lehmkuhl, Sören, Martin Wiese, Lukas Schubert, Mathias Held, Markus Küppers, Matthias Wessling, and Bernhard Blümich. “Continuous Hyperpolarization with Parahydrogen in a Membrane Reactor.” Journal of Magnetic Resonance 291 (June 2018): 8–13. 


Hyperpolarization methods entail a high potential to boost the sensitivity of NMR. Even though the ‘‘Signal Amplification by Reversible Exchange” (SABRE) approach uses para-enriched hydrogen, p-H2, to repeatedly achieve high polarization levels on target molecules without altering their chemical structure, such studies are often limited to batch experiments in NMR tubes. Alternatively, this work introduces a continuous flow setup including a membrane reactor for the p-H2, supply and consecutive detection in a 1 T NMR spectrometer. Two SABRE substrates pyridine and nicotinamide were hyperpolarized, and more than 1000-fold signal enhancement was found. Our strategy combines low-field NMR spectrometry and a membrane flow reactor. This enables precise control of the experimental conditions such as liquid and gas pressures, and volume flow for ensuring repeatable maximum polarization.

Friday, September 7, 2018

Diastereoisomers of L-proline-linked trityl-nitroxide biradicals: synthesis and effect of chiral configurations on exchange interactions #DNPNMR

Zhai, Weixiang, Yalan Feng, Huiqiang Liu, Antal Rockenbauer, Deni Mance, Shaoyong Li, Yuguang Song, Marc Baldus, and Yangping Liu. “Diastereoisomers of L-Proline-Linked Trityl-Nitroxide Biradicals: Synthesis and Effect of Chiral Configurations on Exchange Interactions.” Chemical Science 9, no. 19 (May 16, 2018): 4381–91.


The exchange (J) interaction of organic biradicals is a crucial factor controlling their physiochemical properties and potential applications and can be modulated by changing the nature of the linker. In the present work, we for the first time demonstrate the effect of chiral configurations of radical parts on the J values of trityl-nitroxide (TN) biradicals. Four diastereoisomers (TNT1, TNT2, TNL1 and TNL2) of TN biradicals were synthesized and purified by the conjugation of a racemic (R/S) nitroxide with the racemic (M/P) trityl radical viaL-proline. The absolute configurations of these diastereoisomers were assigned by comparing experimental and calculated electronic circular dichroism (ECD) spectra as (M, S, S) for TNT1, (P, S, S) for TNT2, (M, S, R) for TNL1 and (P, S, R) for TNL2. Electron paramagnetic resonance (EPR) results showed that the configuration of the nitroxide part instead of the trityl part is dominant in controlling the exchange interactions and the order of the J values at room temperature is TNT1 (252 G) > TNT2 (127 G) ≫ TNL2 (33 G) > TNL1 (14 G). Moreover, the J values of TNL1/TNL2 with the S configuration in the nitroxide part vary with temperature and the polarity of solvents due to their flexible linker, whereas the J values of TNT1/TNT2 are almost insensitive to these two factors due to the rigidity of their linkers. The distinct exchange interactions between TNT1,2 and TNL1,2 in the frozen state led to strongly different high-field dynamic nuclear polarization (DNP) enhancements with ε = 7 for TNT1,2 and 40 for TNL1,2 under 800 MHz DNP conditions.

Wednesday, September 5, 2018

Perspective of Overhauser dynamic nuclear polarization for the study of soft materials #DNPNMR

Biller, Joshua R., Ryan Barnes, and Songi Han. “Perspective of Overhauser Dynamic Nuclear Polarization for the Study of Soft Materials.” Current Opinion in Colloid & Interface Science 33 (January 1, 2018): 72–85.


Solution state Overhauser dynamic nuclear polarization (ODNP) has been studied for 60years, but only in recent years has found applications of broad interest to biophysical sciences of hydration dynamics (HD-ODNP) around biomolecules and surfaces. In this review we describe state-of-the-art HD-ODNP methods and experiments, and identify technological and conceptual advances necessary to broadly disseminate HD-ODNP, as well as broaden its scope. Specifically, incomplete treatment of the saturation factor leads to the use of high microwave powers that induce temperature-dependent effects in HD-ODNP that can be detrimental to the stability and property of the sample and/or data interpretation, and thus must be corrected for. Furthermore, direct measurements of the electron spin relaxation times for the nitroxide radical-based spin labels used in HD-ODNP have recently caught up with the ambient solution conditions of relevance to HD-ODNP experiment, allowing us to envision an explicit treatment of the saturation factor. This would enable “single-shot” HD-ODNP at one or two concentrations and power levels, cutting down experimental times from the typical hours to minutes. With the development of a user-friendly and robust operation, the application of HD-ODNP experiments can be broadened for the study of biomolecules, biomaterials, soft polymer materials (i.e. hydrogels) and surfaces. In fact, any hydrated materials that can be viably spin labeled can yield information on local water dynamics and interfaces, and so guide the design of soft materials for medical and pharmaceutical uses. A brief introduction to spin-labeling, and exemplary applications to soft materials is discussed to serve as inspiration for future studies.

Monday, September 3, 2018

Frequency-Swept Integrated and Stretched Solid Effect Dynamic Nuclear Polarization #DNPNMR

Can, T. V., J. E. McKay, R. T. Weber, C. Yang, T. Dubroca, J. van Tol, S. Hill, and R. G. Griffin. “Frequency-Swept Integrated and Stretched Solid Effect Dynamic Nuclear Polarization.” The Journal of Physical Chemistry Letters 9, no. 12 (June 21, 2018): 3187–92. 


We investigate a new time domain approach to dynamic nuclear polarization (DNP), the frequency-swept integrated solid effect (FS-ISE), utilizing a high power, broadband 94 GHz (3.35 T) pulse EPR spectrometer. The bandwidth of the spectrometer enabled measurement of the DNP Zeeman frequency/field profile that revealed two dominant polarization mechanisms, the expected ISE, and a recently observed mechanism, the stretched solid effect (S2E). At 94 GHz, despite the limitations in the microwave chirp pulse length (10 μs) and the repetition rate (2 kHz), we obtained signal enhancements up to ∼70 for the S2E and ∼50 for the ISE. The results successfully demonstrate the viability of the FS-ISE and S2E DNP at a frequency 10 times higher than previous studies. Our results also suggest that these approaches are candidates for implementation at higher magnetic fields.