Apr 27, 2018

Frequency-agile gyrotron for electron decoupling and pulsed dynamic nuclear polarization #DNPNMR

Scott, F.J., et al., Frequency-agile gyrotron for electron decoupling and pulsed dynamic nuclear polarization. J Magn Reson, 2018. 289: p. 45-54.

We describe a frequency-agile gyrotron which can generate frequency-chirped microwave pulses. An arbitrary waveform generator (AWG) within the NMR spectrometer controls the microwave frequency, enabling synchronized pulsed control of both electron and nuclear spins. We demonstrate that the acceleration of emitted electrons, and thus the microwave frequency, can be quickly changed by varying the anode voltage. This strategy results in much faster frequency response than can be achieved by changing the potential of the electron emitter, and does not require a custom triode electron gun. The gyrotron frequency can be swept with a rate of 20MHz/mus over a 670MHz bandwidth in a static magnetic field. We have already implemented time-domain electron decoupling with dynamic nuclear polarization (DNP) magic angle spinning (MAS) with this device. In this contribution, we show frequency-swept DNP enhancement profiles recorded without changing the NMR magnet or probe. The profile of endofullerenes exhibits a DNP profile with a <10MHz linewidth, indicating that the device also has sufficient frequency stability, and therefore phase stability, to implement pulsed DNP mechanisms such as the frequency-swept solid effect. We describe schematics of the mechanical and vacuum construction of the device which includes a novel flanged sapphire window assembly. Finally, we discuss how commercially available continuous-wave gyrotrons can potentially be converted into similar frequency-agile high-power microwave sources.

Apr 25, 2018

Effect of heavy atoms on photochemically induced dynamic nuclear polarization in liquids

Okuno, Y. and S. Cavagnero, Effect of heavy atoms on photochemically induced dynamic nuclear polarization in liquids. Journal of Magnetic Resonance, 2018. 286: p. 172-187.

Given its short hyperpolarization time (∼10−6 s) and mostly non-perturbative nature, photo-chemically induced dynamic nuclear polarization (photo-CIDNP) is a powerful tool for sensitivity enhancement in nuclear magnetic resonance. In this study, we explore the extent of 1H-detected 13C nuclear hyperpolarization that can be gained via photo-CIDNP in the presence of small-molecule additives containing a heavy atom. The underlying rationale for this methodology is the well-known external-heavy-atom (EHA) effect, which leads to significant enhancements in the intersystem-crossing rate of selected photosensitizer dyes from photoexcited singlet to triplet. We exploited the EHA effect upon addition of moderate amounts of halogen-atom-containing cosolutes. The resulting increase in the transient triplet-state population of the photo-CIDNP sensitizer fluorescein resulted in a significant increase in the nuclear hyperpolarization achievable via photo-CIDNP in liquids. We also explored the internal-heavy-atom (IHA) effect, which is mediated by halogen atoms covalently incorporated into the photosensitizer dye. Widely different outcomes were achieved in the case of EHA and IHA, with EHA being largely preferable in terms of net hyperpolarization.

Apr 23, 2018

Magnetic-Field-Dependent Lifetimes of Hyperpolarized 13C Spins at Cryogenic Temperature

Niedbalski, P., et al., Magnetic-Field-Dependent Lifetimes of Hyperpolarized 13C Spins at Cryogenic Temperature. The Journal of Physical Chemistry B, 2018. 122(6): p. 1898-1904.

Using a home-built cryogen-free dynamic nuclear polarization (DNP) system with a variable magnetic field capability, 13C spin–lattice T1 relaxation times of hyperpolarized [1-13C] carboxylates (sodium acetate, glycine, sodium pyruvate, and pyruvic acid) doped with trityl OX063 free radical were systematically measured for the first time at different field strengths up to 9 T at T = 1.8 K. Our data reveal that the 13C T1 values of these frozen hyperpolarized 13C samples vary drastically with the applied magnetic field B according to an apparent empirical power-law dependence (13C T1 ∝ Bα, 2.3 < α < 3.1), with relaxation values ranging from a few hundred seconds at 1 T to over 200,000 s at fields close to 9 T. This low temperature relaxation behavior can be ascribed approximately to a model that accounts for the combined effect of 13C–1H intramolecular dipolar interaction and the relaxation contribution from the paramagnetic impurities present in the DNP sample. Since the lifetime or T1 storage of the hyperpolarized state is intimately linked to DNP efficiency, these 13C relaxation data at cryogenic temperature have important theoretical and experimental implications as the DNP of 13C-labeled biomolecules is pushed to higher magnetic fields.

Apr 20, 2018

Primary Transfer Step in the Light-Driven Ion Pump Bacteriorhodopsin: An Irreversible U-Turn Revealed by Dynamic Nuclear Polarization-Enhanced Magic Angle Spinning NMR #DNPNMR

Ni, Q.Z., et al., Primary Transfer Step in the Light-Driven Ion Pump Bacteriorhodopsin: An Irreversible U-Turn Revealed by Dynamic Nuclear Polarization-Enhanced Magic Angle Spinning NMR. J. Am. Chem. Soc., 2018. 140(11): p. 4085-4091.

Despite much attention, the path of the highly consequential primary proton transfer in the light-driven ion pump bacteriorhodopsin (bR) remains mysterious. Here we use DNP-enhanced magic angle spinning (MAS) NMR to study critical elements of the active site just before the Schiff base (SB) deprotonates (in the L intermediate), immediately after the SB has deprotonated and Asp85 has become protonated (in the Mo intermediate), and just after the SB has reprotonated and Asp96 has deprotonated (in the N intermediate). An essential feature that made these experiments possible is the 75-fold signal enhancement through DNP. (15)N(SB)-(1)H correlations reveal that the newly deprotonated SB is accepting a hydrogen bond from an alcohol and (13)C-(13)C correlations show that Asp85 draws close to Thr89 before the primary proton transfer. Concurrently, (15)N-(13)C correlations between the SB and Asp85 show that helices C and G draw closer together just prior to the proton transfer and relax thereafter. Together, these results indicate that Thr89 serves to relay the SB proton to Asp85 and that creating this pathway involves rapprochement between the C and G helices as well as chromophore torsion.

Apr 18, 2018

Direct hyperpolarization of micro- and nanodiamonds for bioimaging applications - Considerations on particle size, functionalization and polarization loss

Kwiatkowski, G., et al., Direct hyperpolarization of micro- and nanodiamonds for bioimaging applications - Considerations on particle size, functionalization and polarization loss. J Magn Reson, 2018. 286: p. 42-51.

Due to the inherently long relaxation time of (13)C spins in diamond, the nuclear polarization enhancement obtained with dynamic nuclear polarization can be preserved for a time on the order of about one hour, opening up an opportunity to use diamonds as a new class of long-lived contrast agents. The present communication explores the feasibility of using (13)C spins in directly hyperpolarized diamonds for MR imaging including considerations for potential in vivo applications.

Postdoc position at Utrecht University: solid-state NMR-based design of membrane-active peptide-antibiotics

A 2.5 years postdoc position in the structural characterisation and design of novel peptide-antibiotics is available in the Weingarth group at Utrecht University, the Netherlands. 

The project is set within the context of the rapid development of antimicrobial resistance that urgently calls for novel antibiotics with unexploited mechanisms. 

We aim to structurally characterise a highly promising class of membrane-binding peptide-antibiotics that can kill the most refractory pathogens at nanomolecular concentrations and that are robust to antimicrobial resistance. Together with collaborators from Utrecht pharmacy department, we eventually aim to rationally develop novel and improved antibiotics. For the structural characterisation of membrane-binding peptide-antibiotics, our team uses recombinant expression techniques and sensitivity-enhanced solid-state NMR methods such as 1H-detection and DNP. 

We are embedded within the excellently-equipped Utrecht NMR facility (solid state NMR: 950, 800, 700, 500 MHz // 800 & 400 MHz DNP // 900 MHz solution NMR magnet). A 1.2 GHz magnet will be installed in the near future. 

The ideal candidate should be experienced in recombinant expression techniques and solid-state OR solution NMR. Experience in NMR structure calculation is a plus. Experience in other biophysical techniques (ITC, spectroscopic techniques) would also be valuable. 

Applicants with strong knowledge in molecular biology and a further, alternative background could also be considered. 

The NWO-funded position is to be filled asap, and the selected candidate will be part of several antibiotics characterisation/design projects that have been successfully implemented in my lab. Applications are accepted until 01.06.2018. 

To apply, and for further inquiries, please contact m.h.weingarth@uu.nl

Recent publications: 

2. Medeiros-Silva, J. et al, (2016) Angew. Chem. 55, 13606, 1H-detected solid-state NMR of water-inaccessible proteins in vitro and in situ

Conditions of employment: The candidate is offered a full/part-time position for 2.5 years. 
Salary: The gross salary is in the range between € 3.111- , and maximum € 4.084- per month. 

The salary is supplemented with a holiday bonus of 8% and an end-of-year bonus of 8,3% per year. In addition, we offer: a pension scheme, a partially paid parental leave, flexible employment conditions. Conditions are based on the Collective Labour Agreement Dutch Universities. The research group will provide the candidate with necessary support on all aspects of the project. More information: http://www.uu.nl/EN/informationfor/jobseekers/Working-for-Utrecht-University/terms-of-employment/Pages/default.aspx

Apr 13, 2018

Discovery of Intermediates of lacZ beta-Galactosidase Catalyzed Hydrolysis Using dDNP NMR

Kjeldsen, C., J.H. Ardenkjaer-Larsen, and J.O. Duus, Discovery of Intermediates of lacZ beta-Galactosidase Catalyzed Hydrolysis Using dDNP NMR. J. Am. Chem. Soc., 2018. 140(8): p. 3030-3034.

Using dissolution dynamic nuclear polarization, the sensitivity of single scan solution state (13)C NMR can be improved up to 4 orders of magnitude. In this study, the enzyme lacZ beta-galactosidase from Escherichia coli was subjected to hyperpolarized substrate, and previously unknown reaction intermediates were observed, including a 1,1-linked disaccharide. The enzyme is known for making 1,6-transglycosylation, producing products like allolactose, that are also substrates. To analyze the kinetics, a simple kinetic model was developed and used to determine relative transglycosylation and hydrolysis rates of each of the intermediates, and the novel transglycosylation intermediates were determined as better substrates than the 1,6-linked one, explaining their transient nature. These findings suggest that hydrolysis and transglycosylation might be more complex than previously described.

Apr 11, 2018

3D hyperpolarized C-13 EPI with calibrationless parallel imaging

Gordon, J.W., et al., 3D hyperpolarized C-13 EPI with calibrationless parallel imaging. J Magn Reson, 2018. 289: p. 92-99.

With the translation of metabolic MRI with hyperpolarized (13)C agents into the clinic, imaging approaches will require large volumetric FOVs to support clinical applications. Parallel imaging techniques will be crucial to increasing volumetric scan coverage while minimizing RF requirements and temporal resolution. Calibrationless parallel imaging approaches are well-suited for this application because they eliminate the need to acquire coil profile maps or auto-calibration data. In this work, we explored the utility of a calibrationless parallel imaging method (SAKE) and corresponding sampling strategies to accelerate and undersample hyperpolarized (13)C data using 3D blipped EPI acquisitions and multichannel receive coils, and demonstrated its application in a human study of [1-(13)C]pyruvate metabolism.

Apr 6, 2018

High-power sub-terahertz source with a record frequency stability at up to 1 Hz #DNPNMR

Fokin, A., et al., High-power sub-terahertz source with a record frequency stability at up to 1 Hz. Sci Rep, 2018. 8(1): p. 4317.

Many state-of-the-art fundamental and industrial projects need the use of terahertz radiation with high power and small linewidth. Gyrotrons as radiation sources provide the desired level of power in the sub-THz and THz frequency range, but have substantial free-running frequency fluctuations of the order of 10(-4). Here, we demonstrate that the precise frequency stability of a high-power sub-THz gyrotron can be achieved by a phase-lock loop in the anode voltage control. The relative width of the frequency spectrum and the frequency stability obtained for a 0.263 THz/100 W gyrotron are 4 x 10(-12) and 10(-10), respectively, and these parameters are better than those demonstrated so far with high-power sources by almost three orders of magnitude. This approach confirms its potential for ultra-high precision spectroscopy, the development of sources with large-scale radiating apertures, and other new projects.

Apr 4, 2018

Low-temperature magnetic resonance imaging with 2.8 μm isotropic resolution

Chen, H.-Y. and R. Tycko, Low-temperature magnetic resonance imaging with 2.8 μm isotropic resolution. Journal of Magnetic Resonance, 2018. 287: p. 47-55.

We demonstrate the feasibility of high-resolution 1H magnetic resonance imaging (MRI) at low temperatures by obtaining an MRI image of 20 μm diameter glass beads in glycerol/water at 28 K with 2.8 μm isotropic resolution. The experiments use a recently-described MRI apparatus (Moore and Tycko, 2015) with minor modifications. The sample is contained within a radio-frequency microcoil with 150 μm inner diameter. Sensitivity is additionally enhanced by paramagnetic doping, optimization of the sample temperature, three-dimensional phase-encoding of k-space data, pulsed spin-lock detection of 1H nuclear magnetic resonance signals, and spherical sampling of k-space. We verify that the actual image resolution is 2.7 ± 0.3 μm by quantitative comparisons of experimental and calculated images. Our imaging approach is compatible with dynamic nuclear polarization, providing a path to significantly higher resolution in future experiments.

Apr 2, 2018

Studies to enhance the hyperpolarization level in PHIP-SAH-produced C13-pyruvate #DNPNMR

Cavallari, E., et al., Studies to enhance the hyperpolarization level in PHIP-SAH-produced C13-pyruvate. J Magn Reson, 2018. 289: p. 12-17.

The use of [1-(13)C]pyruvate, hyperpolarized by dissolution-Dynamic Nuclear Polarization (d-DNP), in in vivo metabolic studies has developed quickly, thanks to the imaging probe's diagnostic relevance. Nevertheless, the cost of a d-DNP polarizer is quite high and the speed of hyperpolarization process is relatively slow, meaning that its use is limited to few research laboratories. ParaHydrogen Induced Polarization Side Arm Hydrogenation (PHIP-SAH) (Reineri et al., 2015) is a cost effective and easy-to-handle method that produces (13)C-MR hyperpolarization in [1-(13)C]pyruvate and other metabolites. This work aims to identify the main determinants of the hyperpolarization levels observed in C13-pyruvate using this method. By dissecting the various steps of the PHIP-SAH procedure, it has been possible to assess the role of several experimental parameters whose optimization must be pursued if this method is to be made suitable for future translational steps. The search for possible solutions has led to improvements in the polarization of sodium [1-(13)C]pyruvate from 2% to 5%. Moreover, these results suggest that observed polarization levels could be increased considerably by an automatized procedure which would reduce the time required for the work-up passages that are currently carried out manually. The results reported herein mean that the attainment of polarization levels suitable for the metabolic imaging applications of these hyperpolarized substrates show significant promise.