Friday, March 28, 2014

Hyperpolarized H2O MR angiography


Ardenkjaer-Larsen, J.H., et al., Hyperpolarized H2 O MR angiography. Magn Reson Med, 2014. 71(1): p. 50-6.


PURPOSE: The aim of this study was to demonstrate that dissolution- dynamic nuclear polarization is capable of hyperpolarizing water protons and that the signal from the hyperpolarized bolus injection can be exploited in angiographic applications. METHODS: We hyperpolarized water/glycerol using dynamic nuclear polarization followed by dissolution in D2 O. RESULTS: A water (1) H signal enhancement of 77 times compared with 4.7 Tesla was obtained. This corresponds to a polarization of 3.5% for the 3.9 mol/L (1) H in D2 O . Moreover, a T1 in excess of 20 s was achieved. CONCLUSION: The use of hyperpolarized water as a contrast agent presents a new opportunity to obtain MRA images with high contrast-to-noise in a fraction of a second.

Wednesday, March 26, 2014

Reverse micelles as a platform for dynamic nuclear polarization in solution NMR of proteins


Valentine, K.G., et al., Reverse micelles as a platform for dynamic nuclear polarization in solution NMR of proteins. J Am Chem Soc, 2014. 136(7): p. 2800-7.


Despite tremendous advances in recent years, solution NMR remains fundamentally restricted due to its inherent insensitivity. Dynamic nuclear polarization (DNP) potentially offers significant improvements in this respect. The basic DNP strategy is to irradiate the EPR transitions of a stable radical and transfer this nonequilibrium polarization to the hydrogen spins of water, which will in turn transfer polarization to the hydrogens of the macromolecule. Unfortunately, these EPR transitions lie in the microwave range of the electromagnetic spectrum where bulk water absorbs strongly, often resulting in catastrophic heating. Furthermore, the residence times of water on the surface of the protein in bulk solution are generally too short for efficient transfer of polarization. Here we take advantage of the properties of solutions of encapsulated proteins dissolved in low viscosity solvents to implement DNP in liquids. Such samples are largely transparent to the microwave frequencies required and thereby avoid significant heating. Nitroxide radicals are introduced into the reverse micelle system in three ways: attached to the protein, embedded in the reverse micelle shell, and free in the aqueous core. Significant enhancements of the water resonance ranging up to approximately -93 at 0.35 T were observed. We also find that the hydration properties of encapsulated proteins allow for efficient polarization transfer from water to the protein. These and other observations suggest that merging reverse micelle encapsulation technology with DNP offers a route to a significant increase in the sensitivity of solution NMR spectroscopy of proteins and other biomolecules.

Monday, March 24, 2014

High speed 3D overhauser-enhanced MRI using combined b-SSFP and compressed sensing


Sarracanie, M., et al., High speed 3D overhauser-enhanced MRI using combined b-SSFP and compressed sensing. Magn Reson Med, 2013. 71(2): p. 735-745.


PURPOSE: Overhauser-enhanced MRI is a promising technique for imaging the distribution and dynamics of free radicals. A key challenge for Overhauser-enhanced MRI is attaining high spatial and temporal resolution while simultaneously limiting resonator and sample heating due to the long, high power radio-frequency pulses needed to saturate the electron resonance. METHODS: The approach presented here embeds EPR pulses within a balanced steady state free precession sequence. Unlike other Overhauser-enhanced MRI methods, no separate Overhauser prepolarization step is required. This steady-state approach also eliminates the problem of time-varying Overhauser-enhanced signal and provides constant polarization in the sample during the acquisition. A further increase in temporal resolution was achieved by incorporating undersampled k-space strategies and compressed sensing reconstruction. RESULTS: We demonstrate 1 x 2 x 3.5 mm3 resolution at 6.5 mT across a 54 x 54 x 110 mm3 sample in 33 s while sampling 30% of k-space. CONCLUSION: The work presented here overcomes the main limitations of Overhauser enhanced MRI as previously described in the literature, drastically improving speed and resolution, and enabling new opportunities for the measurement of free radicals in living organisms, and for the study of dynamic processes such as metabolism and flow. Magn Reson Med, 2013. (c) 2013 Wiley Periodicals, Inc.

Wednesday, March 19, 2014

Hyperpolarized singlet lifetimes of pyruvate in human blood and in the mouse


Marco-Rius, I., et al., Hyperpolarized singlet lifetimes of pyruvate in human blood and in the mouse. NMR Biomed, 2013. 26(12): p. 1696-704.


Hyperpolarized NMR is a promising technique for non-invasive imaging of tissue metabolism in vivo. However, the pathways that can be studied are limited by the fast T1 decay of the nuclear spin order. In metabolites containing pairs of coupled nuclear spins-1/2, the spin order may be maintained by exploiting the non-magnetic singlet (spin-0) state of the pair. This may allow preservation of the hyperpolarization in vivo during transport to tissues of interest, such as tumors, or to detect slower metabolic reactions. We show here that in human blood and in a mouse in vivo at millitesla fields the (13)C singlet lifetime of [1,2-(13)C2]pyruvate was significantly longer than the (13)C T1, although it was shorter than the T1 at field strengths of several tesla. We also examine the singlet-derived NMR spectrum observed for hyperpolarized [1,2-(13)C2]lactate, originating from the metabolism of [1,2-(13)C2]pyruvate.

Monday, March 17, 2014

Reproducibility study for free-breathing measurements of pyruvate metabolism using hyperpolarized 13C in the heart


Lau, A.Z., et al., Reproducibility study for free-breathing measurements of pyruvate metabolism using hyperpolarized (13) C in the heart. Magn Reson Med, 2013. 69(4): p. 1063-71.


Spatially resolved images of hyperpolarized (13) C substrates and their downstream products provide insight into real-time metabolic processes occurring in vivo. Recently, hyperpolarized (13) C pyruvate has been used to characterize in vivo cardiac metabolism in the rat and pig, but accurate and reproducible measurements remain challenging due to the limited period available for imaging as well as physiological motion. In this article, time-resolved cardiac- and respiratory-gated images of [1-(13) C] pyruvate, [1-(13) C] lactate, and (13) C bicarbonate in the heart are acquired without the need for a breathhold. The robustness of these free-breathing measurements is demonstrated using the time-resolved data to produce a normalized metric of pyruvate dehydrogenase and lactate dehydrogenase activity in the heart. The values obtained are reproducible in a controlled metabolic state. In a 60-min ischemia/reperfusion model, significant differences in hyperpolarized bicarbonate and lactate, normalized using the left ventricular pyruvate signal, were detected between scans performed at baseline and 45 min after reperfusion. The sequence is anticipated to improve quantitative measurements of cardiac metabolism, leading to feasible validation studies using fewer subjects, and potentially improved diagnosis, serial monitoring, and treatment of cardiac disease in patients.

Friday, March 14, 2014

An Alderman-Grant resonator for S-Band Dynamic Nuclear Polarization


Neudert, O., et al., An Alderman-Grant resonator for S-Band Dynamic Nuclear Polarization. J Magn Reson, 2014. 242C(0): p. 79-85.


An Alderman-Grant resonator with resonance at 2GHz (S-Band) was simulated, developed and constructed for Dynamic Nuclear Polarization (DNP) experiments at 73mT. The resonator fits into magnet bores with a minimum diameter of 20mm and is compatible with standard 3mm NMR tubes. The compact resonator design achieves good separation of electric and magnetic fields and therefore can be used with comparatively large sample volumes with only small sample heating effects comparable to those obtained with optimized X- and W-Band DNP setups. The saturation efficiency and sample heating effects were investigated for Overhauser DNP experiments of aqueous solutions of TEMPOL radical, showing relative saturation better than 0.9 and sample heating not exceeding a few Kelvin even at high microwave power and long irradiation time. An application is demonstrated, combining the DNP setup with a commercial fast field cycling NMR relaxometer. Using this resonator design at low microwave frequencies can provide DNP polarization for a class of low-field and time-domain NMR experiments and therefore may enable new applications that benefit from increased sensitivity.

Wednesday, March 12, 2014

Sensitivity enhancement and contrasting information provided by free radicals in oriented-sample NMR of bicelle-reconstituted membrane proteins


Tesch, D.M. and A.A. Nevzorov, Sensitivity enhancement and contrasting information provided by free radicals in oriented-sample NMR of bicelle-reconstituted membrane proteins. J Magn Reson, 2014. 239(0): p. 9-15.


Elucidating structure and topology of membrane proteins (MPs) is essential for unveiling functionality of these important biological constituents. Oriented-sample solid-state NMR (OS-NMR) is capable of providing such information on MPs under nearly physiological conditions. However, two dimensional OS-NMR experiments can take several days to complete due to long longitudinal relaxation times combined with the large number of scans to achieve sufficient signal sensitivity in biological samples. Here, free radicals 5-DOXYL stearic acid, TEMPOL, and CAT-1 were added to uniformly (15)N-labeled Pf1 coat protein reconstituted in DMPC/DHPC bicelles, and their effect on the longitudinal relaxation times (T1Z) was investigated. The dramatically shortened T1Z's allowed for the signal gain per unit time to be used for either: (i) up to a threefold reduction of the total experimental time at 99% magnetization recovery or (ii) obtaining up to 74% signal enhancement between the control and radical samples during constant experimental time at "optimal" relaxation delays. In addition, through OS-NMR and high-field EPR studies, free radicals were able to provide positional constraints in the bicelle system, which provide a description of the location of each residue in Pf1 coat protein within the bicellar membranes. This information can be useful in the determination of oligomerization states and immersion depths of larger membrane proteins.

Tuesday, March 11, 2014

Open position at Bridge12: Microwave and THz Systems Scientist

We are again looking to expand our team. Check out our new job posting for a Microwave and Terahertz Scientist. Your specific role will include development of novel systems such as:
  • Traveling wave tubes (TWT) from E-band (71-76 GHz) up to 500 GHz
  • Gyrotron oscillators and amplifiers from W-Band (75-110 GHz) up to 500 GHz
  • Development of electron guns for vacuum electron devices
  • High-power, low-loss transmission line systems for microwave and terahertz propagation
More information visit: http://www.bridge12.com/career

Monday, March 10, 2014

A system for accurate and automated injection of hyperpolarized substrate with minimal dead time and scalable volumes over a large range


Reynolds, S., et al., A system for accurate and automated injection of hyperpolarized substrate with minimal dead time and scalable volumes over a large range. J Magn Reson, 2014. 239(0): p. 1-8.


Over recent years hyperpolarization by dissolution dynamic nuclear polarization has become an established technique for studying metabolism in vivo in animal models. Temporal signal plots obtained from the injected metabolite and daughter products, e.g. pyruvate and lactate, can be fitted to compartmental models to estimate kinetic rate constants. Modeling and physiological parameter estimation can be made more robust by consistent and reproducible injections through automation. An injection system previously developed by us was limited in the injectable volume to between 0.6 and 2.4ml and injection was delayed due to a required syringe filling step. An improved MR-compatible injector system has been developed that measures the pH of injected substrate, uses flow control to reduce dead volume within the injection cannula and can be operated over a larger volume range. The delay time to injection has been minimized by removing the syringe filling step by use of a peristaltic pump. For 100mul to 10.000ml, the volume range typically used for mice to rabbits, the average delivered volume was 97.8% of the demand volume. The standard deviation of delivered volumes was 7mul for 100mul and 20mul for 10.000ml demand volumes (mean S.D. was 9 ul in this range). In three repeat injections through a fixed 0.96mm O.D. tube the coefficient of variation for the area under the curve was 2%. For in vivo injections of hyperpolarized pyruvate in tumor-bearing rats, signal was first detected in the input femoral vein cannula at 3-4s post-injection trigger signal and at 9-12s in tumor tissue. The pH of the injected pyruvate was 7.1+/-0.3 (mean+/-S.D., n=10). For small injection volumes, e.g. less than 100mul, the internal diameter of the tubing contained within the peristaltic pump could be reduced to improve accuracy. Larger injection volumes are limited only by the size of the receiving vessel connected to the pump.

Friday, March 7, 2014

Dynamic nuclear polarization enhanced NMR spectroscopy for pharmaceutical formulations


Rossini, A.J., et al., Dynamic nuclear polarization enhanced NMR spectroscopy for pharmaceutical formulations. J Am Chem Soc, 2014. 136(6): p. 2324-34.


Dynamic nuclear polarization (DNP) enhanced solid-state NMR spectroscopy at 9.4 T is demonstrated for the detailed atomic-level characterization of commercial pharmaceutical formulations. To enable DNP experiments without major modifications of the formulations, the gently ground tablets are impregnated with solutions of biradical polarizing agents. The organic liquid used for impregnation (here 1,1,2,2-tetrachloroethane) is chosen so that the active pharmaceutical ingredient (API) is minimally perturbed. DNP enhancements (epsilon) of between 40 and 90 at 105 K were obtained for the microparticulate API within four different commercial formulations of the over-the-counter antihistamine drug cetirizine dihydrochloride. The different formulations contain between 4.8 and 8.7 wt % API. DNP enables the rapid acquisition with natural isotopic abundances of one- and two-dimensional (13)C and (15)N solid-state NMR spectra of the formulations while preserving the microstructure of the API particles. Here this allowed immediate identification of the amorphous form of the API in the tablet. API-excipient interactions were observed in high-sensitivity (1)H-(15)N correlation spectra, revealing direct contacts between povidone and the API. The API domain sizes within the formulations were determined by measuring the variation of epsilon as a function of the polarization time and numerically modeling nuclear spin diffusion. Here we measure an API particle radius of 0.3 mum with a single particle model, while modeling with a Weibull distribution of particle sizes suggests most particles possess radii of around 0.07 mum.

Wednesday, March 5, 2014

DNP-Enhanced MAS NMR of Bovine Serum Albumin Sediments and Solutions


Ravera, E., et al., DNP-Enhanced MAS NMR of Bovine Serum Albumin Sediments and Solutions. J Phys Chem B, 2014.


Protein sedimentation sans cryoprotection is a new approach to magic angle spinning (MAS) and dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy of proteins. It increases the sensitivity of the experiments by a factor of approximately 4.5 in comparison to the conventional DNP sample preparation and circumvents intense background signals from the cryoprotectant. In this paper, we investigate sedimented samples and concentrated frozen solutions of natural abundance bovine serum albumin (BSA) in the absence of a glycerol-based cryoprotectant. We observe DNP signal enhancements of epsilon approximately 66 at 140 GHz in a BSA pellet sedimented from an aqueous solution containing the biradical polarizing agent TOTAPOL and compare this with samples prepared using the conventional protocol (i.e., dissolution of BSA in a glycerol/water cryoprotecting mixture). The dependence of DNP parameters on the radical concentration points to the presence of an interaction between TOTAPOL and BSA, so much so that a frozen solution sans cryoprotectant still gives epsilon approximately 50. We have studied the interaction of BSA with another biradical, SPIROPOL, that is more rigid than TOTAPOL and has been reported to give higher enhancements. SPIROPOL was also found to interact with BSA, and to give epsilon approximately 26 close to its maximum achievable concentration. Under the same conditions, TOTAPOL gives epsilon approximately 31, suggesting a lesser affinity of BSA for SPIROPOL with respect to TOTAPOL. Altogether, these results demonstrate that DNP is feasible in self-cryoprotecting samples.

Monday, March 3, 2014

Structural factors controlling the spin-spin exchange coupling: EPR spectroscopic studies of highly asymmetric trityl-nitroxide biradicals


Liu, Y., et al., Structural factors controlling the spin-spin exchange coupling: EPR spectroscopic studies of highly asymmetric trityl-nitroxide biradicals. J Am Chem Soc, 2013. 135(6): p. 2350-6.


Highly asymmetric exchange-coupled biradicals, e.g., the trityl-nitroxides (TNs), possess particular magnetic properties that have opened new possibilities for their application in biophysical, physicochemical, and biological studies. In the present work, we investigated the effect of the linker length on the spin-spin coupling interaction (J) in TN biradicals using the newly synthesized biradicals CT02-GT, CT02-AT, CT02-VT, and CT02-PPT as well as the previously reported biradicals TNN14 and TN1. The results show that the magnitude of J can be easily tuned from ~4 G (conformer 1 in CT02-PPT) to >1200 G (in TNN14) by varying the linker separating the two radical moieties and changing the temperature. Computer simulations of EPR spectra were carried out to estimate J values of the TN biradicals directly. In addition to the spin-spin coupling interaction of TN biradicals, their g, hyperfine-splitting, and zero-field-splitting interactions were explored at low temperature (220 K). Our present study clearly shows that varying the spin-spin interaction as a function of linker distance and temperature provides an effective strategy for the development of new TN biradicals that can find wide applications in relevant fields.