Wednesday, September 30, 2015

X-Band DNP Hyperpolarization of Viscous Liquids and Polymer Melts

Neudert, O., et al., X-Band DNP Hyperpolarization of Viscous Liquids and Polymer Melts. Macromol Rapid Commun, 2015. 36(10): p. 885-9.

NMR studies of synthetic polymers and biomacromolecules, which provide insight into the conformation and dynamics of these materials, can benefit strongly from the increased sensitivity offered by dynamic nuclear polarization (DNP) and other hyperpolarizing methods. In this study (1) H DNP nuclear spin hyperpolarization of two polybutadiene samples, representing a supercooled liquid and an entangled polymer melt, is demonstrated at 0.35 T magnetic field strength and at temperatures between -80 and +50 degrees C. Electron spin polarization transfer from the alpha,gamma-bisdiphenylene-beta-phenylallyl radical to the sample nuclei is achieved by the Overhauser and solid effect. DNP signal enhancements are studied, varying the electron spin resonance offset, microwave power, and sample temperature. The influence of spin relaxation times, line widths, and molecular dynamics are discussed. The results show promising, up to 15-fold NMR signal enhancements using noncryogenic temperatures and an inexpensive setup that is less technically demanding than current high-field DNP setups.

Monday, September 28, 2015

Natural Abundance (17)O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy

Perras, F.A., T. Kobayashi, and M. Pruski, Natural Abundance (17)O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy. J Am Chem Soc, 2015. 137(26): p. 8336-9.

Due to its extremely low natural abundance and quadrupolar nature, the (17)O nuclide is very rarely used for spectroscopic investigation of solids by NMR without isotope enrichment. Additionally, the applicability of dynamic nuclear polarization (DNP), which leads to sensitivity enhancements of 2 orders of magnitude, to (17)O is wrought with challenges due to the lack of spin diffusion and low polarization transfer efficiency from (1)H. Here, we demonstrate new DNP-based measurements that extend (17)O solid-state NMR beyond its current capabilities. The use of the PRESTO technique instead of conventional (1)H-(17)O cross-polarization greatly improves the sensitivity and enables the facile measurement of undistorted line shapes and two-dimensional (1)H-(17)O HETCOR NMR spectra as well as accurate internuclear distance measurements at natural abundance. This was applied for distinguishing hydrogen-bonded and lone (17)O sites on the surface of silica gel; the one-dimensional spectrum of which could not be used to extract such detail. Lastly, this greatly enhanced sensitivity has enabled, for the first time, the detection of surface hydroxyl sites on mesoporous silica at natural abundance, thereby extending the concept of DNP surface-enhanced NMR spectroscopy to the (17)O nuclide.

Friday, September 25, 2015

Feasibility of multianimal hyperpolarized (13) C MRS

Ramirez, M.S., et al., Feasibility of multianimal hyperpolarized (13) C MRS. Magn Reson Med, 2015. 73(5): p. 1726-32.

PURPOSE: There is great potential for real-time investigation of metabolism with MRS and hyperpolarized (HP) (13) C agents. Unfortunately, HP technology has high associated costs and efficiency limitations that may constrain in vivo studies involving many animals. To improve the throughput of preclinical investigations, we evaluate the feasibility of performing HP MRS on multiple animals simultaneously. METHODS: Simulations helped assess the viability of a dual-coil strategy for spatially localized multivolume MRS. A dual-mouse system was assembled and characterized with bench- and scanner-based experiments. Enzyme phantoms mixed with HP [1-(13) C] pyruvate emulated real-time metabolism and offered a controlled mechanism for evaluating system performance. Finally, a normal mouse and a mouse bearing a subcutaneous xenograft of colon cancer were simultaneously scanned in vivo using an agent containing HP [1-(13) C] pyruvate. RESULTS: Geometric separation/rotation, active decoupling, and use of low input impedance preamplifiers permitted an encode-by-channel approach for spatially localized MRS. A precalibrated shim allowed straightforward metabolite differentiation in enzyme phantom and in vivo experiments at 7 Tesla, with performance similar to conventional acquisitions. CONCLUSION: The initial feasibility of multi-animal HP (13) C MRS was established. Throughput scales with the number of simultaneously scanned animals, demonstrating the potential for significant improvements in study efficiency.

Thursday, September 24, 2015

Nanometer-scale water- and proton-diffusion heterogeneities across water channels in polymer electrolyte membranes

Song, J., O.H. Han, and S. Han, Nanometer-scale water- and proton-diffusion heterogeneities across water channels in polymer electrolyte membranes. Angew Chem Int Ed Engl, 2015. 54(12): p. 3615-20.

Nafion, the most widely used polymer for electrolyte membranes (PEMs) in fuel cells, consists of a fluorocarbon backbone and acidic groups that, upon hydration, swell to form percolated channels through which water and ions diffuse. Although the effects of the channel structures and the acidic groups on water/ion transport have been studied before, the surface chemistry or the spatially heterogeneous diffusivity across water channels has never been shown to directly influence water/ion transport. By the use of molecular spin probes that are selectively partitioned into heterogeneous regions of the PEM and Overhauser dynamic nuclear polarization relaxometry, this study reveals that both water and proton diffusivity are significantly faster near the fluorocarbon and the acidic groups lining the water channels than within the water channels. The concept that surface chemistry at the (sub)nanometer scale dictates water and proton diffusivity invokes a new design principle for PEMs.

Monday, September 21, 2015

Characterization and optimization of the visualization performance of continuous flow overhauser DNP hyperpolarized water MRI: Inversion recovery approach

Terekhov, M., et al., Characterization and optimization of the visualization performance of continuous flow overhauser DNP hyperpolarized water MRI: Inversion recovery approach. Magn Reson Med, 2015: p. n/a-n/a.

PURPOSE: Overhauser dynamic nuclear polarization (DNP) allows the production of liquid hyperpolarized substrate inside the MRI magnet bore as well as its administration in continuous flow mode to acquire MR images with enhanced signal-to-noise ratio. We implemented inversion recovery preparation in order to improve contrast-to-noise ratio and to quantify the overall imaging performance of Overhauser DNP-enhanced MRI. METHOD: The negative enhancement created by DNP in combination with inversion recovery (IR) preparation allows canceling selectively the signal originated from Boltzmann magnetization and visualizing only hyperpolarized fluid. The theoretical model describing gain of MR image intensity produced by steady-state continuous flow DNP hyperpolarized magnetization was established and proved experimentally. RESULTS: A precise quantification of signal originated purely from DNP hyperpolarization was achieved. A temperature effect on longitudinal relaxation had to be taken into account to fit experimental results with numerical prediction. CONCLUSION: Using properly adjusted IR preparation, the complete zeroing of thermal background magnetization was achieved, providing an essential increase of contrast-to-noise ratio of DNP-hyperpolarized water images. To quantify and optimize the steady-state conditions for MRI with continuous flow DNP, an approach similar to that incorporating transient-state thermal magnetization equilibrium in spoiled fast field echo imaging sequences can be used. Magn Reson Med, 2015. (c) 2015 Wiley Periodicals, Inc.

Friday, September 18, 2015

Hyperpolarized [ C]ketobutyrate, a molecular analog of pyruvate with modified specificity for LDH isoforms

von Morze, C., et al., Hyperpolarized [ C]ketobutyrate, a molecular analog of pyruvate with modified specificity for LDH isoforms. Magn Reson Med, 2015: p. n/a-n/a.

PURPOSE: The purpose of this study was to investigate 13 C hyperpolarization of alpha-ketobutyrate (alphaKB), an endogenous molecular analog of pyruvate, and its in vivo enzymatic conversion via lactate dehydrogenase (LDH) using localized MR spectroscopy. METHODS: Hyperpolarized (HP) 13 C MR experiments were conducted using [13 C]alphaKB with rats in vivo and with isolated LDH enzyme in vitro, along with comparative experiments using [13 C]pyruvate. Based on differences in the kinetics of its reaction with individual LDH isoforms, HP [13 C]alphaKB was investigated as a novel MR probe, with added specificity for activity of LDHB-expressed H ("heart"-type) subunits of LDH (e.g., constituents of LDH-1 isoform). RESULTS: Comparable T1 and polarization values to pyruvate were attained (T1 = 52 s at 3 tesla [T], polarization = 10%, at C1 ). MR experiments showed rapid enzymatic conversion with substantially increased specificity. Formation of product HP [13 C]alpha-hydroxybutyrate (alphaHB) from alphaKB in vivo was increased 2.7-fold in cardiac slabs relative to liver and kidney slabs. In vitro studies resulted in 5.0-fold higher product production from alphaKB with bovine heart LDH-1, as compared with pyruvate. CONCLUSIONS: HP [13 C]alphaKB may be a useful MR probe of cardiac metabolism and other applications where the role of H subunits of LDH is significant (e.g., renal cortex and brain). Magn Reson Med, 2015. (c) 2015 Wiley Periodicals, Inc.

Wednesday, September 16, 2015

Dynamic nuclear polarization of nucleic acid with endogenously bound manganese

Wenk, P., et al., Dynamic nuclear polarization of nucleic acid with endogenously bound manganese. J Biomol NMR, 2015: p. 1-13.

We report the direct dynamic nuclear polarization (DNP) of 13C nuclei of a uniformly [13C,15N]-labeled, paramagnetic full-length hammerhead ribozyme (HHRz) complex with Mn2+ where the enhanced polarization is fully provided by the endogenously bound metal ion and no exogenous polarizing agent is added. A 13C enhancement factor of epsilon = 8 was observed by intra-complex DNP at 9.4 T. In contrast, "conventional" indirect and direct DNP experiments were performed using AMUPol as polarizing agent where we obtained a 1H enhancement factor of epsilon approximately 250. Comparison with the diamagnetic (Mg2+) HHRz complex shows that the presence of Mn2+ only marginally influences the (DNP-enhanced) NMR properties of the RNA. Furthermore two-dimensional correlation spectra (15N-13C and 13C-13C) reveal structural inhomogeneity in the frozen, amorphous state indicating the coexistence of several conformational states. These demonstrations of intra-complex DNP using an endogenous metal ion as well as DNP-enhanced MAS NMR of RNA in general yield important information for the development of new methods in structural biology.

Monday, September 14, 2015

Successive Stages of Amyloid-beta Self-Assembly Characterized by Solid-State Nuclear Magnetic Resonance with Dynamic Nuclear Polarization

Potapov, A., et al., Successive Stages of Amyloid-beta Self-Assembly Characterized by Solid-State Nuclear Magnetic Resonance with Dynamic Nuclear Polarization. J Am Chem Soc, 2015. 137(25): p. 8294-307.

Self-assembly of amyloid-beta (Abeta) peptides in human brain tissue leads to neurodegeneration in Alzheimer's disease (AD). Amyloid fibrils, whose structures have been extensively characterized by solid state nuclear magnetic resonance (ssNMR) and other methods, are the thermodynamic end point of Abeta self-assembly. Oligomeric and protofibrillar assemblies, whose structures are less well-understood, are also observed as intermediates in the assembly process in vitro and have been implicated as important neurotoxic species in AD. We report experiments in which the structural evolution of 40-residue Abeta (Abeta40) is monitored by ssNMR measurements on frozen solutions prepared at four successive stages of the self-assembly process. Measurements on transient intermediates are enabled by ssNMR signal enhancements from dynamic nuclear polarization (DNP) at temperatures below 30 K. DNP-enhanced ssNMR data reveal a monotonic increase in conformational order from an initial state comprised primarily of monomers and small oligomers in solution at high pH, to larger oligomers near neutral pH, to metastable protofibrils, and finally to fibrils. Surprisingly, the predominant molecular conformation, indicated by (13)C NMR chemical shifts and by side chain contacts between F19 and L34 residues, is qualitatively similar at all stages. However, the in-register parallel beta-sheet supramolecular structure, indicated by intermolecular (13)C spin polarization transfers, does not develop before the fibril stage. This work represents the first application of DNP-enhanced ssNMR to the characterization of peptide or protein self-assembly intermediates.

Friday, September 11, 2015

Nuclear spin hyperpolarization of the solvent using signal amplification by reversible exchange (SABRE)

Moreno, K.X., et al., Nuclear spin hyperpolarization of the solvent using signal amplification by reversible exchange (SABRE). J Magn Reson, 2015. 257: p. 15-23.

Here we report the polarization of the solvent OH protons by SABRE using standard iridium-based catalysts under slightly acidic conditions. Solvent polarization was observed in the presence of a variety of structurally similar N-donor substrates while no solvent enhancement was observed in the absence of substrate or para-hydrogen (p-H2). Solvent polarization was sensitive to the polarizing field and catalyst:substrate ratio in a manner similar to that of substrate protons. SABRE experiments with pyridine-d5 suggest a mechanism where hyperpolarization is transferred from the free substrate to the solvent by chemical exchange while measured hyperpolarization decay times suggest a complimentary mechanism which occurs by direct coordination of the solvent to the catalytic complex. We found the solvent hyperpolarization to decay nearly 3 times more slowly than its characteristic spin-lattice relaxation time suggesting that the hyperpolarized state of the solvent may be sufficiently long lived ( approximately 20s) to hyperpolarize biomolecules having exchangeable protons. This route may offer future opportunities for SABRE to impact metabolic imaging.

Wednesday, September 9, 2015

Dynamic UltraFast 2D EXchange SpectroscopY (UF-EXSY) of hyperpolarized substrates

Leon Swisher, C., et al., Dynamic UltraFast 2D EXchange SpectroscopY (UF-EXSY) of hyperpolarized substrates. J Magn Reson, 2015. 257: p. 102-9.

In this work, we present a new ultrafast method for acquiring dynamic 2D EXchange SpectroscopY (EXSY) within a single acquisition. This technique reconstructs two-dimensional EXSY spectra from one-dimensional spectra based on the phase accrual during echo times. The Ultrafast-EXSY acquisition overcomes long acquisition times typically needed to acquire 2D NMR data by utilizing sparsity and phase dependence to dramatically undersample in the indirect time dimension. This allows for the acquisition of the 2D spectrum within a single shot. We have validated this method in simulations and hyperpolarized enzyme assay experiments separating the dehydration of pyruvate and lactate-to-pyruvate conversion. In a renal cell carcinoma cell (RCC) line, bidirectional exchange was observed. This new technique revealed decreased conversion of lactate-to-pyruvate with high expression of monocarboxylate transporter 4 (MCT4), known to correlate with aggressive cancer phenotypes. We also showed feasibility of this technique in vivo in a RCC model where bidirectional exchange was observed for pyruvate-lactate, pyruvate-alanine, and pyruvate-hydrate and were resolved in time. Broadly, the technique is well suited to investigate the dynamics of multiple exchange pathways and applicable to hyperpolarized substrates where chemical exchange has shown great promise across a range of disciplines.

Monday, September 7, 2015

Improved Stability and Spectral Quality in Ex Situ Dissolution DNP Using an Improved Transfer Device

Katsikis, S., et al., Improved Stability and Spectral Quality in Ex Situ Dissolution DNP Using an Improved Transfer Device. Appl. Magn. Reson., 2015. 46(7): p. 723-729.

Dissolution dynamic nuclear polarization (DNP) has become one of the predominant implementations for DNP. However, the technical implementation of transferring the sample from the polarizer to the nuclear magnetic resonance (NMR) system remains challenging. There is a need for additional technical optimizations in order to use dissolution DNP for biochemical and chemical applications. Here we show how a newly designed pressure dissolution kit considerably improves spectral quality and stability by enabling highly reliable and fast sample transfer to the NMR system.

Friday, September 4, 2015

Brute-Force Hyperpolarization for NMR and MRI

Hirsch, M.L., et al., Brute-Force Hyperpolarization for NMR and MRI. J Am Chem Soc, 2015. 137(26): p. 8428-34.

Hyperpolarization (HP) of nuclear spins is critical for ultrasensitive nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). We demonstrate an approach for >1500-fold enhancement of key small-molecule metabolites: 1-(13)C-pyruvic acid, 1-(13)C-sodium lactate, and 1-(13)C-acetic acid. The (13)C solution NMR signal of pyruvic acid was enhanced 1600-fold at B = 1 T and 40 degrees C by pre-polarizing at 14 T and approximately 2.3 K. This "brute-force" approach uses only field and temperature to generate HP. The noted 1 T observation field is appropriate for benchtop NMR and near the typical 1.5 T of MRI, whereas high-field observation scales enhancement as 1/B. Our brute-force process ejects the frozen, solid sample from the low-T, high-B polarizer, passing it through low field (B < 100 G) to facilitate "thermal mixing". That equilibrates (1)H and (13)C in hundreds of milliseconds, providing (13)C HP from (1)H Boltzmann polarization attained at high B/T. The ejected sample arrives at a room-temperature, permanent magnet array, where rapid dissolution with 40 degrees C water yields HP solute. Transfer to a 1 T NMR system yields (13)C signals with enhancements at 80% of ideal for noted polarizing conditions. High-resolution NMR of the same product at 9.4 T had consistent enhancement plus resolution of (13)C shifts and J-couplings for pyruvic acid and its hydrate. Comparable HP was achieved with frozen aqueous lactate, plus notable enhancement of acetic acid, demonstrating broader applicability for small-molecule NMR and metabolic MRI. Brute-force avoids co-solvated free-radicals and microwaves that are essential to competing methods. Here, unadulterated samples obviate concerns about downstream purity and also exhibit slow solid-state spin relaxation, favorable for transporting HP samples.

Wednesday, September 2, 2015

Time domain DNP with the NOVEL sequence

Can, T.V., et al., Time domain DNP with the NOVEL sequence. J Chem Phys, 2015. 143(5): p. 054201.

We present results of a pulsed dynamic nuclear polarization (DNP) study at 0.35 T (9.7 GHz/14.7 MHz for electron/(1)H Larmor frequency) using a lab frame-rotating frame cross polarization experiment that employs electron spin locking fields that match the (1)H nuclear Larmor frequency, the so called NOVEL (nuclear orientation via electron spin locking) condition. We apply the method to a series of DNP samples including a single crystal of diphenyl nitroxide (DPNO) doped benzophenone (BzP), 1,3-bisdiphenylene-2-phenylallyl (BDPA) doped polystyrene (PS), and sulfonated-BDPA (SA-BDPA) doped glycerol/water glassy matrices. The optimal Hartman-Hahn matching condition is achieved when the nutation frequency of the electron matches the Larmor frequency of the proton, omega1S = omega0I, together with possible higher order matching conditions at lower efficiencies. The magnetization transfer from electron to protons occurs on the time scale of approximately 100 ns, consistent with the electron-proton couplings on the order of 1-10 MHz in these samples. In a fully protonated single crystal DPNO/BzP, at 270 K, we obtained a maximum signal enhancement of epsilon = 165 and the corresponding gain in sensitivity of epsilonT1/TB (1/2)=230 due to the reduction in the buildup time under DNP. In a sample of partially deuterated PS doped with BDPA, we obtained an enhancement of 323 which is a factor of approximately 3.2 higher compared to the protonated version of the same sample and accounts for 49% of the theoretical limit. For the SA-BDPA doped glycerol/water glassy matrix at 80 K, the sample condition used in most applications of DNP in nuclear magnetic resonance, we also observed a significant enhancement. Our findings demonstrate that pulsed DNP via the NOVEL sequence is highly efficient and can potentially surpass continuous wave DNP mechanisms such as the solid effect and cross effect which scale unfavorably with increasing magnetic field. Furthermore, pulsed DNP is also a promising avenue for DNP at high temperature.