Friday, February 28, 2014

Hyperpolarized Xenon-Based Molecular Sensors for Label-Free Detection of analytes


Garimella, P.D., et al., Hyperpolarized Xenon-Based Molecular Sensors for Label-Free Detection of analytes. J. Am. Chem. Soc., 2013. 136(1): p. 164-168.


Nuclear magnetic resonance (NMR) can reveal the chemical constituents of a complex mixture without resorting to chemical modification, separation, or other perturbation. Recently, we and others have developed magnetic resonance agents that report on the presence of dilute analytes by proportionately altering the response of a more abundant or easily detected species, a form of amplification. One example of such a sensing medium is xenon gas, which is chemically inert and can be optically hyperpolarized, a process that enhances its NMR signal by up to 5 orders of magnitude. Here, we use a combinatorial synthetic approach to produce xenon magnetic resonance sensors that respond to small molecule analytes. The sensor responds to the ligand by producing a small chemical shift change in the Xe NMR spectrum. We demonstrate this technique for the dye, Rhodamine 6G, for which we have an independent optical assay to verify binding. We thus demonstrate that specific binding of a small molecule can produce a xenon chemical shift change, suggesting a general approach to the production of xenon sensors targeted to small molecule analytes for in vitro assays or molecular imaging in vivo.

Wednesday, February 26, 2014

Direct Enhancement of Nuclear Singlet Order by Dynamic Nuclear Polarization

I don't think I posted this one already.


Tayler, M.C.D., et al., Direct Enhancement of Nuclear Singlet Order by Dynamic Nuclear Polarization. J. Am. Chem. Soc., 2012. 134(18): p. 7668-7671.


Hyperpolarized singlet order is available immediately after dissolution DNP, avoiding need for additional preparation steps. We demonstrate this procedure on a sample of [1,2?13C2]pyruvic acid.

Monday, February 24, 2014

On Mechanisms of Dynamic Nuclear Polarization in Solids


Thurber, K.R. and R. Tycko, On Mechanisms of Dynamic Nuclear Polarization in Solids. Israel Journal of Chemistry, 2013: p. n/a-n/a.


Dynamic nuclear polarization (DNP) can dramatically increase the signal in nuclear magnetic resonance (NMR) experiments by transferring spin polarization from electrons to nuclei. We discuss quantum mechanical descriptions of the “solid effect” and “cross effect” mechanisms of DNP that typically occur in solid state NMR at high magnetic field, both with and without magic angle spinning (MAS). We present several extensions of recent theoretical descriptions, including (i) treatments of solid effect DNP and cross effect DNP with MAS, in which two nuclei change spin states simultaneously and (ii) a treatment of solid effect DNP in a two-spin system without MAS, in which the ratio of microwave field strength to hyperfine coupling strength can be arbitrary. We also briefly discuss factors that influence DNP efficiency in experiments, beyond the basic principles contained in simple quantum mechanical models.

Real-time DNP NMR observations of acetic acid uptake, intracellular acidification, and of consequences for glycolysis and alcoholic fermentation in yeast


Jensen, P.R., et al., Real-time DNP NMR observations of acetic acid uptake, intracellular acidification, and of consequences for glycolysis and alcoholic fermentation in yeast. Chemistry, 2013. 19(40): p. 13288-93.


Uptake and upshot in vivo: Straightforward methods that permit the real-time observation of organic acid influx, intracellular acidification, and concomitant effects on cellular-reaction networks are crucial for improved bioprocess monitoring and control. Herein, dynamic nuclear polarization (DNP) NMR is used to observe acetate influx, ensuing intracellular acidification and the metabolic consequences on alcoholic fermentation and glycolysis in living cells.

Friday, February 21, 2014

15N-permethylated amino acids as efficient probes for MRI-DNP applications


Chiavazza, E., et al., 15N-permethylated amino acids as efficient probes for MRI-DNP applications. Contrast Media Mol Imaging, 2013. 8(5): p. 417-21.


The synthesis, NMR properties and preliminary polarization tests on protonated and perdeuterated forms of alpha-trimethylglutamine (NMe3Gln), alpha-trimethylglutamate (NMe3Glu) and epsilon-trimethyllysine (NMe3Lys) are reported. The (15)N-permethylated, perdeuterated amino acids display very long (15)N-T1 values, ranging between 190 and 330 s, are well polarized by the dynamic nuclear polarization (DNP) procedure, yielding good polarization levels (10%), and appear to be well tolerated by cells and mice. The obtained results make perdeuterated amino acids excellent candidates for innovative DNP (15)N-MRI applications such as perfusion or targeting studies.

Wednesday, February 19, 2014

In vivo single-shot C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding


Schmidt, R., et al., In vivo single-shot C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding. J Magn Reson, 2014. 240C(0): p. 8-15.


Hyperpolarized metabolic imaging is a growing field that has provided a new tool for analyzing metabolism, particularly in cancer. Given the short life times of the hyperpolarized signal, fast and effective spectroscopic imaging methods compatible with dynamic metabolic characterizations are necessary. Several approaches have been customized for hyperpolarized 13C MRI, including CSI with a center-out k-space encoding, EPSI, and spectrally selective pulses in combination with spiral EPI acquisitions. Recent studies have described the potential of single-shot alternatives based on spatiotemporal encoding (SPEN) principles, to derive chemical-shift images within a sub-second period. By contrast to EPSI, SPEN does not require oscillating acquisition gradients to deliver chemical-shift information: its signal encodes both spatial as well as chemical shift information, at no extra cost in experimental complexity. SPEN MRI sequences with slice-selection and arbitrary excitation pulses can also be devised, endowing SPEN with the potential to deliver single-shot multi-slice chemical shift images, with a temporal resolution required for hyperpolarized dynamic metabolic imaging. The present work demonstrates this with initial in vivo results obtained from SPEN-based imaging of pyruvate and its metabolic products, after injection of hyperpolarized [1-13C]pyruvate. Multi-slice chemical-shift images of healthy rats were obtained at 4.7T in the region of the kidney, and 4D (2D spatial, 1D spectral, 1D temporal) data sets were obtained at 7T from a murine lymphoma tumor model.

Friday, February 14, 2014

Measuring long-lived (13)C2 state lifetimes at natural abundance


Claytor, K., et al., Measuring long-lived (13)C2 state lifetimes at natural abundance. J Magn Reson, 2014. 239(0): p. 81-6.


Long-lived disconnected eigenstates (for example, the singlet state in a system with two nearly equivalent carbons, or the singlet-singlet state in a system with two chemically equivalent carbons and two chemically equivalent hydrogens) hold the potential to drastically extend the lifetime of hyperpolarization in molecular tracers for in vivo magnetic resonance imaging (MRI). However, a first-principles calculation of the expected lifetime (and thus selection of potential imaging agents) is made very difficult because of the large variety of relevant intra- and intermolecular relaxation mechanisms. As a result, all previous measurements relied on costly and time consuming syntheses of (13)C labeled compounds. Here we show that it is possible to determine (13)C singlet state lifetimes by detecting the naturally abundant doubly-labeled species. This approach allows for rapid and low cost screening of potential molecular biomarkers bearing long-lived states.

Wednesday, February 12, 2014

Long-lived nuclear spin states in methyl groups and quantum-rotor-induced polarization


Meier, B., et al., Long-lived nuclear spin states in methyl groups and quantum-rotor-induced polarization. J Am Chem Soc, 2013. 135(50): p. 18746-9.


Substances containing rapidly rotating methyl groups may exhibit long-lived states (LLSs) in solution, with relaxation times substantially longer than the conventional spin-lattice relaxation time T1. The states become long-lived through rapid internal rotation of the CH3 group, which imposes an approximate symmetry on the fluctuating nuclear spin interactions. In the case of very low CH3 rotational barriers, a hyperpolarized LLS is populated by thermal equilibration at liquid helium temperature. Following dissolution, cross-relaxation of the hyperpolarized LLS, induced by heteronuclear dipolar couplings, generates strongly enhanced antiphase NMR signals. This mechanism explains the NMR signal enhancements observed for (13)C-gamma-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1-3).

Monday, February 10, 2014

Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments


Takahashi, H., et al., Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments. J Magn Reson, 2013. 239C(0): p. 91-99.


Thanks to instrumental and theoretical development, notably the access to high-power and high-frequency microwave sources, high-field dynamic nuclear polarization (DNP) on solid-state NMR currently appears as a promising solution to enhance nuclear magnetization in many different types of systems. In magic-angle-spinning DNP experiments, systems of interest are usually dissolved or suspended in glass-forming matrices doped with polarizing agents and measured at low temperature (down to approximately 100K). In this work, we discuss the influence of sample conditions (radical concentration, sample temperature, etc.) on DNP enhancements and various nuclear relaxation times which affect the absolute sensitivity of DNP spectra, especially in multidimensional experiments. Furthermore, DNP-enhanced solid-state NMR experiments performed at 9.4 T are complemented by high-field CW EPR measurements performed at the same magnetic field. Microwave absorption by the DNP glassy matrix is observed even below the glass transition temperature caused by softening of the glass. Shortening of electron relaxation times due to glass softening and its impact in terms of DNP sensitivity is discussed.

Friday, February 7, 2014

Shortening spin-lattice relaxation using a copper-chelated lipid at low-temperatures - A magic angle spinning solid-state NMR study on a membrane-bound protein


This article is not about DNP. However, the authors describe how to use paramagnetic relaxation enhancers to speed up the data acquisition and with this increase the sensitivity. A similar effect happens when a paramagnetic polarization agent is used in a DNP-NMR experiment and often the only reason why it is actually possible to run 1H-DNP-NMR experiments with recycling delays of several seconds is because of the relaxation enhancing nature of the polarizing agent.



Yamamoto, K., et al., Shortening spin-lattice relaxation using a copper-chelated lipid at low-temperatures - A magic angle spinning solid-state NMR study on a membrane-bound protein. J Magn Reson, 2013. 237(0): p. 175-81.


Inherent low sensitivity of NMR spectroscopy has been a major disadvantage, especially to study biomolecules like membrane proteins. Recent studies have successfully demonstrated the advantages of performing solid-state NMR experiments at very low and ultralow temperatures to enhance the sensitivity. However, the long spin-lattice relaxation time, T1, at very low temperatures is a major limitation. To overcome this difficulty, we demonstrate the use of a copper-chelated lipid for magic angle spinning solid-state NMR measurements on cytochrome-b5 reconstituted in multilamellar vesicles. Our results on multilamellar vesicles containing as small as 0.5mol% of a copper-chelated lipid can significantly shorten T1 of protons, which can be used to considerably reduce the data collection time or to enhance the signal-to-noise ratio. We also monitored the effect of slow cooling on the resolution and sensitivity of (13)C and (15)N signals from the protein and (13)C signals from lipids.

Wednesday, February 5, 2014

Paramagnet Induced Signal Quenching in MAS-DNP Experiments in Homogeneous Solutions


Corzilius, B., et al., Paramagnet Induced Signal Quenching in MAS-DNP Experiments in Homogeneous Solutions. J. Magn. Reson., (0).


The effects of nuclear signal quenching induced by the presence of a paramagnetic polarizing agent are documented for conditions used in magic angle spinning (MAS)-dynamic nuclear polarization (DNP) experiments on homogeneous solutions. In particular, we present a detailed analysis of three time constants: (1) the longitudinal build-up time constant TB for 1H; (2) the rotating frame relaxation time constant T1ρ for 1H and 13C and (3) T2 of 13C, the transverse relaxation time constant in the laboratory frame. These relaxation times were measured during microwave irradiation at a magnetic field of 5 T (140 GHz) as a function of the concentration of four polarizing agents: TOTAPOL, 4-amino-TEMPO, trityl (OX063), and Gd-DOTA and are compared to those obtained for a sample lacking paramagnetic doping. We also report the EPR relaxation time constants T1S and T2S, the DNP enhancements, ε, and the parameter E, defined below, which measures the sensitivity enhancement for the four polarizing agents as a function of the electron concentration. We observe substantial intensity losses (paramagnetic quenching) with all of the polarizing agents due to broadening mechanisms and cross relaxation during MAS. In particular, the monoradical trityl and biradical TOTAPOL induce ∼40 and 50% loss of signal intensity. In contrast there is little suppression of signal intensity in static samples containing these paramagnetic species. Despite the losses due to quenching, we find that all of the polarizing agents provide substantial gains in signal intensity, and in particular that the net enhancement is optimal for biradicals that operate with the cross effect. We discuss the possibility that much of this polarization loss can be regained with the development of instrumentation and methods to perform electron decoupling.

Monday, February 3, 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, 2013. 239C(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 15N-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.