Thursday, November 17, 2011

A spinning thermometer to monitor microwave heating and glass transitions in DNP

Miéville, P.; Vitzthum, V.; Caporini, M. A.; Jannin, S.; Gerber-Lemaire, S.; Bodenhausen, G. Magn. Reson. Chem. 2011, 49, 689.

As previously demonstrated by Thurber and Tycko, the peak position of 79Br in potassium bromide (KBr) allows one to determine the temperature of a spinning sample. We propose to adapt the original design by using a compact KBr tablet placed at the bottom of the magic angle spinning rotor, separated from the sample under investigation by a thin disk made of polytetrafluoroethylene (or ‘Teflon’®). This design allows spinning the sample up to at least 16 kHz. The KBr tablet can remain in the rotor when changing the sample under investigation. Calibration in the range of 98 < T < 320 K has been carried out in a static rotor by inserting a platinum thermometer. The accuracy is better than ± 0.9 K, even in the presence of microwave irradiation. Irradiation with 5 W microwaves at 263 GHz leads to a small temperature increase of 3.6 ± 1.4 K in either static or spinning samples. The dynamic nuclear polarization enhancement decreases with increasing temperature, in particular when a frozen glassy sample undergoes a glass transition.

Tuesday, November 15, 2011

Hot Topics in Spin-Hyperpolarization

Nuclear Magnetic Resonance (NMR) spectroscopy, microscopy and imaging techniques (MRI) play a crucial role in numerous fields of science ranging from physics, chemistry, material sciences, biology to medicine. The high information content of modern multi-dimensional NMR spectroscopy makes it possible to obtain structural and dynamical information with atomic resolution. In addition, owing to the low energy excitation in the MHz frequency region, the method is non-invasive, making it one of the most important imaging modalities in medical diagnostics. However, despite all its versatility, the key issue is frequently sensitivity, which limits the applicability of NMR spectroscopy and imaging techniques in the case of fast dynamical processes and detection of lowly concentrated molecules in both in vitro and in vivo applications.

Several strategies for spin-hyperpolarization are used to increase substantially NMR sensitivity. Two of these strategies are based on the transfer of the much bigger electron spin polarization onto the nuclear spin system either during a chemical reaction (Chemical Induced Dynamic Nuclear Polarization, CIDNP) or by using microwave fields (Dynamic Nuclear Polarization, DNP).Parahydrogen Induced Polarization (PHIP) is based on the correlation between a particular quantum mechanical spin state and a rotational state in diatomic hydrogen. In a chemical reaction the hydrogen spin state can be used to generate target molecules with high spin polarization. In spin exchange optical pumping (SEOP) the polarization of excited electrons is transferred onto noble gas atoms such as Helium, Xenon and Krypton to generate highly polarized gases for MRI application to lung studies, characterization of porous media and surfaces.

Although the hyperpolarization strategies differ in their underlying physico-chemical principles they have a number of problems in common. Recently, a network across Europe and associated states has been launched within the ESF COST programme (Action TD1103) to stimulate the communication between technology developers and users of the different hyperpolarisationtechniques. The Lorentz workshop will provide an overview of the current state-of-the-art in spinhyperpolarisation and aims to identify common problems and mutual points of interest to initiate communication and collaborative projects within the COST Action.

Tuesday, November 1, 2011

Non-aqueous solvents for DNP surface enhanced NMR spectroscopy

Zagdoun, A.; Rossini, A. J.; Gajan, D.; Bourdolle, A.; Ouari, O.; Rosay, M.; Maas, W. E.; Tordo, P.; Lelli, M.; Emsley, L.; Lesage, A.; Coperet, C. Chemical Communications 2011.

A series of non-aqueous solvents combined with the exogenous biradical bTbK are developed for DNP NMR that yield enhancements comparable to the best available water based systems. 1,1,2,2-tetrachloroethane appears to be one of the most promising organic solvents for DNP solid-state NMR. Here this results in a reduction in experimental times by a factor of 1000. These new solvents are demonstrated with the first DNP surface enhanced NMR characterization of an organometallic complex supported on a hydrophobic surface.

Factors Affecting DNP NMR in Polycrystalline Diamond Samples

Casabianca, L. B.; Shames, A. I.; Panich, A. M.; Shenderova, O.; Frydman, L. The Journal of Physical Chemistry C 2011, 115, 19041.

This work examines several polycrystalline diamond samples for their potential as polarizing agents for dynamic nuclear polarization (DNP) in NMR. Diamond samples of various origin and particle sizes ranging from a few nanometers to micrometers were examined by EPR, solid-state NMR and DNP techniques. A correlation was found between the size of the diamond particles and the electron spin-lattice relaxation time, the 13C nuclear spin-lattice relaxation times in room temperature magic-angle-spinning experiments, and the ability of the diamond carbons to be hyperpolarized by irradiating unpaired electrons of inherent defects by microwaves at cryogenic temperatures. As the size of the diamond particles approaches that of bulk diamond, both electron and nuclear relaxation times become longer. NMR signal enhancement through DNP was found to be very efficient only for these larger size diamond samples. The reasons and implications of these results are briefly discussed, in the light of these EPR, DNP, and NMR observations.

Analysis of Cancer Metabolism by Imaging Hyperpolarized Nuclei: Prospects for Translation to Clinical Research

Kurhanewicz, J.; Vigneron, D. B.; Brindle, K. M.; Chekmenev, E. Y.; Comment, A.; Cunningham, C.; DeBerardinis, R. J.; Green, G. G. R.; Leach, M. O.; Rajan, S. S.; Rizi, R. R.; Ross, B. D.; Warren, W.; Malloy, C. R. Neoplasia 2011, 13, 81.

A major challenge in cancer biology is to monitor and understand cancer metabolism in vivo with the goal of improved diagnosis and perhaps therapy. Because of the complexity of biochemical pathways, tracer methods are required for detecting specific enzyme-catalyzed reactions. Stable isotopes such as 13C or 15N with detection by nuclear magnetic resonance provide the necessary information about tissue biochemistry, but the crucial metabolites are present in low concentration and therefore are beyond the detection threshold of traditional magnetic resonance methods. A solution is to improve sensitivity by a factor of 10,000 or more by temporarily redistributing the populations of nuclear spins in a magnetic field, a process termed hyperpolarization. Although this effect is short-lived, hyperpolarized molecules can be generated in an aqueous solution and infused in vivo where metabolism generates products that can be imaged. This discovery lifts the primary constraint on magnetic resonance imaging for monitoring metabolism—poor sensitivity—while preserving the advantage of biochemical information. The purpose of this report was to briefly summarize the known abnormalities in cancer metabolism, the value and limitations of current imaging methods for metabolism, and the principles of hyperpolarization. Recent preclinical applications are described. Hyperpolarization technology is still in its infancy, and current polarizer equipment and methods are suboptimal. Nevertheless, there are no fundamental barriers to rapid translation of this exciting technology to clinical research and perhaps clinical care.