Friday, July 20, 2018

Quantum-rotor-induced polarization

Meier, Benno. “Quantum-Rotor-Induced Polarization.” Magnetic Resonance in Chemistry 0, no. 0 (2018).

Quantum-rotor-induced polarization is closely related to para-hydrogen-induced polarization. In both cases, the hyperpolarized spin order derives from rotational interaction and the Pauli principle by which the symmetry of the rotational ground state dictates the symmetry of the associated nuclear spin state. In quantum-rotor-induced polarization, there may be several spin states associated with the rotational ground state, and the hyperpolarization is typically generated by hetero-nuclear cross-relaxation. This review discusses preconditions for quantum-rotor-induced polarization for both the 1-dimensional methyl rotor and the asymmetric rotor H217O@C60, that is, a single water molecule encapsulated in fullerene C60. Experimental results are presented for both rotors.

Wednesday, July 18, 2018

Dynamic Nuclear Polarization NMR Spectroscopy of Polymeric Carbon Nitride Photocatalysts: Insights into Structural Defects and Reactivity #DNPNMR

Li, Xiaobo, Ivan V. Sergeyev, Fabien Aussenac, Anthony F. Masters, Thomas Maschmeyer, and James M. Hook. “Dynamic Nuclear Polarization NMR Spectroscopy of Polymeric Carbon Nitride Photocatalysts: Insights into Structural Defects and Reactivity.” Angewandte Chemie International Edition, May 8, 2018.

Metal-free polymeric carbon nitrides (PCNs) are promising photocatalysts for solar hydrogen production, but their structurephotoactivity relationship remains elusive. Here, we characterize two PCNs by dynamic nuclear polarization-enhanced solid-state NMR spectroscopy, that circumvents the need for specific labeling with either 13C- or 15N-enriched precursors. This allows rapid 1-D and 2-D data acquisition, providing insights into the structural contrasts of the PCNs. Compared to PCN_B with lower performance, PCN_P, the porous and more active photocatalyst, is richer in terminal Nhydrogens not associated with inter-polymer chains, which are further proposed to act as efficient carrier traps and reaction sites.

Monday, July 16, 2018

Stable Isotope-Resolved Analysis with Quantitative Dissolution Dynamic Nuclear Polarization

Lerche, M. H., D. Yigit, A. B. Frahm, J. H. Ardenkjaer-Larsen, R. M. Malinowski, and P. R. Jensen. “Stable Isotope-Resolved Analysis with Quantitative Dissolution Dynamic Nuclear Polarization.” Analytical Chemistry 90 (January 2, 2018): 674–78. 

Metabolite profiles and their isotopomer distributions can be studied noninvasively in complex mixtures with NMR. The advent of dissolution Dynamic Nuclear Polarization (dDNP) and isotope enrichment add sensitivity and resolution to such metabolic studies. Metabolic pathways and networks can be mapped and quantified if protocols that control and exploit the ex situ signal enhancement are created. We present a sample preparation method, including cell incubation, extraction and signal enhancement, to obtain reproducible and quantitative dDNP (qdDNP) NMR-based stable isotope-resolved analysis. We further illustrate how qdDNP was applied to gain metabolic insights into the phenotype of aggressive cancer cells.

Friday, July 13, 2018

Reversal of Paramagnetic Effects by Electron Spin Saturation #DNPNMR

Jain, Sheetal K., Ting A. Siaw, Asif Equbal, Christopher B. Wilson, Ilia Kaminker, and Songi Han. “Reversal of Paramagnetic Effects by Electron Spin Saturation.” The Journal of Physical Chemistry C 122, no. 10 (March 15, 2018): 5578–89.

We present a study in which both significant dynamic nuclear polarization (DNP) enhancement of 7Li NMR and reversal of the paramagnetic effects (PEs) are achieved by microwave (μw) irradiation-induced electron spin saturation of nitroxide radicals at liquid-helium temperatures. The reversal of the PE was manifested in significant narrowing of the 7Li NMR line and reversal of the paramagnetic chemical shift under DNP conditions. The extent of the PE was found to decrease with increased saturation of the electron paramagnetic resonance line, modulated as a function of microwave (μw) power, frequency, duration of irradiation, and gating time between μw irradiation and NMR detection. The defining observation was the shortening of the electron phase memory time, Tm, of the excited observer spins with increasing μw irradiation and concurrent electron spin saturation of the electron spin bath. This and a series of corroborating studies reveal the origin of the NMR line narrowing to be the reversal of paramagnetic relaxation enhancement (PRE), leading us to debut the term REversal of PRE by electron Spin SaturatION (REPRESSION). The shortening of electron Tm of any paramagnetic system as a function of electron spin saturation has not been reported to date, making REPRESSION a discovery of this study. The reversal of the paramagnetic dipolar shift is due to the decrease in electron spin order, also facilitated by electron spin saturation. This study offers new fundamental insights into PE under DNP conditions and a method to detect and identify NMR signal proximal to paramagnetic sites with reduced or minimal line broadening.

Wednesday, July 11, 2018

Hyperpolarized NMR: d-DNP, PHIP, and SABRE

Kovtunov, Kirill Viktorovich, Ekaterina Pokochueva, Oleg Salnikov, Samuel Cousin, Dennis Kurzbach, Basile Vuichoud, Sami Jannin, et al. “Hyperpolarized NMR: D-DNP, PHIP, and SABRE.” Chemistry – An Asian Journal 0, no. ja (2018).

NMR signals intensities can be enhanced by several orders of magnitude via utilization of techniques for hyperpolarization of different molecules, and it allows one to overcome the main sensitivity challenge of modern NMR/MRI techniques. Hyperpolarized fluids can be successfully used in different applications of material science and biomedicine. This focus review covers the fundamentals of the preparation of hyperpolarized liquids and gases via dissolution dynamic nuclear polarization (d-DNP) and parahydrogen-based techniques such as signal amplification by reversible exchange (SABRE) and parahydrogen-induced polarization (PHIP) in both heterogeneous and homogeneous processes. The different novel aspects of hyperpolarized fluids formation and utilization along with the possibility of NMR signal enhancement observation are described.

Monday, July 9, 2018

Applications of dissolution dynamic nuclear polarization in chemistry and biochemistry

Zhang, Guannan, and Christian Hilty. “Applications of Dissolution Dynamic Nuclear Polarization in Chemistry and Biochemistry.” Magnetic Resonance in Chemistry 0, no. 0 (2018). 

Sensitivity of detection is one of the most limiting aspects when applying NMR spectroscopy to current problems in the molecular sciences. A number of hyperpolarization methods exist for increasing the population difference between nuclear spin Zeeman states and enhance the signal-to-noise ratio by orders of magnitude. Among these methods, dissolution dynamic nuclear polarization (D-DNP) is unique in its capability of providing high spin polarization for many types of molecules in the liquid state. Originally proposed for biomedical applications including in vivo imaging, applications in high resolution NMR spectroscopy are now emerging. These applications are the focus of the present review. Using D-DNP, a small sample aliquot is first hyperpolarized as a frozen solid at low temperature, followed by dissolution into the liquid state. D-DNP extends the capabilities of liquid state NMR spectroscopy towards shorter timescales and enables the study of nonequilibrium processes, such as the kinetics and mechanisms of reactions. It allows the determination of intermolecular interactions, in particular based on spin relaxation parameters. At the same time, a challenge in the application of this hyperpolarization method is that spin polarization is nonrenewable. Substantial effort has been devoted to develop methods for enabling rapid correlation spectroscopy, the measurement of time-dependent signals, and the extension of the observable time window. With these methods, D-DNP has the potential to open new application areas in the chemical and biochemical sciences.

Friday, July 6, 2018

Many-body kinetics of dynamic nuclear polarization by the cross effect #DNPNMR

Karabanov, A., D. Wiśniewski, F. Raimondi, I. Lesanovsky, and W. Köckenberger. “Many-Body Kinetics of Dynamic Nuclear Polarization by the Cross Effect.” Physical Review A 97 (26 2018): 031404.

Dynamic nuclear polarization (DNP) is an out-of-equilibrium method for generating nonthermal spin polarization which provides large signal enhancements in modern diagnostic methods based on nuclear magnetic resonance. A particular instance is cross-effect DNP, which involves the interaction of two coupled electrons with the nuclear spin ensemble. Here we develop a theory for this important DNP mechanism and show that the nonequilibrium nuclear polarization buildup is effectively driven by three-body incoherent Markovian dissipative processes involving simultaneous state changes of two electrons and one nucleus.We identify different parameter regimes for effective polarization transfer and discuss under which conditions the polarization dynamics can be simulated by classical kinetic Monte Carlo methods. Our theoretical approach allows simulations of the polarization dynamics on an individual spin level for ensembles consisting of hundreds of nuclear spins. The insight obtained by these simulations can be used to find optimal experimental conditions for cross-effect DNP and to design tailored radical systems that provide optimal DNP efficiency.