Thursday, October 6, 2011

Optimization of 1H spin density for dynamic nuclear polarization using photo-excited triplet electron spins

Kagawa, A.; Murokawa, Y.; Takeda, K.; Kitagawa, M. J. Magn. Reson. 2009, 197, 9.

In dynamic nuclear polarization (DNP) using photo-excited triplet electron spins, known as Microwave-Induced Optical Nuclear Polarization (MIONP), the attainable 1H polarization is determined by the ratio of the buildup rate and the spin-lattice relaxation rate, in turn depend on the 1H spin density. It is shown that the final 1H polarization can be enhanced by diluting the 1H spins with partial deuteration. The DNP experiments are demonstrated in 0.05 mol% pentacene-doped p-terphenyl for various 1H abundances. It is also shown that the 1H spin diffusion coefficient can be determined by examining the initial buildup rate of 1H polarization for various repetition rates of the DNP sequence.

Tensors and rotations in NMR

This article does not cover DNP. However, for those of us who still like to write their own simulation programs it is a very helpful article.

Mueller, L. J. Concepts in Magnetic Resonance Part A 2011, 38A, 221.

The transformation of second-rank Cartesian tensors under rotation plays a fundamental role in the theoretical description of nuclear magnetic resonance experiments, providing the framework for describing anisotropic phenomena such as single crystal rotation patterns, tensor powder patterns, sideband intensities under magic-angle sample spinning, and as input for relaxation theory. Here, two equivalent procedures for effecting this transformation—direct rotation in Cartesian space and the decomposition of the Cartesian tensor into irreducible spherical tensors that rotate in subgroups of rank 0, 1, and 2—are reviewed. In a departure from the standard formulation, the explicit use of the spherical tensor basis for the decomposition of a spatial Cartesian tensor is introduced, helping to delineate the rotational properties of the basis states from those of the matrix elements. The result is a uniform approach to the rotation of a physical system and the corresponding transformation of the spatial components of the NMR Hamiltonian, expressed as either Cartesian or spherical tensors. This clears up an apparent inconsistency in the NMR literature, where the rotation of a spatial tensor in spherical tensor form has typically been partnered with the inverse rotation in Cartesian form to produce equivalent transformations. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part A 38: 221–235, 2011.

Dynamic Nuclear Polarization in NMR

Chandrakumar, N. Journal of the Indian Institure of Science 2010, 90, 133.

Dynamic nuclear polarization was first predicted — and, shortly thereafter, established experimentally — in 1953, the first demonstration being on Lithium metal. The basic approach involves the saturation of the ESR of a paramagnetic species in the system, while the NMR is observed. Initial applications of DNP involved low and moderate field studies that focused especially on investigations of molecular hydrodynamics. Applications to MRI provided a subsequent fillip to the technique. In the meanwhile, the closely related nuclear Overhauser effect (NOE) — which involves saturation, as well as observation of different NMR signals — had become an essential technique for the structure elucidation of both small molecules, as well as biomolecules. Most recently, DNP is witnessing rejuvenation, with high field applications to sensitivity enhancement in NMR. We present in the following an overview of Dynamic nuclear polarization (DNP). The elementary general theory of the phenomenon is discussed. Four different DNP mechanisms that are currently recognized are briefly introduced and different modes of the experiment — involving either cw ESR irradiation, or pulsed ESR excitation — are pointed out. A brief run down of various possible implementations is presented, including our own early work at moderate fields in cw mode, as well as hardware configurations and requirements for high field DNP. Different current implementations of DNP experiments are summarized, including solid state, as well as in situ and ex situ dissolution DNP variants. Typical results of DNP enhanced high resolution NMR are then briefly discussed, including the results of our own early work on differential 19F enhancements at moderate fields. Design of free radicals that satisfy the requirements to establish an efficient cross effect DNP is discussed. Recent experiments that have succeeded in detecting an intermediate in the photocycle of bacteriorhodopsin are alluded to. Finally, the implementation of ultrafast multi-dimensional NMR techniques under DNP conditions is briefly discussed, as an approach to further exploitation of the prospects that are on offer.

Wednesday, October 5, 2011

Stacked rings for terahertz wave-guiding

de Rijk, E.; Macor, A.; Hogge, J.; Alberti, S.; Ansermet, J. Review of Scientific Instruments 2011, 82, 066102.

We demonstrate the construction of corrugated waveguides using stacked rings to propagate terahertz frequencies. The waveguide allows propagation of the same fundamental mode as an optical-fiber, namely, the HE11 mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications.

Monday, October 3, 2011

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

Hu, K.-N., Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids. Solid State Nuclear Magnetic Resonance, 2011. 40(2): p. 31-41.

This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin-spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin-spin interactions for significant electron-nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in high-field DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (< 100 K). For example, large DNP enhancements (~ 300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at < 20 K, nuclear polarization using lower microwave power (< 100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.