Category Archives: solution-state DNP

L-band Overhauser dynamic nuclear polarization

I must have missed that article from 2010, describing L-Band ODNP experiments. This actually looks like a very nice setup that could be used for teaching purposes.

Garcia, S., et al., L-band Overhauser dynamic nuclear polarization. J Magn Reson, 2010. 203(1): p. 138-43.

We present the development of an Overhauser dynamic nuclear polarization (DNP) instrument at 0.04 T using 1.1 GHz (L-band) electron spin resonance frequencies (ESR) and 1.7 MHz (1)H nuclear magnetic resonance frequencies. Using this home-built DNP system, the electron-nucleus coupling factor of 4-oxo-TEMPO dissolved in water was determined as 0.39+/-0.06 at 0.04 T. The higher coupling factor obtained at this field compared to higher magnetic fields, such as 0.35 T, directly translates to higher enhancement of the NMR signal and opens up a wider time scale window for observing water dynamics interacting with macromolecular systems, including proteins, polymers or lipid vesicles. The higher enhancements obtained will facilitate the observation of water dynamics at correlation times up to 10 ns, that corresponds to more than one order of magnitude slower dynamics than accessible at 0.35 T using X-band ESR frequencies.

DNP-enhanced NMR on aligned Lipid Bilayers at ambient Temperature

Jakdetchai, O., et al., DNP-enhanced NMR on aligned Lipid Bilayers at ambient Temperature. J Am Chem Soc, 2014.

DNP-enhanced solid-state NMR has been shown to hold great potential for functional studies of membrane proteins at low temperatures due to its great sensitivity improvement. There are however numerous applications, for which experiments at ambient temperature are desirable and which would also benefit from DNP signal enhancement. Here, we demonstrate as a proof of concept that a significant signal increase for lipid bilayers under room temperature conditions can be achieved by utilizing the Overhauser effect. Experiments were carried out on aligned bilayers at 400 MHz/263 GHz using a stripline structure combined with a Fabry-Perot microwave resonator. A signal enhancement of protons of up to -10 was observed. Our results demonstrate that Overhauser DNP at high field pro-vides efficient polarization transfer within insoluble samples, which is driven by fast local molecular fluc-tuations. Furthermore, our experimental setup offers an attractive option for DNP-enhanced solid-state NMR on ordered membranes and provides a general perspective towards DNP at ambient temperatures.

High-field liquid state NMR hyperpolarization: a combined DNP/NMRD approach

Neugebauer, P., et al., High-field liquid state NMR hyperpolarization: a combined DNP/NMRD approach. Phys Chem Chem Phys, 2014. 16(35): p. 18781-18787.

Here we show how fast dynamics between radicals and solvent molecules in liquid solutions can be detected by comparison of coupling factors determined by nuclear magnetic relaxation dispersion (NMRD) measurements and dynamic nuclear polarization (DNP) enhancement measurements at high magnetic field (9.2 T). This is important for a theoretical understanding of the Overhauser DNP mechanism at high magnetic fields and thus for optimization of the DNP agent/target system for high resolution liquid state NMR applications. Mixtures of the solution of TEMPOL radicals in water, toluene, acetone and DMSO have been investigated. The results are compared to the classical hard-sphere model and molecular dynamic simulations. Our results clearly indicate that fast sub-ps dynamics, which are not related to classical rotational or translational motion of the molecules, significantly contribute to the Overhauser DNP mechanism at high magnetic fields.

High DNP efficiency of TEMPONE radicals in liquid toluene at low concentrations

Enkin, N., et al., High DNP efficiency of TEMPONE radicals in liquid toluene at low concentrations. Phys Chem Chem Phys, 2014. 16(19): p. 8795-800.

We show that at low concentrations (</=5 mM) TEMPONE radicals in liquid toluene exhibit higher DNP efficiency than in water. In spite of reduced coupling factors, the improved DNP performance in toluene results from favourable saturation and leakage factors, as determined by pulse electron-electron double resonance (ELDOR) and NMR relaxation, respectively. The extracted coupling factors at 0.35 Tesla support theoretical predictions of the Overhauser mechanism.

Liquid state dynamic nuclear polarization of ethanol at 3.4 T (95 GHz)

van der Heijden, G.H., A.P. Kentgens, and P.J. van Bentum, Liquid state dynamic nuclear polarization of ethanol at 3.4 T (95 GHz). Phys Chem Chem Phys, 2014. 16(18): p. 8493-502.

Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between an unpaired electron spin of a radical and proton spins of the solvent molecules. The Overhauser hard sphere model is able to predict quantitatively the DNP enhancement for water TEMPOL solutions. The increase in temperature due to dielectric heating of the sample acts to reduce the correlation times and allows a substantial Overhauser DNP. In this paper we extend the work done on water towards other small molecules, such as ethanol. Experimentally we observe a similar enhancement for all three proton groups in the ethanol molecule. The classical interpretation of the low field Overhauser experiments on ethanol invokes a very fast anisotropic rotation of the hydrogen bonded TEMPOL-ethanol complex to explain the fast relaxation of the OH proton. Here we will discuss W-band relaxation and DNP enhancement within this classical model. Although the description can be made quantitative, the invoked parameters do not seem to be realistic. We will propose an alternative model based on the dynamic interaction both in free collision and due to modulation of the hydrogen bond length of the complex.

High-resolution NMR spectroscopy of encapsulated proteins dissolved in low-viscosity fluids

Some more information on encapsulated proteins, an approach that was shown recently to be a very interesting path for solution-state DNP by the same group.

Nucci, N.V., K.G. Valentine, and A.J. Wand, High-resolution NMR spectroscopy of encapsulated proteins dissolved in low-viscosity fluids. J Magn Reson, 2014. 241(0): p. 137-47.

High-resolution multi-dimensional solution NMR is unique as a biophysical and biochemical tool in its ability to examine both the structure and dynamics of macromolecules at atomic resolution. Conventional solution NMR approaches, however, are largely limited to examinations of relatively small (<25kDa) molecules, mostly due to the spectroscopic consequences of slow rotational diffusion. Encapsulation of macromolecules within the protective nanoscale aqueous interior of reverse micelles dissolved in low viscosity fluids has been developed as a means through which the \’slow tumbling problem\’ can be overcome. This approach has been successfully applied to diverse proteins and nucleic acids ranging up to 100kDa, considerably widening the range of biological macromolecules to which conventional solution NMR methodologies may be applied. Recent advances in methodology have significantly broadened the utility of this approach in structural biology and molecular biophysics.

Sensitivity enhancement in solution NMR: emerging ideas and new frontiers

Lee, J.H., Y. Okuno, and S. Cavagnero, Sensitivity enhancement in solution NMR: emerging ideas and new frontiers. J Magn Reson, 2014. 241(0): p. 18-31.

Modern NMR spectroscopy has reached an unprecedented level of sophistication in the determination of biomolecular structure and dynamics at atomic resolution in liquids. However, the sensitivity of this technique is still too low to solve a variety of cutting-edge biological problems in solution, especially those that involve viscous samples, very large biomolecules or aggregation-prone systems that need to be kept at low concentration. Despite the challenges, a variety of efforts have been carried out over the years to increase sensitivity of NMR spectroscopy in liquids. This review discusses basic concepts, recent developments and future opportunities in this exciting area of research.

Rationalizing Overhauser DNP of nitroxide radicals in water through MD simulations

Sezer, D., Rationalizing Overhauser DNP of nitroxide radicals in water through MD simulations. Phys Chem Chem Phys, 2014. 16(3): p. 1022-32.

The recently introduced methodology (Sezer, Phys. Chem. Chem. Phys., 2013, 15, 526) for calculating dynamic nuclear polarization (DNP) coupling factors through synergistic use of molecular dynamics (MD) simulations and the analytical model of diffusing hard spheres with spins at their centers (HSCS) is applied to several nitroxides in water. Computations with one TEMPONE and one TEMPOL in water agree with experiments at 0.35 T and 3.4 T, respectively. At 9.2 T the predicted coupling factors are larger by about 50% than the experimental numbers obtained with 1 M TEMPOL solution. MD simulations at this elevated concentration reveal nanoscopic TEMPOL clusters and qualitatively explain the lower experimental values. Comparing the dynamics from the MD simulations with those of the HSCS model, the assumption of centered spins is shown to be too limiting even for small molecules like TEMPOL and water. Using the available extension of the HSCS model to off-centered spins, the current procedure for analyzing hydration water dynamics from Overhauser DNP measurements on spin-labeled proteins is revisited.

Asymmetric Collapse in Biomimetic Complex Coacervates Revealed by Local Polymer and Water Dynamics

Ortony, J.H., et al., Asymmetric collapse in biomimetic complex coacervates revealed by local polymer and water dynamics. Biomacromolecules, 2013. 14(5): p. 1395-402.

Complex coacervation is a phenomenon characterized by the association of oppositely charged polyelectrolytes into micrometer-scale liquid condensates. This process is the purported first step in the formation of underwater adhesives by sessile marine organisms, as well as the process harnessed for the formation of new synthetic and protein-based contemporary materials. Efforts to understand the physical nature of complex coacervates are important for developing robust adhesives, injectable materials, or novel drug delivery vehicles for biomedical applications; however, their internal fluidity necessitates the use of in situ characterization strategies of their local dynamic properties, capabilities not offered by conventional techniques such as X-ray scattering, microscopy, or bulk rheological measurements. Herein, we employ the novel magnetic resonance technique Overhauser dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP), together with electron paramagnetic resonance (EPR) line shape analysis, to concurrently quantify local molecular and hydration dynamics, with species- and site-specificity. We observe striking differences in the structure and dynamics of the protein-based biomimetic complex coacervates from their synthetic analogues, which is an asymmetric collapse of the polyelectrolyte constituents. From this study we suggest charge heterogeneity within a given polyelectrolyte chain to be an important parameter by which the internal structure of complex coacervates may be tuned. Acquiring molecular-level insight to the internal structure and dynamics of dynamic polymer complexes in water through the in situ characterization of site- and species-specific local polymer and hydration dynamics should be a promising general approach that has not been widely employed for materials characterization.

A comparative study of 1H and 19F Overhauser DNP in fluorinated benzenes

Neudert, O., et al., A comparative study of 1H and 19F Overhauser DNP in fluorinated benzenes. Phys Chem Chem Phys, 2013. 15(47): p. 20717-26.

Hyperpolarization techniques, such as Overhauser dynamic nuclear polarization (DNP), can provide a dramatic increase in the signal obtained from nuclear magnetic resonance experiments and may therefore enable new applications where sensitivity is a limiting factor. In this contribution, studies of the (1)H and (19)F Overhauser dynamic nuclear polarization enhancements at 345 mT are presented for three different aromatic solvents with the TEMPO radical for a range of radical concentrations. Furthermore, nuclear magnetic relaxation dispersion measurements of the same solutions are analyzed, showing contributions from dipolar and scalar coupling modulated by translational diffusion and different coupling efficiency for different solvents and nuclei. Measurements of the electron paramagnetic resonance linewidth are included to support the analysis of the DNP saturation factor for varying radical concentration. The results of our study give an insight into the characteristics of nitroxide radicals as polarizing agents for (19)F Overhauser DNP of aromatic fluorinated solvents. Furthermore, we compare our results with the findings of the extensive research on Overhauser DNP that was conducted in the past for a large variety of other radicals.

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