X-Band DNP Hyperpolarization of Viscous Liquids and Polymer Melts

13C Hyperpolarization of Viscous Liquids by Transfer of Solid-Effect 1H Dynamic Nuclear Polarization at High Magnetic Field

Dynamic nuclear polarization (DNP) is routinely used as a method for increasing the sensitivity to nuclear magnetic resonance (NMR). Recently, high-field solid-effect DNP in viscous liquids on 1H nuclei was demonstrated using narrow-line polarizing agents. Here we expand the applicability of DNP in viscous media to 13C nuclei. To hyperpolarize 13C nuclei, we combined solid-effect 1H DNP with a subsequent transfer of the 1H polarization to 13C via insensitive nuclei enhanced by polarization transfer (INEPT). We demonstrate this approach using a triarylmethyl radical as a polarizing agent and glycerol-13C3 as an analyte. We achieved 13C enhancement factors of up to 45 at a magnetic field of 9.4 T and room temperature.

Solid-Effect Dynamic Nuclear Polarization in Viscous Liquids at 9.4 T Using Narrow-Line Polarizing Agents

Dynamic nuclear polarization (DNP) is a hyperpolarization method that is widely used for increasing the sensitivity of nuclear magnetic resonance (NMR) experiments. DNP is efficient in solid-state and liquid-state NMR, but its implementation in the intermediate state, namely, viscous media, is still less explored. Here, we show that a ¹H DNP enhancement of over 50 can be obtained in viscous liquids at a magnetic field of 9.4 T and a temperature of 315 K. This was accomplished by using narrow-line polarizing agents in glycerol, both the water-soluble α,γ-bisdiphenylen-β-phenylallyl (BDPA) and triarylmethyl radicals, and a microwave/RF double-resonance probehead. We observed DNP enhancements with a field profile indicative of the solid effect and investigated the influence of microwave power, temperature, and concentration on the ¹H NMR results. To demonstrate potential applications of this new DNP approach for chemistry and biology, we show hyperpolarized ¹H NMR spectra of tripeptides, triglycine, and glypromate, in glycerol-d₈.

Solid-like Dynamic Nuclear Polarization Observed in the Fluid Phase of Lipid Bilayers at 9.4 T

Dynamic nuclear polarization (DNP) is a powerful method to enhance NMR sensitivity. Much progress has been achieved recently to optimize DNP performance at high magnetic fields in solid-state samples, mostly by utilizing the solid or the cross effect. In liquids, only the Overhauser mechanism is active, which exhibits a DNP field profile matching the EPR line shape of the radical, distinguishable from other DNP mechanisms. Here, we observe DNP enhancements with a field profile indicative of the solid effect and thermal mixing at ∼320 K and a magnetic field of 9.4 T in the fluid phase of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers doped with the radical BDPA (1,3-bis(diphenylene)-2-phenylallyl). This interesting observation might open up new perspectives for DNP applications in macromolecular systems at ambient temperatures.

Three mechanisms of room temperature dynamic nuclear polarization occur simultaneously in an ionic liquid

For the first time, several mechanisms of dynamic nuclear polarization, namely Overhauser, solid effect and cross effect/thermal mixing, have been identified in an ionic liquid with a nitroxide radical at ambient temperatures.

Molecular Dynamics in Ionic Liquid/Radical Systems

Molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Emim-Tf₂N) with either of the four organic stable radicals, TEMPO, 4-benzoyloxy-TEMPO, BDPA, and DPPH, is studied by using Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarization (DNP). In complex fluids at ambient temperature, NMR signal enhancement by DNP is frequently obtained by a combination of several mechanisms, where the Overhauser effect and solid effect are the most common. Understanding the interactions of free radicals with ionic liquid molecules is of particular significance due to their complex dynamics in these systems, influencing the properties of the ion-radical interaction. A combined analysis of EPR, DNP, and NMR relaxation dispersion is carried out for cations and anions containing, respectively, the NMR active nuclei ¹H or ¹⁹F. Depending on the size and the chemical properties of the radical, different interaction processes are distinguished, namely, the Overhauser effect and solid effect, driven by dominating dipolar or scalar interactions. The resulting NMR relaxation dispersion is decomposed into rotational and translational contributions, allowing the identification of the corresponding correlation times of motion and interactions. The influence of electron relaxation time and electron-nuclear spin hyperfine coupling is discussed.

Field-cycling NMR and DNP - A friendship with benefits

Field-cycling relaxometry, or rather its electronic version with a resistive magnet which requires signal detection at a field strength of 1 Tesla or below, remains an inherently insensitive technique due to the construction compromise that goes along with the need for a fast-switching, low-inductance magnet. For the same reasons, signal lifetime is short and frequency resolution is typically not given, at least for the predominantly used hydrogen nuclei. Dynamic Nuclear Polarization (DNP) bears the potential to circumvent these disadvantages: not only has it been demonstrated to enhance magnetization by up to three orders of magnitude beyond its thermal value, but it also provides the possibility to address particular parts of a molecule, thus generating selectivity even in the absence of spectral resolution. At the same time, DNP requires the introduction of stable radicals giving rise to additional relaxation contributions. This article presents a straightforward way to recover the native relaxation rates of the undisturbed system, and shows examples in different research fields where field-cycling relaxometry is traditionally used for refining models of molecular dynamics and interactions.

Hyperpolarization by DNP and Molecular Dynamics: Eliminating the Radical Contribution in NMR Relaxation Studies

Fast field cycling NMR relaxation dispersion represents a versatile method to elucidate the distribution of timescales of molecular motion for systems as diverse as polymers, proteins, and complex fluids. While electronic field switching accesses magnetic field strengths between about 1 T and Earth field, the method remains fundamentally insensitive and unspecific due to the low signal intensity at low fields and the inherently large field inhomogeneity that prohibits spectral resolution for most nuclei. These conditions limit the accessible concentrations and the detection of insensitive X-nuclei. Dynamic nuclear polarization (DNP) has been demonstrated to significantly enhance sensitivity, favoring low-field applications due to the increase in enhancement factors under these conditions. However, the required presence of radicals adds a significant and often dominating relaxivity to the system of nuclei, which has mostly precluded relaxation studies under DNP because of the need to separate several competing relaxation mechanisms. In this study, we present proof that the intrinsic relaxation dispersion of a substance can be completely recovered from experiments with different concentrations of radicals, irrespective of the nature of the DNP effect. This approach not only enhances detection sensitivity by at least one order of magnitude but also provides information about selective radical/molecule interaction that allows the separation of contributions from different molecular moieties from their differential enhancement and relaxation time.

X-nuclei hyperpolarization for studying molecular dynamics by DNP-FFC

Dynamic Nuclear Polarization methods are used for improving the quality of the NMR data, opening new possibilities by increasing both the sensitivity and the selectivity in NMR relaxation experiments. Recently, Fast Field Cycling relaxometry combined with DNP was introduced, demonstrating that molecular dynamics studies in the presence of natural or artificial radicals are indeed feasible under conditions where the signal-to-noise ratio is frequently critical. In this work, the extension of NMR relaxation dispersion beyond ¹H NMR, by hyperpolarization of X-nuclei, is demonstrated. Overhauser effect via nitroxide radicals in simple (low viscous) liquids and saline solutions was observed for ²H, ⁷Li and ¹³C nuclei at ambient temperature. Substantial NMR signal enhancement up to several hundred was achieved for the studied samples. An advanced approach for reconstructing of the original relaxation dispersion of pure substances is used to eliminate the effect of the additional radical relaxivity of the X-nuclei.

Non-destructive analysis of polymers and polymer-based materials by compact NMR

Low-field nuclear magnetic resonance (NMR) based on permanent magnet technologies is currently experiencing a considerable growth of popularity in studying polymer materials. Various bulk properties can be probed with compact NMR tabletop instruments by placing the sample of interest inside the magnet. Contrary to this, compact NMR sensors with open geometries give access to depth-dependent properties of polymer samples and objects of different sizes and shapes truly non-destructively by performing measurements in the inhomogeneous stray-field outside the magnet system. Some of the sensors are also portable being thus well suited for onsite measurements. The gain of both bulk and depth-dependent microscopic properties are important for establishing improved structure-property relationships needed for the rational design of new polymer formulations. Selected recent applications will be presented to illustrate this potential of compact NMR.

Spectroscopic methods for assessing the molecular origins of macroscopic solution properties of highly concentrated liquid protein solutions

In cases of subcutaneous injection of therapeutic monoclonal antibodies, high protein concentrations (>50 mg/ml) are often required. During the development of these high concentration liquid formulations (HCLF), challenges such as aggregation, gelation, opalescence, phase separation, and high solution viscosities are more prone compared to low concentrated protein formulations. These properties can impair manufacturing processes, as well as protein stability and shelf life. To avoid such unfavourable solution properties, a detailed understanding about the nature of these properties and their driving forces are required. However, the fundamental mechanisms that lead to macroscopic solution properties, as above mentioned, are complex and not fully understood, yet. Established analytical methods for assessing the colloidal stability, i.e. the ability of a native protein to remain dispersed in solution, are restricted to dilute conditions and provide parameters such as the second osmotic virial coefficient, B₂₂, and the diffusion interaction coefficient, k_D. These parameters are routinely applied for qualitative estimations and identifications of proteins with challenging solution behaviours, such as high viscosities and aggregation, although the assays are prepared for low protein concentration conditions, typically between 0.1 and 20 mg/ml ("ideal" solution conditions). Quantitative analysis of samples of high protein concentration is difficult and it is hard to obtain information about the driving forces of such solution properties and corresponding protein-protein self-interactions. An advantage of using specific spectroscopic methods is the potential of directly analysing highly concentrated protein solutions at different solution conditions. This allows for collecting/gaining valuable information about the fundamental mechanisms of solution properties of the high protein concentration regime. In addition, the derived parameters might be more predictive as compared to the parameters originating from assays which are optimized for the low protein concentration range. The provided information includes structural data, molecular dynamics at various timescales and protein-solvent interactions, which can be obtained at molecular resolution. Herein, we provide an overview about spectroscopic techniques for analysing the origins of macroscopic solution behaviours in general, with a specific focus on pharmaceutically relevant high protein concentration and formulation conditions.

Molecular dynamics-based selectivity for Fast-Field-Cycling relaxometry by Overhauser and solid effect dynamic nuclear polarization

In the last decade nuclear spin hyperpolarization methods, especially Dynamic Nuclear Polarization (DNP), have provided unprecedented possibilities for various NMR techniques by increasing the sensitivity by several orders of magnitude. Recently, in-situ DNP-enhanced Fast Field Cycling (FFC) relaxometry was shown to provide appreciable NMR signal enhancements in liquids and viscous systems. In this work, a measurement protocol for DNP-enhanced NMR studies is introduced which enables the selective detection of nuclear spin hyperpolarized by either Overhauser effect or solid effect DNP. Based on field-cycled DNP and relaxation studies it is shown that these methods allow for the independent measurement of polymer and solvent nuclear spins in a concentrated solution of high molecular weight polybutadiene in benzene doped with α,γ-bisdiphenylene-β-phenylallyl radical. Appreciable NMR signal enhancements of about 10-fold were obtained for both constituents. Moreover, qualitative information about the dynamics of the radical and solvent was obtained. Selective DNP-enhanced FFC relaxometry is applied for the measurement of the ¹H nuclear magnetic relaxation dispersion of both constituents with improved precision. The introduced method is expected to greatly facilitate NMR studies of complex systems with multiple overlapping signal contributions that cannot be distinguished by standard methods.

A compact X-Band resonator for DNP-enhanced Fast-Field-Cycling NMR

A new probehead was developed enabling Dynamic Nuclear Polarization (DNP)-enhanced Fast-Field-Cycling relaxometry at 340mT polarization field strength. It is based on a dielectric cavity resonator operating in the TM110 mode at 9.5GHz, which is suitable for both transverse and axial magnet geometries with a bore access of at least 20mm. The probehead includes a planar radio frequency coil for NMR detection and is compatible with standard 3mm NMR tubes. The resonator was assessed in terms of the microwave conversion factor and microwave-induced sample heating effects. Due to the compact size of the cavity, appreciable microwave magnetic field strengths were observed even with only moderate quality factors. Exemplary DNP experiments at 9.5GHz and 2.0GHz microwave frequency are compared for three different viscous samples, demonstrating the advantage of DNP at 9.5GHz for such systems. This new probehead enables new applications of DNP-enhanced Fast-Field-Cycling relaxometry of viscous and solid systems.

DNP sensitivity of 19 F-NMR signals in hexafluorobenzene depending on polarizing agent type

Low field dynamic nuclear polarization or low field magnetic double resonance technique enables enhanced nuclear magnetic resonance signals to be detected without increasing the strength of the polarizing field. The study reports that the dynamic nuclear polarization of ¹⁹ F nuclei in hexafluorobenzene solutions doped with nitroxide, BDPA, MC800 asphaltene and MC30 asphaltene free radicals at 15 G. The ¹⁹ F nuclei in all solutions gave positive DNP enhancements changing between 3.42 and 189.54, corresponding to predominantly scalar interactions with the unpaired electrons in the radicals. DNP sensitivity of ¹⁹ F nuclei in hexafluorobenzene was observed to be changed significantly depending on the radical type. Nitroxide was found to have the best DNP performance among the polarizing agents. Copyright © 2016 John Wiley & Sons, Ltd.

Overhauser Dynamic Nuclear Polarization Studies on Local Water Dynamics

Overhauser dynamic nuclear polarization (ODNP) is an emerging technique for quantifying translational water dynamics in the vicinity (<1 nm) of stable radicals that can be chemically attached to macromolecules of interest. This has led to many in-depth and enlightening studies of hydration water of biomolecules, revolving around the role of solvent dynamics in the structure and function of proteins, nucleic acids, and lipid bilayer membranes. Still to date, a complete and fully automated ODNP instrument is not commercialized. The purpose of this chapter is to share the technical know-how of the hardware, theory, measurement, and data analysis method needed to successfully utilize and disseminate the ODNP technique.