Characterization of frequency-chirped dynamic nuclear polarization in rotating solids

Frequency-Chirped Magic Angle Spinning Dynamic Nuclear Polarization Combined with Electron Decoupling

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) increases the signal intensity of solid-state nuclear magnetic resonance. DNP typically uses continuous wave (CW) microwave irradiation close to the resonance frequency of unpaired electron spins. In this study, we demonstrate that frequency-chirped microwaves improve DNP performance under MAS. By modulating the gyrotron anode potential, we generate a train of microwave chirps with a maximum bandwidth of 310 MHz and a maximum incident power on the spinning sample of 18 W. We characterize the efficiency of chirped DNP using the following polarizing agents: TEMTriPol-1, AsymPolPOK, AMUPol, and Finland trityl. The effects of different chirp widths and periods are analyzed at different MAS frequencies and microwave powers. Furthermore, we show that chirped DNP can be combined with electron decoupling to improve signal intensity by 59%, compared to CW DNP without electron decoupling, using Finland trityl as a polarizing agent.

Cellular Applications of DNP Solid-State NMR - State of the Art and a Look to the Future

Sensitivity enhanced dynamic nuclear polarization solid-state NMR is emerging as a powerful technique for probing the structural properties of conformationally homogenous and heterogenous biomolecular species irrespective of size at atomic resolution within their native environments. Herein we detail advancements that have made acquiring such data, specifically within the confines of intact bacterial and eukaryotic cell a reality and further discuss the type of structural information that can presently be garnered by the technique's exploitation. Subsequently, we discuss bottlenecks that have thus far curbed cellular DNP-ssNMR's broader adoption namely due a lack of sensitivity and spectral resolution. We also explore possible solutions ranging from utilization of new pulse sequences, design of better performing polarizing agents, and application of additional biochemical/ cell biological methodologies.

Polarizing agents for efficient high field DNP solid-state NMR spectroscopy under magic-angle spinning: from design principles to formulation strategies

Dynamic Nuclear Polarization (DNP) has recently emerged as a cornerstone approach to enhance the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS), opening unprecedented analytical opportunities in chemistry and biology. DNP relies on a polarization transfer from unpaired electrons (present in endogenous or exogenous polarizing agents) to nearby nuclei. Developing and designing new polarizing sources for DNP solid-state NMR spectroscopy is currently an extremely active research field per se, that has recently led to significant breakthroughs and key achievements, in particular at high magnetic fields. This review describes recent developments in this area, highlighting key design principles that have been established over time and led to the introduction of increasingly more efficient polarizing sources. After a short introduction, Section 2 presents a brief history of solid-state DNP, highlighting the main polarization transfer schemes. The third section is devoted to the development of dinitroxide radicals, discussing the guidelines that were progressively established to design the fine-tuned molecular structures in use today. In Section 4, we describe recent efforts in developing hybrid radicals composed of a narrow EPR line radical covalently linked to a nitroxide, highlighting the parameters that modulate the DNP efficiency of these mixed structures. Section 5 reviews recent advances in the design of metal complexes suitable for DNP MAS NMR as exogenous electron sources. In parallel, current strategies that exploit metal ions as endogenous polarization sources are discussed. Section 6 briefly describes the recent introduction of mixed-valence radicals. In the last part, experimental aspects regarding sample formulation are reviewed to make best use of these polarizing agents in a broad panel of application fields.

Coherent Dynamic Nuclear Polarization using Chirped Pulses

This paper presents a study of coherent dynamic nuclear polarization (DNP) using frequency swept pulses at 94 GHz which optimize the polarization transfer efficiency. Accordingly, an enhancement ε ∼ 496 was observed using 10 mM trityl-OX063 as the polarizing agent in a standard 6:3:1 d₈-glycerol/D₂O/H₂O glassing matrix at 70 K. At present, this is the largest DNP enhancement reported at this microwave frequency and temperature. Furthermore, the frequency swept pulses enhance the nuclear magnetic resonance (NMR) signal and reduce the recycle delay, accelerating the NMR signal acquisition.

Highly bioresistant, hydrophilic and rigidly linked trityl-nitroxide biradicals for cellular high-field dynamic nuclear polarization

Cellular dynamic nuclear polarization (DNP) has been an effective means of overcoming the intrinsic sensitivity limitations of solid-state nuclear magnetic resonance (ssNMR) spectroscopy, thus enabling atomic-level biomolecular characterization in native environments. Achieving DNP signal enhancement relies on doping biological preparations with biradical polarizing agents (PAs). Unfortunately, PA performance within cells is often limited by their sensitivity to the reductive nature of the cellular lumen. Herein, we report the synthesis and characterization of a highly bioresistant and hydrophilic PA (StaPol-1) comprising the trityl radical OX063 ligated to a gem-diethyl pyrroline nitroxide via a rigid piperazine linker. EPR experiments in the presence of reducing agents such as ascorbate and in HeLa cell lysates demonstrate the reduction resistance of StaPol-1. High DNP enhancements seen in small molecules, proteins and cell lysates at 18.8 T confirm that StaPol-1 is an excellent PA for DNP ssNMR investigations of biomolecular systems at high magnetic fields in reductive environments.

Room temperature DNP of diamond powder using frequency modulation

Dynamic nuclear polarization (DNP) is a method of enhancing NMR signals via the transfer of polarization from electron spins to nuclear spins using microwave (MW) irradiation. In most cases, monochromatic continuous-wave (MCW) MW irradiation is used. Recently, several groups have shown that frequency modulation of the MW irradiation can result in an additional increase in DNP enhancement above that obtained with MCW. The effect of frequency modulation on the solid effect (SE) and the cross effect (CE) has previously been studied using the stable organic radical 4-hydroxy TEMPO (TEMPOL) at temperatures under 20 K. Here, in addition to the SE and CE, we discuss the effect of frequency modulation on the Overhauser effect (OE) and the truncated CE (tCE) in the room-temperature ¹³C-DNP of diamond powders. We recently showed that diamond powders can exhibit multiple DNP mechanisms simultaneously due to the heterogeneity of P1 (substitutional nitrogen) environments within diamond crystallites. We explore how the two parameters that define the frequency modulation: (i) the Modulation frequency, f_m (how fast the microwave frequency is varied) and (ii) the Modulation amplitude, Δω (the magnitude of the change in microwave frequency) influence the enhancement obtained via each mechanism. Frequency modulation during DNP not only allows us to improve DNP enhancement, but also gives us a way to control which DNP mechanism is most active. By choosing the appropriate modulation parameters, we can selectively enhance some mechanisms while simultaneously suppressing others.

A transition from solid effect to indirect cross effect with broadband microwave irradiation

Dynamic nuclear polarization (DNP) at high magnetic fields has become a prominent technique for signal enhancement in nuclear magnetic resonance (NMR). In static samples, the highest DNP enhancement is usually observed for high radical concentrations in the range of 15-40 mM. Under these conditions, the dominant DNP mechanism for broad-line radicals is the electron-electron spectral-diffusion-based indirect cross effect (iCE). To further increase the DNP performance, broadband microwave irradiation is often applied. Until now, the theory of iCE was not rigorously combined with broadband microwave irradiation. This paper fills this gap by extending the iCE theory to explicitly include broadband irradiation. We demonstrate that our theory allows for quantitative fitting of the DNP spectra lineshapes using four different datasets acquired at 3.4 T and 7 T. We find that the DNP mechanism changes with an increase in the excitation bandwidth. While with narrowband continuous-wave irradiation the DNP mechanism is a combination of the solid effect (SE) and iCE, it shifts toward iCE with increasing excitation bandwidth until, at high bandwidth, the iCE completely dominates the DNP spectrum - this effect was not accounted for previously.

Role of electron spin dynamics and coupling network in designing dynamic nuclear polarization

Dynamic nuclear polarization (DNP) has emerged as a powerful sensitivity booster of nuclear magnetic resonance (NMR) spectroscopy for the characterization of biological solids, catalysts and other functional materials, but is yet to reach its full potential. DNP transfers the high polarization of electron spins to nuclear spins using microwave irradiation as a perturbation. A major focus in DNP research is to improve its efficiency at conditions germane to solid-state NMR, at high magnetic fields and fast magic-angle spinning. In this review, we highlight three key strategies towards designing DNP experiments: time-domain "smart" microwave manipulation to optimize and/or modulate electron spin polarization, EPR detection under operational DNP conditions to decipher the underlying electron spin dynamics, and quantum mechanical simulations of coupled electron spins to gain microscopic insights into the DNP mechanism. These strategies are aimed at understanding and modeling the properties of the electron spin dynamics and coupling network. The outcome of these strategies is expected to be key to developing next-generation polarizing agents and DNP methods.