Rational Design of Dinitroxide Polarizing Agents for Dynamic Nuclear Polarization to Enhance Overall NMR Sensitivity.

We evaluate the overall sensitivity gains provided by a series of eighteen nitroxide biradicals for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and 100 K, including eight new biradicals. We find that in the best performing group the factors contributing to the overall sensitivity gains, namely the DNP enhancement, the build-up time, and the contribution factor, often compete with each other leading to very similar overall sensitivity across a range of biradicals. NaphPol and HydroPol are found to provide the best overall sensitivity factors, in organic and aqueous solvents respectively. One of the new biradicals, AMUPolCbm, provides high sensitivity for all three solvent formulations measured here, and can be considered to be a "universal" polarizing agent.

Large cross-effect dynamic nuclear polarisation enhancements with kilowatt inverting chirped pulses at 94 GHz.

Dynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei by applying resonant microwave radiation, and has been widely used to improve the sensitivity of nuclear magnetic resonance (NMR). Here we demonstrate new levels of performance for static cross-effect proton DNP using high peak power chirped inversion pulses at 94 GHz to create a strong polarisation gradient across the inhomogeneously broadened line of the mono-radical 4-amino TEMPO. Enhancements of up to 340 are achieved at an average power of a few hundred mW, with fast build-up times (3 s). Experiments are performed using a home-built wideband kW pulsed electron paramagnetic resonance (EPR) spectrometer operating at 94 GHz, integrated with an NMR detection system. Simultaneous DNP and EPR characterisation of other mono-radicals and biradicals, as a function of temperature, leads to additional insights into limiting relaxation mechanisms and give further motivation for the development of wideband pulsed amplifiers for DNP at higher frequencies.

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.

Optically Enhanced Solid-State 1H NMR Spectroscopy.

Low sensitivity is the primary limitation to extending nuclear magnetic resonance (NMR) techniques to more advanced chemical and structural studies. Photochemically induced dynamic nuclear polarization (photo-CIDNP) is an NMR hyperpolarization technique where light is used to excite a suitable donor-acceptor system, creating a spin-correlated radical pair whose evolution drives nuclear hyperpolarization. Systems that exhibit photo-CIDNP in solids are not common, and this effect has, up to now, only been observed for 13C and 15N nuclei. However, the low gyromagnetic ratio and natural abundance of these nuclei trap the local hyperpolarization in the vicinity of the chromophore and limit the utility for bulk hyperpolarization. Here, we report the first example of optically enhanced solid-state 1H NMR spectroscopy in the high-field regime. This is achieved via photo-CIDNP of a donor-chromophore-acceptor molecule in a frozen solution at 0.3 T and 85 K, where spontaneous spin diffusion among the abundant strongly coupled 1H nuclei relays polarization through the whole sample, yielding a 16-fold bulk 1H signal enhancement under continuous laser irradiation at 450 nm. These findings enable a new strategy for hyperpolarized NMR beyond the current limits of conventional microwave-driven DNP.

Deuterated TEKPol Biradicals and the Spin-Diffusion Barrier in MAS DNP.

The sensitivity of NMR spectroscopy is considerably enhanced by dynamic nuclear polarization (DNP). In DNP polarization is transferred from unpaired electrons of a polarizing agent to nearby proton spins. In solids, this transfer is followed by the transport of hyperpolarization to the bulk via 1 H-1 H spin diffusion. The efficiency of these steps is critical to obtain high sensitivity gains, but the pathways for polarization transfer in the region near the unpaired electron spins are unclear. Here we report a series of seven deuterated and one fluorinated TEKPol biradicals to probe the effect of deprotonation on MAS DNP at 9.4 T. The experimental results are interpreted with numerical simulations, and our findings support that strong hyperfine couplings to nearby protons determine high transfer rates across the spin diffusion barrier to achieve short build-up times and high enhancements. Specifically, 1 H DNP build-up times increase substantially with TEKPol isotopologues that have fewer hydrogen atoms in the phenyl rings, suggesting that these protons play a crucial role transferring the polarization to the bulk. Based on this new understanding, we have designed a new biradical, NaphPol, which yields significantly increased NMR sensitivity, making it the best performing DNP polarizing agent in organic solvents to date.

1H detection and dynamic nuclear polarization-enhanced NMR of Aß1-42 fibrils.

Several publications describing high-resolution structures of amyloid-ß (Aß) and other fibrils have demonstrated that magic-angle spinning (MAS) NMR spectroscopy is an ideal tool for studying amyloids at atomic resolution. Nonetheless, MAS NMR suffers from low sensitivity, requiring relatively large amounts of samples and extensive signal acquisition periods, which in turn limits the questions that can be addressed by atomic-level spectroscopic studies. Here, we show that these drawbacks are removed by utilizing two relatively recent additions to the repertoire of MAS NMR experiments-namely, 1H detection and dynamic nuclear polarization (DNP). We show resolved and sensitive two-dimensional (2D) and three-dimensional (3D) correlations obtained on 13C,15N-enriched, and fully protonated samples of M0Aß1-42 fibrils by high-field 1H-detected NMR at 23.4 T and 18.8 T, and 13C-detected DNP MAS NMR at 18.8 T. These spectra enable nearly complete resonance assignment of the core of M0Aß1-42 (K16-A42) using submilligram sample quantities, as well as the detection of numerous unambiguous internuclear proximities defining both the structure of the core and the arrangement of the different monomers. An estimate of the sensitivity of the two approaches indicates that the DNP experiments are currently ∼6.5 times more sensitive than 1H detection. These results suggest that 1H detection and DNP may be the spectroscopic approaches of choice for future studies of Aß and other amyloid systems.

Efficient Dynamic Nuclear Polarization up to 230 K with Hybrid BDPA-Nitroxide Radicals at a High Magnetic Field.

Pairing the spectral resolution provided by high magnetic fields at ambient temperature with the enhanced sensitivity offered by dynamic nuclear polarization (DNP) is a major goal of modern solid-state NMR spectroscopy, which will allow one to unlock ever-challenging applications. This study demonstrates that, by combining HyTEK2, a hybrid BDPA-nitroxide biradical polarizing agent, with ortho-terphenyl (OTP), a rigid DNP matrix, enhancement factors as high as 65 can be obtained at 230 K, 40 kHz magic angle spinning (MAS), and 18.8 T. The temperature dependence of the DNP enhancement and its behavior around the glass transition temperature (Tg) of the matrix is investigated by variable-temperature EPR measurements of the electron relaxation properties and numerical simulations. A correlation is suggested between the decrease in enhancement at the passage of the Tg and the concomitant drop of both transverse electron relaxation times in the biradical.

Structural Analysis of an Antigen Chemically Coupled on Virus-Like Particles in Vaccine Formulation.

Structure determination of adjuvant-coupled antigens is essential for rational vaccine development but has so far been hampered by the relatively low antigen content in vaccine formulations and by their heterogeneous composition. Here we show that magic-angle spinning (MAS) solid-state NMR can be used to assess the structure of the influenza virus hemagglutinin stalk long alpha helix antigen, both in its free, unformulated form and once chemically coupled to the surface of large virus-like particles (VLPs). The sensitivity boost provided by high-field dynamic nuclear polarization (DNP) and proton detection at fast MAS rates allows to overcome the penalty associated with the antigen dilution. Comparison of the MAS NMR fingerprints between the free and VLP-coupled forms of the antigen provides structural evidence of the conservation of its native fold upon bioconjugation. This work demonstrates that high-sensitivity MAS NMR is ripe to play a major role in vaccine design, formulation studies, and manufacturing process development.

Dynamic Nuclear Polarization Enhancement of 200 at 21.15 T Enabled by 65 kHz Magic Angle Spinning.

Solid-state nuclear magnetic resonance under magic angle spinning (MAS) enhanced with dynamic nuclear polarization (DNP) is a powerful approach to characterize many important classes of materials, allowing access to previously inaccessible structural and dynamic parameters. Here, we present the first DNP MAS experiments using a 0.7 mm MAS probe, which allows us to reach spinning frequencies of 65 kHz, with microwave irradiation, at 100 K. At the highest magnetic field available for DNP today (21.1 T), we find that the polarizing agent HyTEK2 provides DNP enhancements as high as 200 at a spinning rate of 65 kHz at 100 K, and BDPA yields an enhancement of 106 under the same conditions. Fast spinning rates enable excellent DNP performance, but they also yield unprecedented 1H resolution under DNP conditions. We report well-resolved 1H-detected 1H-13C and 1H-15N correlation spectra of microcrystalline histidine·HCl·H2O.

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance.

The development of magic-angle spinning dynamic nuclear polarization (MAS DNP) has allowed atomic-level characterization of materials for which conventional solid-state NMR is impractical due to the lack of sensitivity. The rapid progress of MAS DNP has been largely enabled through the understanding of rational design concepts for more efficient polarizing agents (PAs). Here, we identify a new design principle which has so far been overlooked. We find that the local geometry around the unpaired electron can change the DNP enhancement by an order of magnitude for two otherwise identical conformers. We present a set of 13 new stable mono- and dinitroxide PAs for MAS DNP NMR where this principle is demonstrated. The radicals are divided into two groups of isomers, named open (O-) and closed (C-), based on the ring conformations in the vicinity of the N-O bond. In all cases, the open conformers exhibit dramatically improved DNP performance as compared to the closed counterparts. In particular, a new urea-based biradical named HydrOPol and a mononitroxide O-MbPyTol yield enhancements of 330 ± 60 and 119 ± 25, respectively, at 9.4 T and 100 K, which are the highest enhancements reported so far in the aqueous solvents used here. We find that while the conformational changes do not significantly affect electron spin-spin distances, they do affect the distribution of the exchange couplings in these biradicals. Electron spin echo envelope modulation (ESEEM) experiments suggest that the improved performance of the open conformers is correlated with higher solvent accessibility.

Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene-Trityl Radical.

Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from mS-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.

TinyPols: a family of water-soluble binitroxides tailored for dynamic nuclear polarization enhanced NMR spectroscopy at 18.8 and 21.1 T.

Dynamic Nuclear Polarization (DNP) has recently emerged as a key method to increase the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS). While efficient binitroxide polarizing agents such as AMUPol have been developed for MAS DNP NMR at magnetic fields up to 9.4 T, their performance drops rapidly at higher fields due to the unfavorable field dependence of the cross-effect (CE) mechanism and AMUPol-like radicals were so far disregarded in the context of the development of polarizing agents for very high-field DNP. Here, we introduce a new family of water-soluble binitroxides, dubbed TinyPols, which have a three-bond non-conjugated flexible amine linker allowing sizable couplings between the two unpaired electrons. We show that this adjustment of the linker is crucial and leads to unexpectedly high DNP enhancement factors at 18.8 T and 21.1 T: an improvement of about a factor 2 compared to AMUPol is reported for spinning frequencies ranging from 5 to 40 kHz, with ε H of up to 90 at 18.8 T and 38 at 21.1 T for the best radical in this series, which are the highest MAS DNP enhancements measured so far in aqueous solutions at these magnetic fields. This work not only breathes a new momentum into the design of binitroxides tailored towards high magnetic fields, but also is expected to push the application frontiers of high-resolution DNP MAS NMR, as demonstrated here on a hybrid mesostructured silica material.

BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T.

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, α,γ-bisdiphenylene-ß-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field 27Al and 29Si DNP surface enhanced NMR spectra of amorphous aluminosilicates and 17O NMR on silica nanoparticles.

A Bioresistant Nitroxide Spin Label for In-Cell EPR Spectroscopy: In Vitro and In Oocytes Protein Structural Dynamics Studies.

Approaching protein structural dynamics and protein-protein interactions in the cellular environment is a fundamental challenge. Owing to its absolute sensitivity and to its selectivity to paramagnetic species, site-directed spin labeling (SDSL) combined with electron paramagnetic resonance (EPR) has the potential to evolve into an efficient method to follow conformational changes in proteins directly inside cells. Until now, the use of nitroxide-based spin labels for in-cell studies has represented a major hurdle because of their short persistence in the cellular context. The design and synthesis of the first maleimido-proxyl-based spin label (M-TETPO) resistant towards reduction and being efficient to probe protein dynamics by continuous wave and pulsed EPR is presented. In particular, the extended lifetime of M-TETPO enabled the study of structural features of a chaperone in the absence and presence of its binding partner at endogenous concentration directly inside cells.

Tailoring of Polarizing Agents in the bTurea Series for Cross-Effect Dynamic Nuclear Polarization in Aqueous Media.

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ɛ ((1)H) in cross-effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP-optimized glycerol/water matrix ("DNP juice") have been studied. We observe that ɛ ((1)H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e-e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e-e distance or too long a T1e can dramatically limit ɛ ((1)H). Our study also shows that the molecular structure of AMUPol is not optimal and its ɛ ((1)H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better ɛ ((1)H) than AMUPol itself (by a factor of ca. 1.2).

Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization.

A series of 37 dinitroxide biradicals have been prepared and their performance studied as polarizing agents in cross-effect DNP NMR experiments at 9.4 T and 100 K in 1,1,2,2-tetrachloroethane (TCE). We observe that in this regime the DNP performance is strongly correlated with the substituents on the polarizing agents, and electron and nuclear spin relaxation times, with longer relaxation times leading to better enhancements. We also observe that deuteration of the radicals generally leads to better DNP enhancement but with longer build-up time. One of the new radicals introduced here provides the best performance obtained so far under these conditions.

Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature.

Efficient dynamic nuclear polarization (DNP) in solids, which enables very high sensitivity NMR experiments, is currently limited to temperatures of around 100 K and below. Here we show how by choosing an adequate solvent, (1)H cross effect DNP enhancements of over 80 can be obtained at 240 K. To achieve this we use the biradical TEKPol dissolved in a glassy phase of ortho-terphenyl (OTP). We study the solvent DNP enhancement of both TEKPol and BDPA in OTP in the range from 100 to 300 K at 9.4 and 18.8 T. Surprisingly, we find that the DNP enhancement decreases only relatively slowly for temperatures below the glass transition of OTP (Tg = 243 K), and (1)H enhancements around 15-20 at ambient temperature can be observed. We use this to monitor molecular dynamic transitions in the pharmaceutically relevant solids Ambroxol and Ibuprofen.

EPR Studies of the Binding Properties, Guest Dynamics, and Inner-Space Dimensions of a Water-Soluble Resorcinarene Capsule.

Nitroxide free radicals have been used to study the inner space of one of Rebek's water-soluble capsules. EPR and (1) H NMR spectroscopy, ESI-MS, and DFT calculations showed a preference for the formation of 1:2 complexes. EPR titrations allowed us to determine binding constants (Ka ) in the order of 10(7) M(-2) . EPR spectral-shape analysis provided information on the guest rotational dynamics within the capsule. The interplay between optimum hydrogen bonding upon capsule formation and steric strain for guest accommodation highlights some degree of flexibility for guest inclusion, particularly at the center of the capsule where the hydrogen bond seam can be barely distorted or slightly disturbed.

Spin exchange monitoring of the strong positive homotropic allosteric binding of a tetraradical by a synthetic receptor in water.

The flexible tetranitroxide 4T has been prepared and was shown to exhibit a nine line EPR spectrum in water, characteristic of significant through space spin exchange (J(ij)) between four electron spins interacting with four nitrogen nuclei (J(ij) ≫ a(N)). Addition of CB[8] to 4T decreases dramatically all the Jij couplings, and the nine line spectrum is replaced by the characteristic three line spectrum of a mononitroxide. The supramolecular association between 4T and CB[8] involves a highly cooperative asymmetric complexation by two CB[8] (K1 = 4027 M(-1); K2 = 202,800 M(-1); α = 201) leading to a rigid complex with remote nitroxide moieties. The remarkable enhancement for the affinity of the second CB[8] corresponds to an allosteric interaction energy of ≈13 kJ mol(-1), which is comparable to that of the binding of oxygen by hemoglobin. These results are confirmed by competition and reduction experiments, DFT and molecular dynamics calculations, mass spectrometry, and liquid state NMR of the corresponding reduced complex bearing hydroxylamine moieties. This study shows that suitably designed molecules can generate allosteric complexation with CB[8]. The molecule must (i) carry several recognizable groups for CB[8] and (ii) be folded so that the first binding event reorganizes the molecule (unfold) for a better subsequent recognition. The presence of accessible protonable amines and H-bond donors to fit with the second point are also further stabilizing groups of CB[8] complexation. In these conditions, the spin exchange coupling between four radicals has been efficiently and finely tuned and the resulting allosteric complexation induced a dramatic stabilization enhancement of the included paramagnetic moieties in highly reducing conditions through the formation of the supramolecular 4T@CB[8]2 complex.

Silica-surface reorganization during organotin grafting evidenced by ¹¹9Sn DNP SENS: a tandem reaction of gem-silanols and strained siloxane bridges.

Grafting reactive molecular complexes on dehydroxylated amorphous silica is a strategy to develop "single-site" heterogeneous catalysts. In general, only the reactivity of isolated silanols is invoked for silica dehydroxylated at 700 °C ([SiO(2-700)]), though ca. 10% of the surface silanols are in fact geminal Q2-silanols. Here we report the reaction of allyltributylstannane with [SiO(2-700)] and find that the geminal Q2-silanols react to form products that would formally arise from vicinal Q3-silanols that are not present on [SiO(2-700)], indicating that a surface rearrangement occurs. The reorganization of the silica surface is unique to silica dehydroxylated at 700 °C or above. The findings were identified using Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) combined with DFT calculations.