Large-enhancement nanoscale dynamic nuclear polarization near a silicon nanowire surface

Dynamic nuclear polarization (DNP) has revolutionized the field of nuclear magnetic resonance spectroscopy, expanding its reach and capabilities to investigate diverse materials, biomolecules, and complex dynamic processes. Bringing high-efficiency DNP to the nanometer scale would open exciting avenues for studying nanoscale nuclear spin ensembles, such as single biomolecules, virus particles, and condensed matter systems. Combining pulsed DNP with nanoscale force-detected magnetic resonance measurements, we demonstrated a 100-fold enhancement in the Boltzmann polarization of proton spins in nanoscale sugar droplets at 6 kelvin and 0.33 tesla. Crucially, this enhancement corresponds to a factor of 200 reduction in the averaging time compared to measurements that rely on the detection of statistical fluctuations in nanoscale nuclear spin ensembles. These results substantially advance the capabilities of force-detected magnetic resonance detection as a practical tool for nanoscale imaging.

Resonant Mixing Dynamic Nuclear Polarization

We propose a mechanism for dynamic nuclear polarization that is different from the well-known Overhauser effect, solid effect, cross effect, and thermal mixing processes. We term it Resonant Mixing (RM), and we show that it arises from the evolution of the density matrix for a simple electron-nucleus coupled spin pair subject to weak microwave irradiation, the same interactions as the solid effect. However, the SE is optimal when the microwave field is off-resonance, whereas RM is optimal when the microwave field is on-resonance and involves the mixing of states by the microwave field together with the electron-nuclear coupling. Finally, we argue that this mechanism is responsible for the observed dispersive-shaped DNP field profile for trityl samples near the electron paramagnetic resonance center.

Dynamic nuclear polarization with trityl radicals

Despite the expanding applications of dynamic nuclear polarization (DNP) to problems in biological and materials science, there remain unresolved questions concerning DNP mechanisms. In this paper, we investigate the Zeeman DNP frequency profiles obtained with trityl radicals, OX063 and its partially deuterated analog OX071, in two commonly used glassing matrices based on glycerol and dimethyl sulfoxide (DMSO). When microwave irradiation is applied in the neighborhood of the narrow EPR transition, we observe a dispersive shape in the ¹H Zeeman field and the effects are larger in DMSO than in glycerol. With the help of direct DNP observations on ¹³C and ²H nuclei, we investigate the origin of this dispersive field profile. In particular, we observe a weak nuclear Overhauser effect between ¹H and ¹³C in the sample, which, when irradiating at the positive ¹H solid effect (SE) condition, results in a negative enhancement of ¹³C spins. This observation is not consistent with thermal mixing (TM) being the mechanism responsible for the dispersive shape in the ¹H DNP Zeeman frequency profile. Instead, we propose a new mechanism, resonant mixing, involving mixing of nuclear and electron spin states in a simple two-spin system without invoking electron-electron dipolar interactions.

Radical-Induced Low-Field 1H Relaxation in Solid Pyruvic Acid Doped with Trityl-OX063

In dynamic nuclear polarization (DNP), radicals such as trityl provide a source for high nuclear spin polarization. Conversely, during the low-field transfer of hyperpolarized solids, the radicals' dipolar or Non-Zeeman reservoir may act as a powerful nuclear polarization sink. Here, we report the low-temperature proton spin relaxation in pyruvic acid doped with trityl, for fields from 5 mT to 2 T. We estimate the heat capacity of the radical Non-Zeeman reservoir experimentally and show that a recent formalism by Wenckebach yields a parameter-free, yet quantitative model for the entire field range.

Ox-SLIM: Synthesis of and Site-Specific Labelling with a Highly Hydrophilic Trityl Spin Label

The combination of pulsed dipolar electron paramagnetic resonance spectroscopy (PDS) with site-directed spin labelling is a powerful tool in structural biology. Rational design of trityl-based spin labels has enabled studying biomolecular structures at room temperature and within cells. However, most current trityl spin labels suffer either from aggregation with proteins due to their hydrophobicity, or from bioconjugation groups not suitable for in-cell measurements. Therefore, we introduce here the highly hydrophilic trityl spin label Ox-SLIM. Engineered as a short-linked maleimide, it combines the most recent developments in one single molecule, as it does not aggregate with proteins, exhibits high resistance under in-cell conditions, provides a short linker, and allows for selective and efficient spin labelling via cysteines. Beyond establishing synthetic access to Ox-SLIM, its suitability as a spin label is illustrated and ultimately, highly sensitive PDS measurements are presented down to protein concentrations as low as 45 nm resolving interspin distances of up to 5.5 nm.

Postmodification via Thiol-Click Chemistry Yields Hydrophilic Trityl-Nitroxide Biradicals for Biomolecular High-Field Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) is a powerful method to enhance nuclear magnetic resonance (NMR) signal intensities, enabling unprecedented applications in life and material science. An ultimate goal is to expand the use of DNP-enhanced solid-state NMR to ultrahigh magnetic fields where optimal spectral resolution and sensitivity are integrated. Trityl-nitroxide (TN) biradicals have attracted significant interest in high-field DNP, but their application to complex (bio)molecules has so far been limited. Here we report a novel postmodification strategy for synthesis of hydrophilic TN biradicals in order to improve their use in biomolecular applications. Initially, three TN biradicals (referred to as NATriPols 1-3) with amino-acid linkers were synthesized. EPR studies showed that the α-position of the amino-acid linkers is an ideal modification site for these biradicals since their electron-electron magnetic interactions are marginally affected by the substituents at this position. On the basis of this finding, we synthesized NATriPol-4 with pyridine disulfide appended at the α-position. Postmodification of NATriPol-4 via thiol-click chemistry resulted in various TN biradicals including hydrophilic NATriPol-5 in a quantitative manner. Interestingly, DNP enhancements at 18.8 T of NATriPols for ¹³C,¹⁵N-proline in a glycerol/water matrix are inversely correlated with their hydrophobicity. Importantly, applications of hydrophilic NATriPol-5 and NATriPol-3 to biomolecules including a globular soluble protein and a membrane targeting peptide reveal significantly improved performance compared to TEMTriPol-1 and AMUPol. Our work provides an efficient approach for one-step synthesis of new polarizing agents with tunable physicochemical properties, thus expediting optimization of new biradicals for biomolecular applications at ultrahigh magnetic fields.

Trehalose as an alternative to glycerol as a glassing agent for in vivo DNP MRI

PURPOSE

In dynamic nuclear polarization (DNP), the solution needs to form a glass to attain significant levels of polarization in reasonable time periods. Molecules that do not form glasses by themselves are often mixed with glass forming excipients. Although glassing agents are often essential in DNP studies, they have the potential to perturb the metabolic measurements that are being studied. Glycerol, the glassing agent of choice for in vivo DNP studies, is effective in reducing ice crystal formation during freezing, but is rapidly metabolized, potentially altering the redox and adenosine triphosphate balance of the system.

METHODS

DNP buildup curves of ¹³ C urea and alanine with OX063 in the presence of trehalose, glycerol, and other polyol excipients were measured as a function of concentration. T₁ and T_m relaxation times for OX063 in the presence of trehalose were measured by EPR.

RESULTS

Approximately 15-20 wt% trehalose gives a glass that polarizes samples more rapidly than the commonly used 60%-wt formulation of glycerol and yields similar polarization levels within clinically relevant timeframes.

CONCLUSIONS

Trehalose may be an attractive biologically inert alternative to glycerol for situations where there may be concerns about glycerol's glucogenic potential and possible alteration of the adenosine triphosphate/adenosine diphosphate and redox balance.

Crossover from a Solid Effect to Thermal Mixing 1H Dynamic Nuclear Polarization with Trityl-OX063

Trityl-OX063 is a narrow-line, water-soluble, and biocompatible polarizing agent, widely used for dynamic nuclear polarization (DNP) amplified NMR of ¹³C, but not of the abundant ¹H nuclear spin, for which the ineffective solid effect (SE) is expected to be operational. Surprisingly, we observed a crossover from SE to thermal mixing (TM) DNP of ¹H with increasing Trityl-OX063 concentration at 7 T. We experimentally ascertained diagnostic signatures of TM-DNP that have only been theoretically predicted: (i) an electron paramagnetic resonance (EPR) spectrum that maintains an asymmetrically broadened EPR line from strong e-e couplings and (ii) hyperpolarization, i.e., cooling of select electron-spin populations, manifested in a characteristic pump-probe electron double-resonance spectrum under DNP conditions. Low microwave power requirements, high polarization transfer rates, and efficient DNP at high magnetic fields are the key benefits of TM-DNP.

Photochemistry of tris(2,3,5,6-tetrathiaaryl)methyl radicals in various solutions

A detailed mechanism of TAM photolysis was studied and includes photoionization of the TAM radical with the formation of carbocation and further conversion of the carbocation under aerobic conditions into quinone-methide and under anaerobic conditions supposedly into an aromatic carbene.

Detecting acetylated aminoacids in blood serum using hyperpolarized 13C-1Η-2D-NMR

Dynamic Nuclear Polarization (DNP) can substantially enhance the sensitivity of NMR experiments. Among the implementations of DNP, ex-situ dissolution DNP (dDNP) achieves high signal enhancement levels owing to a combination of a large temperature factor between 1.4 and 300 K with the actual DNP effect in the solid state at 1.4 K. For sufficiently long T₁ relaxation times much of the polarization can be preserved during dissolution with hot solvent, thus enabling fast experiments during the life time of the polarization. Unfortunately, for many metabolites found in biological samples such as blood, relaxation times are too short to achieve a significant enhancement. We have therefore introduced ¹³C-carbonyl labeled acetyl groups as probes into amino acid metabolites using a simple reaction protocol. The advantage of such tags is a sufficiently long T₁ relaxation time, the possibility to enhance signal intensity by introducing ¹³C, and the possibility to identify tagged metabolites in NMR spectra. We demonstrate feasibility for mixtures of amino acids and for blood serum. In two-dimensional dDNP-enhanced HMQC experiments of these samples acquired in 8 s we can identify acetylated amino acids and other metabolites based on small differences in chemical shifts.

Dissolution DNP using trityl radicals at 7 T field

Characterization of direct ¹³C DNP at 1.4 K and 7 T field using trityl radicals.

NMR signal enhancement of >50 000 times in fast dissolution dynamic nuclear polarization

A novel mixed biradical polarizing agent (BDPAesterTEMPO) showing one of the largest NMR signal enhancements to be observed so far in fast dissolution Dynamic Nuclear Polarization (dDNP).

Triarylmethyl Radical: EPR Signal to Noise at Frequencies between 250 MHz and 1.5 GHz and Dependence of Relaxation on Radical and Salt Concentration and on Frequency

In vivo oximetry by pulsed electron paramagnetic resonance is based on measurements of changes in electron spin relaxation rates of probe molecules, such as the triarylmethyl radicals. A series of experiments was performed at frequencies between 250 MHz and 1.5 GHz to assist in the selection of an optimum frequency for oximetry. Electron spin relaxation rates for the triarylmethyl radical OX063 as a function of radical concentration, salt concentration, and resonance frequency were measured by electron spin echo 2-pulse decay and 3-pulse inversion recovery in the frequency range of 250 MHz-1.5 GHz. At constant OX063 concentration, 1/T₁ decreases with increasing frequency because the tumbling dependent processes that dominate relaxation at 250 MHz are less effective at higher frequency. 1/T₂ also decreases with increasing frequency because 1/T₁ is a significant contribution to 1/T₂ for trityl radicals in fluid solution. 1/T₂-1/T₁, the incomplete motional averaging contribution to 1/T₂, increases with increasing frequency. At constant frequency, relaxation rates increase with increasing radical concentration due to contributions from collisions that are more effective for 1/T₂ than 1/T₁. The collisional contribution to relaxation increases as the concentration of counter-ions in solution increases, which is attributed to interactions of cations with the negatively charged radicals that decrease repulsion between trityl radicals. The Signal-to-Noise ratio (S/N) of field-swept echo-detected spectra of OX063 were measured in the frequency range of 400 MHz-1 GHz. S/N values, normalized by √Q, increase as frequency increases. Adding salt to the radical solution decreased S/N because salt lowers the resonator Q. Changing the temperature from 19 to 37 °C caused little change in S/N at 700 MHz. Both slower relaxation rates and higher S/N at higher frequencies are advantageous for oximetry. The potential disadvantage of higher frequencies is the decreased depth of penetration into tissue.

Electron spin dynamics and spin-lattice relaxation of trityl radicals in frozen solutions

Electron spin-lattice relaxation of two trityl radicals, d24-OX063 and Finland trityl, were studied under conditions relevant to their use in dissolution dynamic nuclear polarization (DNP). The dependence of relaxation kinetics on temperature up to 100 K and on concentration up to 60 mM was obtained at X- and W-bands (0.35 and 3.5 Tesla, respectively). The relaxation is quite similar at both bands and for both trityl radicals. At concentrations typical for DNP, relaxation is mediated by excitation transfer and spin-diffusion to fast-relaxing centers identified as triads of trityl radicals that spontaneously form in the frozen samples. These centers relax by an Orbach-Aminov mechanism and determine the relaxation, saturation and electron spin dynamics during DNP.

Facile and High-Yielding Synthesis of TAM Biradicals and Monofunctional TAM Radicals

Facile and high-yielding procedures for synthesis of monocarboxylic acid derivatives of triarylmethyl radicals (TAMs) were developed. Reaction of methyl thioglycolate with tris(2,3,5,6-tetrathiaaryl)methyl cation smoothly afforded the monosubstituted TAM derivative, which was hydrolyzed to a monocarboxylic acid, with the TAM moiety attached to thioglycolic acid via the sulfur atom. Alternatively, the diamagnetic tricarboxylic acid precursor of Finland trityl was transformed to a trimethyl ester and partially hydrolyzed under controlled conditions. The diester product was isolated and the remaining fractions were converted back to the trimethyl ester for production of more diester. The first representatives of TAM biradicals with different TAM cores and interspin distances were obtained by reaction of these new TAM monocaboxylic acids with N,N'-dimethylethylenediamine.

Pulsed Dynamic Nuclear Polarization with Trityl Radicals

Continuous-wave (CW) dynamic nuclear polarization (DNP) is now established as a method of choice to enhance the sensitivity in a variety of NMR experiments. Nevertheless, there remains a need for the development of more efficient methods to transfer polarization from electrons to nuclei. Of particular interest are pulsed DNP methods because they enable a rapid and efficient polarization transfer that, in contrast with CW DNP methods, is not attenuated at high magnetic fields. Here we report nuclear spin orientation via electron spin-locking (NOVEL) experiments using the polarizing agent trityl OX063 in glycerol/water at a temperature of 80 K and a magnetic field of 0.34 T. (1)H NMR signal enhancements up to 430 are observed, and the buildup of the local polarization occurs in a few hundred nanoseconds. Thus, NOVEL can efficiently dynamically polarize (1)H atoms in a system that is of general interest to the solid-state DNP NMR community. This is a first, important step toward the general application of pulsed DNP at higher fields.

Paramagnetic spherical nanoparticles by the self-assembly of persistent trityl radicals

The self-association of a water-soluble persistent trityl radical gives rise to paramagnetic nanoparticles: a new class of non-metallic high spin systems.

Simultaneous DNP enhancements of (1)H and (13)C nuclei: theory and experiments

DNP on heteronuclear spin systems often results in interesting phenomena such as the polarization enhancement of one nucleus during MW irradiation at the "forbidden" transition frequencies of another nucleus or the polarization transfer between the nuclei without MW irradiation. In this work we discuss the spin dynamics in a four-spin model system of the form {ea-eb-((1)H,(13)C)}, with the Larmor frequencies ωa, ωb, ωH and ωC, by performing Liouville space simulations. This spin system exhibits the common (1)H solid effect (SE), (13)C cross effect (CE) and in addition high order CE-DNP enhancements. Here we show, in particular, the "proton shifted (13)C-CE" mechanism that results in (13)C polarization when the model system, at one of its (13)C-CE conditions, is excited by a MW field at the zero quantum or double quantum electron-proton transitions ωMW = ωa ± ωH and ωMW = ωb ± ωH. Furthermore, we introduce the "heteronuclear" CE mechanism that becomes efficient when the system is at one of its combined CE conditions |ωa - ωb| = |ωH ± ωC|. At these conditions, simulations of the four-spin system show polarization transfer processes between the nuclei, during and without MW irradiation, resembling the polarization exchange effects often discussed in the literature. To link the "microscopic" four-spin simulations to the experimental results we use DNP lineshape simulations based on "macroscopic" rate equations describing the electron and nuclear polarization dynamics in large spin systems. This approach is applied based on electron-electron double resonance (ELDOR) measurements that show strong (1)H-SE features outside the EPR frequency range. Simulated ELDOR spectra combined with the indirect (13)C-CE (iCE) mechanism, result in additional "proton shifted (13)C-CE" features that are similar to the experimental ones. These features are also observed experimentally in (13)C-DNP spectra of a sample containing 15 mM of trityl in a glass forming solution of (13)C-glycerol/H2O and are analyzed by calculating the basic (13)C-SE and (13)C-iCE shapes using simulated ELDOR spectra that were fitted to the experimental ones.