Introducing nuclear spin-noise spectroscopy

International audienceNMR spectra are usually obtained by exciting, through a rf field, the nuclear magnetization and then by monitoring the induction, it creates. An alternative approach, named spin noise, exists: it consists in searching for correlations in the noise signal at the probe detection output, a concept up to now used for only a single spin species. Here, we report its extension for looking to small signals in the presence of a major one and show that this technique allows sensitivity enhancement for their detection, in particular when the temperature of detection coil is lower than that of the sample. Signals resulting from small species appear as bumps, superimposed on the dip which results from the main component contribution. For the description and the processing of the experimental spectra, a new analytical equation is introduced. Its derivation is based on fluctuating rf fields due to the preamplifier and coil resistances and magnetization fluctuations and coherent rf field due to the feedback field (radiation damping). Its generality allows analytical explanation of the difference of tuning conditions and the treatment of static magnetic field inhomogeneity, which has an enhanced spectral signature in spin-noise spectra and provides a chemical shift reference. We have combined all these aspects and proved the enhanced detection capability of the nuclear spin-noise approach for characterizing secondary isotopic effect, proving the opening of a new "spin-noise" spectroscopy. Perspectives of this work in terms of classical NMR in the presence of radiation damping but also in terms of unconventional detection scheme will be discussed

Measuring Nano- to Microstructures from Relayed Dynamic Nuclear Polarization NMR

We show how dynamic nuclear polarization (DNP) NMR can be used in combination with models for polarization dynamics to determine the domain sizes in complex materials. By selectively doping a source component with radicals and leaving the target undoped, we Can measure experimental polarization buildup curves which can be compared with simulations based on heterogeneous distributions of polarization-within the sample. The variation of the integrated DNP enhancement as a function of the polarization time is found to be characteristic of the geometry. We demonstrate the method experimentally on four different systems where we successfully determine domain sizes between 200 and 20 000 nm, specifically in powdered histidine hydrochloride monohydrate) pore lengths of mesoporous silica materials, and two domain sizes in two component polymer film coatings. Additionally, we find that even in the apparently homogeneous frozen solutions used as polarization sources in most DNP experiments, polarization is relayed from protons near the radicals to the bulk of the solution by spin diffusion, which explains the experimentally observed buildup times in these samples

Signal enhancement in protein NMR using the spin-noise tuning optimum

We have assessed the potential of an alternative probe tuning strategy based on the spin-noise response for application in common high-resolution multi-dimensional biomolecular NMR experiments with water signal suppression on aqueous and salty samples. The method requires the adjustment of the optimal tuning condition, which may be offset by several 100 kHz from the conventional tuning settings using the noise response of the water protons as an indicator. Although the radio frequency-pulse durations are typically longer under such conditions, signal-to-noise gains of up to 22% were achieved. At salt concentrations up to 100 mM a substantial sensitivity gain was observed

High-Resolution H-1 Solid-State NMR Spectroscopy Using Windowed LG4 Homonuclear Dipolar Decoupling

We investigate the optimal implementation of windowed LG4 decoupling for the direct acquisition of high-resolution H-1 solid-state NMR spectra in the moderate magic-angle spinning regime ((r)<2 x 35kHz). We determine how the optimal value of the azimuth of the LG rotation axis, , is affected by the choice of windowing parameters. We find that for both the windowless and windowed implementation of LG4, the optimal value of is that for which the chemical-shift scaling factor is approximately 0.43. We then provide an analytical expression for estimating the chemical-shift scaling factor as a function of both and the windowed decoupling parameters, which allows the optimal value of to be easily determined for any implementation of LG4. We also introduce a simple and general method for improving the performance of any phase-modulated homonuclear dipolar decoupling sequence by compensating for pulse imperfection errors that accumulate in windowed spectra due to the repeated switching of the RF irradiation. This method involves the introduction of short (<0.5 mu s) compensation pulses at the start and end of each decoupling pulse, the duration and phase of which are chosen to cancel the effects of the amplitude and phase transients that arise as a result of the RF switching. We demonstrate how this method can be implemented for homonuclear dipolar decoupling sequences such as wPMLG and wLG4 to reduce the chemical-shift scaling factor distortions generated by phase transients such that high-quality, high-resolution H-1 NMR spectra can be acquired at arbitrary RF offsets

Determination of the xanthate group distribution on viscose by liquid-state 1H NMR spectroscopy

The final publication is available via https://doi.org/10.1007/s00216-010-4570-2.An analytical method for determination of the xanthate group distribution on viscoses based on liquidstate NMR spectroscopy was developed. Sample preparation involves stabilization of the xanthate group by allylation followed by derivatization of the remaining free hydroxyl groups at the glucose unit. The method was applied for studying (1) the γ-value (number of xanthate groups per 100 glucose units) of viscose, (2) the distribution of the xanthate groups on the anhydroglucose unit (AGU), and (3) changes of the xanthate group distribution during ripening. Results of the γ-value determination are well comparable with reference methods. Elucidation of the xanthate group distribution on the AGU gives the percentage at the C-6 position and a cumulative share of the positions C-2 and C-3. During ripening, xanthate groups at C-2 and C-3 degrade first, while xanthates at C-6 decompose at a slower rate

High-resolution NMR of hydrogen in organic solids by DNP enhanced natural abundance deuterium spectroscopy

International audienceWe demonstrate that high field (9.4T) dynamic nuclear polarization (DNP) at cryogenic (similar to 100 K) sample temperatures enables the rapid acquisition of natural abundance H-1-H-2 cross-polarization magic angle spinning (CPMAS) solid-state NMR spectra of organic solids. Spectra were obtained by impregnating substrates with a solution of the stable DNP polarizing agent TEKPol in tetrachloroethane. Tetrachloroethane is a non-solvent for the solids, and the unmodified substrates are then polarized through spin diffusion. High quality natural abundance H-2 CPMAS spectra of histidine hydrochloride monohydrate, glycylglycine and theophylline were acquired in less than 2 h, providing direct access to hydrogen chemical shifts and quadrupolar couplings. The spectral resolution of the H-2 solid-state NMR spectra is comparable to that of H-1 spectra obtained with state of the art homonuclear decoupling techniques

Nanostructure of Materials Determined by Relayed Paramagnetic Relaxation Enhancement

Particle and domain sizes strongly influence the properties of materials. Here we present an NMR approach based on paramagnetic relaxation enhancement (PRE) relayed by spin diffusion (SD), which allows us to determine lengths in the nm pm range. We demonstrate the method on multicomponent organic polymer mixtures by selectively doping one component with a paramagnetic center in order to measure the domain size in a second component. Using this approach we determine domain sizes in ethyl cellulose/hydroxypropyl cellulose film coatings in pharmaceutical controlled release formulations. Here we measure particle sizes ranging from around 50 to 200 nm

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

International audienceDynamic 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