International audienceThis work compares the overall sensitivity enhancements provided by dynamic nuclear polarization (DNP) for the solid-state NMR characterization of polymer samples doped with biradicals and prepared either by film casting (FC), or by glass forming (GF) using 1,1,2,2-tetrachloroethane as the solvent. Analysis of amorphous and semicrystalline polymers (polystyrene, poly(ethylene oxide), polylactide, poly(methyl methacrylate)) of varying molecular weights showed that GF provided larger sensitivity enhancements than FC but yielded DNP-enhanced 13C CPMAS spectra of lower resolution for semicrystalline polymers, owing to line-broadening due to conformational distribution of the polymer chains in frozen solution. Moreover, use of deuterated solvents significantly reduced the intensity of the solvent signals in the DNP-enhanced 13C CPMAS spectra of polymers prepared by GF, while preserving the sensitivity enhancement observed for the polymer signals. For the polymers investigated here, both FC and GF performed better than incipient wetness impregnation, yielding overall sensitivity enhancements between 5 and 40

International audienceDynamic nuclear polarization (DNP) is shownto greatly improve the solid-state nuclear magnetic resonance(SSNMR) analysis of synthetic polymers by allowing structuralassignment of intrinsically diluted NMR signals, which aretypically not detected in conventional SSNMR. Specifically,SSNMR and DNP SSNMR were comparatively used to studyfunctional polymers for which precise structural elucidation ofchain ends is essential to control their reactivity and toeventually obtain advanced polymeric materials of complexarchitecture. Results show that the polymer chain-end signals,while hardly observable in conventional SSNMR, could be clearly identified in the DNP SSNMR spectrum owing to the increasein sensitivity afforded by the DNP setup (a factor ∼10 was achieved here), hence providing access to detailed structuralcharacterization within realistic experimental times. This sizable gain in sensitivity opens new avenues for the characterization of“smart” functional polymeric materials and new analytical perspectives in polymer science

T 1D , the relaxation time of dipolar order, is sensitive to slow motional processes. Thus T 1D is a probe for membrane dynamics and organization that could be used to characterize myelin, the lipid-rich membrane of axonal fibers. A mono-component T 1D model associated with a modified ihMT sequence was previously proposed for in vivo evaluation of T 1D with MRI. However, experiments have suggested that myelinated tissues exhibit multiple T 1D components probably due to a heterogeneous molecular mobility. A bi-component T 1D model is proposed and implemented. ihMT images of ex-vivo, fixed rat spinal cord were acquired with multiple frequency alternation rate. Fits to data yielded two T 1D s of about 500 µs and 10 ms. The proposed model seems to further explore the complexity of myelin organization compared to the previously reported mono-component T 1D model.

A straightforward method is reported to quantitatively relate structural constraints based on (13)C-(13)C double-quantum build-up curves obtained by dynamic nuclear polarization (DNP) solid-state NMR to the crystal structure of organic powders at natural isotopic abundance. This method relies on the significant gain in NMR sensitivity provided by DNP (approximately 50-fold, lowering the experimental time from a few years to a few days), and is sensitive to the molecular conformation and crystal packing of the studied powder sample (in this case theophylline). This method allows trial crystal structures to be rapidly and effectively discriminated, and paves the way to three-dimensional structure elucidation of powders through combination with powder X-ray diffraction, crystal-structure prediction, and density functional theory computation of NMR chemical shifts.

International audienceHigh-field dynamic nuclear polarization (DNP)may enhance the sensitivity of solid-state NMR experimentson a wide range of systems, including synthetic polymers,owing to the transfer of electron spin polarization from radicalsto nuclei upon microwave irradiation (usually at cryogenictemperatures). Provided that the radicals are homogeneouslydispersed in the sample, a uniform DNP enhancement isexpected for all the signals in the 13C cross-polarization magicangle spinning (CPMAS) spectrum. Here, we show that, in thecase of methyl group containing polymers, a change in thecross-polarization (CP) dynamics induced by the moderate increase in sample temperature due to microwave irradiation maylead to the observation of apparent nonuniform enhancements in the DNP-enhanced 13C CPMAS spectra. This peculiarbehavior should be accounted for when measuring 13C CP DNP enhancements on polymer materials, especially forheterogeneous polymer samples (for which truly nonuniform DNP enhancements could potentially be detected), or whenquantitative results are sought

Melanin is the most widespread pigment in the animal kingdom. Despite its importance, its detailed structure and overall molecular architecture remain elusive. Both eumelanin (black) and pheomelanin (red) occur in the human body. These two melanin compounds show very different responses to UV-radiation exposure, which could relate to their microscopic features. Herein, the structural properties and motional behavior of natural eu- and pheomelanin extracted from black and red human hair are investigated by means of solid-state NMR spectroscopy. Several 1D and 2D NMR spectroscopic techniques were combined to highlight the differences between the two forms of the pigment. The quantitative analysis of the (1) H NMR wide-line spectra extracted from 2D (1) H-(13) C LG-WISE experiments revealed the presence of two dynamically distinguishable components in both forms. Remarkably, the more mobile fraction of the pigment showed a higher mobility with respect to the proteinaceous components that coexist in the melanosome, which is particularly evident for the red pigment. An explanation of the observed effects takes into account the different architecture of the proteinaceous matrix that constitutes the physical substrate onto which melanin polymerizes within the eu- and pheomelanosomes. Further insight into the molecular structure of the more mobile fraction of pheomelanin was also obtained by means of the analysis of 2D (1) H-(13) C INEPT experiments. Our view is that not only structural features inherent in the pure pigment, but also the role of the matrix structure in defining the overall melanin supramolecular arrangement and the resulting dynamic behavior of the two melanin compounds should be taken into account to explain their functions. The reported results could pave a new way toward the explanation of the molecular origin of the differences in the photoprotection activity displayed by black and red melanin pigments.

Giulia 2019. A strategy for probing the evolution of crystallization processes by low-temperature solid-state NMR and dynamic nuclear polarization.

Complementary results from 13C intermolecular nuclear Overhauser effects (NOE), 1H-13C heteronuclear Overhauser spectroscopy (HOSEY) and 1H-NMR diffusion measurements were used for probing the structure of the first solvation shell of uridine in water. It is demonstrated that a cyclic dihydrate is formed. The two water molecules produce two hydrogen bonds with the two oxygen atoms from the pyrimidine ring and accept only one hydrogen bond from the amide proton. The dihydrate has only a short lifetime as compared with the rotational correlation time of the free nucleoside. The chemical exchange constant of the amide proton with water is then estimated by diffusion experiments. The results are consistent with previous data obtained for uracil in water and provide interesting information about water accessibility in nucleic acid bases.