Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy

Dynamic nuclear polarization (DNP) is shown to greatly improve the solid-state nuclear magnetic resonance (SSNMR) analysis of synthetic polymers by allowing structural assignment of intrinsically diluted NMR signals, which are typically not detected in conventional SSNMR. Specifically, SSNMR and DNP SSNMR were comparatively used to study functional polymers for which precise structural elucidation of chain ends is essential to control their reactivity and to eventually obtain advanced polymeric materials of complex architecture. 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 increase in sensitivity afforded by the DNP setup (a factor ∼10 was achieved here), hence providing access to detailed structural characterization 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

Host–Guest Complexes as Water-Soluble High-Performance DNP Polarizing Agents

Dynamic nuclear polarization (DNP) enhances the sensitivity of solid-state NMR (SSNMR) spectroscopy by orders of magnitude and, therefore, opens possibilities for novel applications from biology to materials science. This multitude of opportunities implicates a need for high-performance polarizing agents, which integrate specific physical and chemical features tailored for various applications. Here, we demonstrate that for the biradical bTbK in complex with captisol (CAP), a β-cyclodextrin derivative, host–guest assembling offers a new and easily accessible approach for the development of new polarizing agents. In contrast to bTbK, the CAP-bTbK complex is water-soluble and shows significantly improved DNP performance compared to the commonly used DNP agent TOTAPOL. Furthermore, NMR and EPR data reveal improved electron and nuclear spin relaxation properties for bTbK within the host molecule. The numerous possibilities to functionalize host molecules will permit designing novel radical complexes targeting diverse applications

Word and the Music in the Contemporary Czech Melodrama - the Analytical Basis for the Music Education

Melodrama represents a unique form that combines musical and textual component in a very distinct manner. The submitted doctoral dissertation offers a complex analysis of the contemporary Czech melodrama and places it into a European and indispensable local context. The author emphasizes typical features of this genre and describes the possibilities of its further development by means of complex analysis of melodrama as a work of art together with the evaluation of its status in the contemporary musical culture. Complex analysis of the work of art is absolutely necessary in order to understand the addressed problematics. It is a starting point that offers a further insight into the main characteristics of the form, not only in an artistic sense but also in other contexts. It provides a space for analysis pursued in a pedagogical context, and it offers a base for the pedagogical interpretation of the artwork that is determinative for the analytical perspective of the submitted dissertation. Pedagogical dimension of this doctoral dissertation that focuses on melodrama fully corresponds. Melodrama is, in fact, a very appealing tool for the understanding of music. It allows us to react appropriately to the needs of contemporary education with emphasis on the use of the integrative and poly-aesthetic..

Rigid Orthogonal Bis-TEMPO Biradicals with Improved Solubility for Dynamic Nuclear Polarization

The synthesis and characterization of oxidized bis-thioketal-trispiro dinitroxide biradicals that orient the nitroxides in a rigid, approximately orthogonal geometry are reported. The biradicals show better performance as polarizing agents in dynamic nuclear polarization (DNP) NMR experiments as compared to biradicals lacking the constrained geometry. In addition, the biradicals display improved solubility in aqueous media due to the presence of polar sulfoxides. The results suggest that the orientation of the radicals is not dramatically affected by the oxidation state of the sulfur atoms in the biradical, and we conclude that a biradical polarizing agent containing a mixture of oxidation states can be used for improved solubility without a loss in performance

Rigid Orthogonal Bis-TEMPO Biradicals with Improved Solubility for Dynamic Nuclear Polarization

The synthesis and characterization of oxidized bis-thioketal-trispiro dinitroxide biradicals that orient the nitroxides in a rigid, approximately orthogonal geometry are reported. The biradicals show better performance as polarizing agents in dynamic nuclear polarization (DNP) NMR experiments as compared to biradicals lacking the constrained geometry. In addition, the biradicals display improved solubility in aqueous media due to the presence of polar sulfoxides. The results suggest that the orientation of the radicals is not dramatically affected by the oxidation state of the sulfur atoms in the biradical, and we conclude that a biradical polarizing agent containing a mixture of oxidation states can be used for improved solubility without a loss in performance

Optimizing Sample Preparation Methods for Dynamic Nuclear Polarization Solid-state NMR of Synthetic Polymers

This 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 <i>film casting</i> (FC), or by <i>glass forming</i> (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 <i>incipient wetness impregnation</i>, yielding overall sensitivity enhancements between 5 and 40

Spin Exchange Monitoring of the Strong Positive Homotropic Allosteric Binding of a Tetraradical by a Synthetic Receptor in Water

The flexible tetranitroxide <b>4T</b> has been prepared and was shown to exhibit a nine line EPR spectrum in water, characteristic of significant through space spin exchange (<i>J</i>_<i>ij</i>) between four electron spins interacting with four nitrogen nuclei (<i>J</i>_<i>ij</i> ≫ <i>a</i>_N). Addition of CB[8] to <b>4T</b> decreases dramatically all the <i>J</i>_<i>ij</i> couplings, and the nine line spectrum is replaced by the characteristic three line spectrum of a mononitroxide. The supramolecular association between <b>4T</b> and CB[8] involves a highly cooperative asymmetric complexation by two CB[8] (<i>K</i>₁ = 4027 M^–1; <i>K</i>₂ = 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 <i>reorganizes</i> 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 <b>4T</b>@CB­[8]₂ complex

Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K

A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (ε) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger ε. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in ε between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on ε is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g·mol^–1, is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large ε may be obtained at higher temperatures

Mitochondria-Targeted Spin Traps: Synthesis, Superoxide Spin Trapping, and Mitochondrial Uptake

Development of reliable methods and site-specific detection of free radicals is an active area of research. Here, we describe the synthesis and radical-trapping properties of new derivatives of DEPMPO and DIPPMPO, bearing a mitochondria-targeting triphenyl­phosphonium cationic moiety or guani­dinium cationic group. All of the spin traps prepared have been observed to efficiently trap superoxide radical anions in a cell-free system. The superoxide spin adducts exhibited similar spectral properties, indicating no significant differences in the geometry of the cyclic nitroxide moieties of the spin adducts. The superoxide adduct stability was measured and observed to be highest (<i>t</i>_1/2 = 73 min) for DIPPMPO nitrone linked to triphenyl­phosphonium moiety via a short carbon chain (Mito-DIPPMPO). The experimental results and DFT quantum chemical calculations indicate that the cationic property of the triphenyl­phosphonium group may be responsible for increased superoxide trapping efficiency and adduct stability of Mito-DIPPMPO, as compared to the DIPPMPO spin trap. The studies of uptake of the synthesized traps into isolated mitochondria indicated the importance of both cationic and lipophilic properties, with the DEPMPO nitrone linked to the triphenyl­phosphonium moiety via a long carbon chain (Mito₁₀-DEPMPO) exhibiting the highest mitochondrial uptake. We conclude that, of the synthesized traps, Mito-DIPPMPO and Mito₁₀-DEPMPO are the best candidates for potential mitochondria-specific spin traps for use in biologically relevant systems

Hosting Various Guests Including Fullerenes and Free Radicals in Versatile Organic Paramagnetic <b>bTbk</b> Open Frameworks

The dinitroxide bis­(TEMPO) bisketal (<b>bTbk</b>) was shown to crystallize into open frameworks whose structures were determined by single-crystal X-ray diffraction. We show that <b>bTbk</b> can be used as a supramolecular building block for the hosting of a plethora of guests inside the 1D channels of its paramagnetic framework, including other radicals such as TEMPO or 2-azaadamantane-<i>N</i>-oxyl. C₆₀ and C₇₀ were also found to be easily included in this open framework during its crystallization. This resulted in well-defined, nanostructured assemblies of composite radical crystals (<b>bTbk</b>/toluene/C₆₀ or C₇₀) or (<b>bTbk</b>/toluene/TEMPO) by a very simple dissolution/crystallization process with tunable guest content. Selective C₆₀ extraction was also demonstrated directly from fullerene soot