Side-chain decoupling

Assignment to an individual residue in the amino acid or nucleotide sequence is also possible in favorable cases by purely spectroscopic techniques. The method for such an assignment is as follows (1) By side-chain decoupling identify all lines in the... 

These results demonstrate that side-chain liquid crystalline polymers can be synthesized by polymer analogous reactions from theoretically any polymer backbone. When the polymer backbone is rigid, as in the case of PPO, a long spacer is required both to decrease the Tg of the parent polymer and to partially decouple the... 

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

With diastereomerically pure 32 now available, it was possible to determine its stereochemistry. Regiochemistry about the vinyl silane moiety in 32 was ascertained by nuclear Overhauser enhancement difference (DNOE) experiments. Decoupling experiments with either downfield methylene proton adjacent to the carboxylic acid 8 2.62 (dd, 1 H, J = 9.5,15.0 Hz) or 8 2.48 (dd, 1 H, J = 5.9, 15.0 Hz) identified the C-6 proton resonance 8 2.78 (m, 1 H). Similarly, the C-2 proton resonance was located [8 2.11 (m, 1H)]. Irradiation of the vinylsilane proton singlet at 8 5.38 led to an enhancement of 10% of the C-6 and none of the C-2 proton resonance, thus demonstrating a syn relationship between the vinyl proton and the C-6 proton. The fact that acid 32 was converted to the natural product confirmed that a p-oriented side chain had been produced at C-6 and clinched the stereochemistry of 32 as depicted. 

To make this into a 3D experiment we need to create a third time domain, in this case a time domain that encodes the chemical shift of the Hn proton. We simply stop for a moment on our journey from 15N SQC to Hn SQC to Ha and side-chain H coherence, at the point where we have an Hn coherence, and insert an evolution delay to indirectly record the chemical shift of the Hn- The pulse sequence is shown in Figure 12.46 (center) and the coherence pathway is diagramed in Figure 12.47. The new evolution delay is called f2 because it is the second independent time domain, forcing us to rename the direct time domain of the FID as 3. In the center of the t2 evolution delay there is a 15N 180° pulse to reverse the 1Jnh coupling evolution so that the Hn will not be split by 15N in the F2 dimension, just as the t evolution delay includes a 180° pulse in the center to decouple ... 

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

Prepolymerized lipids form vesicles only if the disentanglement of the polymer main chain ( = hack hone) and the membrane forming side-chains is simplified hy a hydrophilic spacer between them . Efficient decouplings of the motions of the polymeric chain and the polymeric bilayer are thus achieved and stable liposomes with diameters of around 500 nm were formed upon ultrasonication (Figure 4.28a). Their bilayer showed a well-defined melting behaviour in DSC. The ionene polymer with C12, C16 and C20 intermediate chains also produced vesicles upon sonication (Figure 4.28b). Here, the amphiphilic main chain is obviously so simple that ordering to form membranes produces no problems whatsoever . ... 

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