Decoupling homonuclear, 198 also

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

By analogy with homonuclear decoupling above, selective heteronuclear decoupling is also possible. The decoupling of a single proton resonance could in... 

Homonuclear single-frequency decoupling can also be used to simplify the multiplicities of other resonances. Flomonudear decoupling, however, can potentially overload the receiver, because irradiation may... 

We have so far looked at the NOE only in a homonuclear manner, but of course there is also a heteronuclear NOE. Theory tells us that when we are dealing with C-H fragments in small molecules, the decoupling of the proton leads to an increase in the carbon signal intensity by up to almost 200% So signals of protonated carbons should be stronger than those of non-proton-ated carbons. 

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... 

The UV spectrum (/Imax 218, 237, 260,299,325, and 338 nm) of tubingensin B (380) was very similar to that of tubingensin A (379), indicating the presence of a similar carbazole framework. Comparison of the H-NMR spectrum of tubingensin B (380) with that of tubingensin A (379) indicated that these two alkaloids are structural isomers with a similar 2,3-disubstituted carbazole substructure. This was also supported with homonuclear decoupling and COSY experiments. In addition,... 

Undesired homonuclear spin interactions can be also suppressed using suitable multiple-pulse sequences while still exploiting the information content provided by interactions that are not affected. Using a combination of MAS and pulse decoupling it is even possible to reintroduce parts of an interaction that would be averaged out by one of the manipulation techniques alone ( recoupling ) [11]. This high flexibility of solid-state NMR enables one to fully exploit the rich information content provided by the spin interactions. It becomes particularly powerful if such experiments are combined to multidimensional NMR techniques as discussed in Section 14.3. 

The spin-echo pulse sequence can also be applied to produce homonuclear (e.g., H) J-resolved spectra. For such a system it is not possible to apply broadband decoupling, so the F2 dimension might be expected to display the ordinary coupled H spectrum. However, because the coupling information is independently available, it is not difficult to process the data in such a way that only the chemical shifts are displayed in the F2 dimension, as illustrated in Fig. 10.7. 

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