Proton decoupling heteronuclear

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... 

In the case of pharmaceutical solids that are dominated by carbon and proton nuclei, the dipole-dipole interactions may be simplified. The carbon and proton nuclei may be perceived as dilute and abundant based upon then-isotopic natural abundance, respectively (Table 1). Homonuclear 13C—13C dipolar interactions essentially do not exist because of the low concentration of 13C nuclei (natural abundance of 1.1%). On the other hand, H—13C dipolar interactions contribute significantly to the broad resonances, but this heteronuclear interaction may be removed through simple high-power proton decoupling fields, similar to solution-phase techniques. 

Fig. 31. Stacked plot of the heteronuclear two-dimensional J-resolved spectrum of cured, carbon black filled, natural rubber. The proton flip experiment was used with high-power proton decoupling during the detection time. The experiment was performed with the sample spinning at the magic angle (reprinted from Ref. 1911 with permission)...

This is the simple heteronuclear -X COSY (or HETCOR) pulse sequence adapted to long-range correlations simply by adjustment of the Ai and A2 delays. Small coupling constants rule out the use of broad-band proton decoupling during evolution time t by BIRD, and thus the correlation cross-peaks display the structure of proton multiplets, which reduces further the sensitivity of the experiment. 

It is known that, at the HORROR condition, homonuclear dipolar couplings between xHs are reintroduced, leading to partial removal of the heteronuclear coupling.28 29 It is worth noting that at 60 and 120 kHz the proton decoupling is insufficient due to the rotary resonance effect. It has been shown theoretically by Emsley and co-workers,19 that the first and second rotary resonances are partially quenched by the presence of homonuclear couplings. 

When we discussed heteronuclear, 3C-H spin coupling (Section 8.6.1), we noted that all the l3C spectra presented in Chapters 5 and 7 had been proton decoupled, because in the absence of such decoupling the spectra would have been considerably more complex. Now we are ready to see how the effects of l3C-H spin coupling can be erased when acquiring a l3C spectrum. 

The proton-decoupled proton spectra allowed a distinction to be made between homo-and heteronuclear spin couplings, and Laurie and coworkers also demonstrated nulling of residual solvent resonances during the 2D /-resolved NMR of uridine in aqueous solution, wrote software for 45° tilting of the 2D spectra, and developed experimental protocols for multiple data-acquisition and processing, and a method for acquisition of the 2D /-resolved spectra in phase-sensitive mode. Lately, the 2D /-resolved technique has been less used, as it yields little evidence for spectral assignments. 

In the previous article (1) on heteronuclear double resonance in this series it was suggested that such experiments would soon vie for importance with homonuclear ones. In fact, there has been an almost total eclipse partly as a result of the use of proton decoupling in all work, partly because of improved methods of frequency generation and control which have made experimental distinctions between the two types of experiment much less important. The present review therefore deals with both homo- and hetero-nuclear experiments and includes multiple resonance work also. The seven years up to mid-1978 are covered, although it has been impossible to mention every experiment. Emphasis is laid upon new ideas and developments of technique, with some preference for the more recent work. The Chemical Society specialist reports on NMR spectroscopy have included regular articles on multiple resonance (2,3) and a number of reviews deal with various aspects of the subject. (4-19) It has been decided to omit work on the solid state. 

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