Chemical shift dispersion

The third alternative is a more robust, sensitive and specialized fonn of the first, in that only hydrogen nuclei indirectly spin-spin coupled to in a specific molecular configuration are imaged. In achieving selectivity, the technique exploits the much wider chemical shift dispersion of compared to H. The metliod involves cyclic transfer from selected H nuclei to indirectly spin-spin coupled C nuclei and back according to the sequence... 

Phosphorus nuclei have been used for many years in in vivo NMR, especially for intracellular pH measurements. However, because most organic phosphates have similar chemical shifts, compound identification can be difficult without special attention being paid to culture conditions in the NMR tube.15 Carbon NMR also yields significant results because of the large chemical shift dispersion and narrow lines of this nucleus. 13C spectra reflect most of the chemical rearrangements that may take place between substrate and final product. 

Table 1. Chemical shift dispersion in ppm of the designed and naturally occiuring foiu-helix bundle proteins a2D [1], GTD-43 [2], SA-42 [42] and lL-4 [78] ...

The chemical shift dispersion (Table 1) and the temperature dependence of the resonance hne shape provides a qualitative measure of whether the structure is well ordered [2]. However, NMR spectroscopy also provides information relevant to the problem of protein folding in the study of the molten globule states. NMR spectroscopic investigations of molten globules may be more demanding than those of ordered proteins due to spectral overlap arising from poor shift dispersion and to short relaxation times that are due to conformational exchange at intermediate rates on the NMR time scale. 

Significant improvements in the appearance of the NMR spectrum of molten globule states are, however, observed upon addition of small amounts of trifluoroethanol (TFE), less than 10 vol%. Amide proton exchange rates are reduced and resonances are sharpened although the chemical shift dispersion is also decreased [51]. The net result is an increased resolution that simphfies considerably the spectral assignment. The increased spectral resolution is in fact due to the fact that the peptides become more denatured rather than more structured and that, in order to obtain more structural information about the folded state, the peptide is partially unfolded ... 

Fig. 7. One-dimensional NMR spectra of the designed four-helix bundles SA-42 (lower trace) and GTD-43 (top two traces). The chemical shift dispersion of SA-42 in 90% H2O and 10% D2O at 323 K and pH 4.5 is poor and the resonances are severely broadened due to conformational exchange. The chemical shift dispersion of GTD-43 in the same solvent at 288 K and pH 3.0 is comparable to that of the naturally occurring four-helix bundle IL-4 and the resonances are not significantly affected by conformational exchange. Upon raising the temperature to 298 K line broadening is observed (top trace) which shows that GTD-43 is in slow exchange on the NMR time scale, unlike SA-42 where an increased temperature reduces the line width. These spectra are therefore diagnostic of structures with disordered (SA-42) and ordered (GTD-43) hydrophobic cores...

C NMR chemical shifts are calculated for the carbon nuclei in PVA to the pentad and hexad levels of stereosequence for the methine and methylene carbons, respectively. The RIS model developed by Wolf and Surer (A 069) is employed to calculate the frequencies. The relative orders of the observed methine pentad and methylene hexad resonances agree with the calculated chemical shifts, in addition to the agreement between the overall chemical shift dispersions measured and predicted for the methylene carbons. 

Standard (gel-phase) NMR spectra of polymers usually show significant line broadening, mainly because of chemical shift anisotropy and dipolar coupling [98], Only nuclei with strong chemical shift dispersion, e.g. 13C [99-106], 15N [107], 19F [108-112], and 31P [113] give sufficiently resolved gel-phase NMR spectra. The resolution of... 

Both the quality of TFP resin and the resulting polymeric-activated resins may be quantitatively determined by 19F NMR spectroscopy.1 This nondestructive technique takes advantage of the high sensitivity, natural abundance (the 1/2 spin nucleus 19F is 100% abundant), and large chemical shift dispersion ( 200 ppm) of the 19F nucleus. The 19F NMR spectrum of TFP resin shows two resonances at 148 and 165 ppm (Fig. 3), each corresponding to 2 equiv. fluorine nuclei. The 19F NMR spectrum of TFP-activated carboxylate and sulfonate resins shows two resonances at ca. 

In spite of the limitations of this model, the resulting shifts capture many aspects of the observed chemical shift dispersion, but have some key limitations that are discussed in the final section. We believe that this model captures enough of the key physics involved in shift dispersion in DNA duplexes to provide a useful foundation for analyses of new structures and for the development of improved theories. 

As we discussed in the introduction, there are many potential contributions to chemical shift dispersion in nucleic acids. Here we have considered two that are likely to be the most important ring current contributions from the aromatic bases,... 

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