Chemical shifts spectral dispersion

Dispersion interactions have been shown in the absence of other effects to be responsible for gas-to-liquid changes of chemical shifts 1>2). The dispersion contribution to the electric field effect on infrared and ultraviolet spectral transitions has been shown to be proportional to McRae term 10 n)... 

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 ... 

The much wider chemical shift dispersion of 13C results in a significantly higher spectral resolution than in the corresponding H spectra. However, the sensitivity is much lower compared to lU NMR as a consequence of the low natural abundance... 

Proton and phosphorus NMR spectra of DNA in solution show very large line widths and poor spectral resolution despite recent demonstrations of time-averaged flexibility of the doublehelical state (12-15). We have circumvented the sequence dependent dispersion of the chemical shifts by investigating synthetic DNA s of defined repeating nucleotide sequence.[1]... 

The spectral dispersion for organosilicones may be considerable for certain families of compounds. This is reflected in the Si chemical shifts of siloxanes, -(SiRR 0) -, an important class of compounds which includes resins, fluids, room-temperature vulcanized and heat-cured rubber consumer products. The first, Si NMR results (5,78) reported on polydimethylsiloxanes showed that individual resonance... 

There have been some very thorough structural studies conducted on MO, (78,86-88) MT, (78,82) and DT (79) systems using Si NMR. Harris and Newman (87) have looked at the trimethylsilylation products of wollastenite, a form of anhydrous calcium metasilicate, and pseudo-wollastenite, a crystalline modification of that mineral. The chemical shifts of the four model compounds listed in Table XI were compared in order to determine the degree of spectral dispersion that would occur in the many polymeric structures formed from trimethylsilylation of the decomposed calcium silicate. 

The chemical shift range for C is greater Sc = 0—250 ppm) than for ( H = 0—15 ppm) giving greater spectral dispersion, i.e. the peaks for carbons with very sMght differences in chemical shifts are separated and... 

Several homonuclear 3D NMR experiments, such as TOCSY-NOESY, ROESY-COSY, or TOCSY-COSY, have been employed to reduce spectral overlap, however, with limited success due to the generally poor proton chemical shift dispersion of complex oligosaccharides.3 5 10 13 14... 

Similar connectivities can also be established for the (H2, H6/H8) and (H3, H6/H8) cross-peaks, and, in the case of DNA, also for cross-peaks entailing H2" and H6/H8, although the chemical shift dispersion is less favorable for H2 and H3 protons in RNA. Nevertheless, these spectral regions can also be very useful when used in combination with the walk in the anomeric-to-aromatic region. Figure 10 shows examples of ID slices of 2D NOESY spectra through frequencies of aromatic protons for DNA and RNA, and Table 1 lists most common cross-peaks expected for residues in helical regions. 

For unlabeled peptides purified from natural sources, one is generally limited to two-dimensional (2D) NMR experiments (although some 3D experiments are possible as outlined in Section 9.09.3.2.5). Therefore, determination of the structure of even small peptides can become problematic due to spectral overlap. The limit at which we are able to conduct NMR structural studies of peptides depends on the chemical shift dispersion of each individual protein, but generally speaking we can expect unlabeled peptides of up to 10 kDa (90-95 amino acid residues) to be amenable to structure determination using homonuclear NMR. 

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