Chemical shift 246 INDEX

D. S. Wishart, B. D. Sykes and F. M. Richards, The chemical shift index A fast and simple method for the assignment of protein secondary structure through NMR spectroscopy, Biochemistry, 1992, 31, 1647-1651. 

J. S. Fruchart, G. Lippens, R. Warrass, C. Seetharaman, C. Dhalluin and C. Boutillon, The chemical shift index method applied to resin-bound peptides, J. Pept. Res., 2000, 56, 346-351. 

Fig. 6. Structural analysis of Vc 1.1 by NMR. (A) The aH secondary shifts. (B) Structural information from NOEs, chemical shift index (CSI), amide exchange, and scalar coupling, are summarized (adapted from ref. 58). (C) A stereoview of the 20 low-energy structures. Some residues are numbered for orientation. (D) Ribbon representation of the peptide the cysteine residues are in yellow. The N-terminus and C-terminus are marked N and C, respectively. (E) The surface of the peptide with selected residues labeled.

Different methods exist that predict protein secondary stmcture elements from chemical shift values [e.g.. Chemical Shift Index (CSI) (3) and Probability-based protein Secondary Stmcture Identification using combined NMR chemical-shift data (PSSI)] (4). These methods are based on the statistics of database analysis of chemical shift values from a range of peptides and proteins of known secondary stmcture. Chemical shifts have also been used to predict backbone torsion angles by software such as Torsion Angle Likelihood Obtained from Shifts and Sequence similarity (TALOS) (5). 

Figure 2. Chemical Shift Index plots of peptides 1, 2, 3 and 4. Arrows indicate the location of P-sheets in these peptides.

Clusters of three or more positive chemical shift index (CSI) values are indicative of a p-sheet. Overall, these results suggest that it is possible to greatly increase the solubility of this p-sheet model without significantly disrupting the structure. They also suggest that some residues (His in particular) can disrupt the type 11 p-tum and eliminate most of the p-sheet structure. This result also implies that it may be possible to use host-guest techniques (17) to study type IF P-tum propensities with this system. 

Spectra were referenced using TSP in the proton dimension. In N and C dimension the shifts were referenced by calculation from the ratio of the zero point frequencies of the standard and corrected for the temperature (Edison et al. 1994). Chemical shift deviation was calculated for H , C , CO and C nuclei. Consensus chemical shift index was calculated using program CSl (Wishart and Sykes, 1994). 

Figure 3. Chemical shift deviation of the assigned residues compared to the random coil values for the first twelve amino terminal residues. Bottom panel shows the consensus chemical shift index for the complete protein.

NIGS index (= Nucleus-Independent Chemical Shift index)... 

Fig. 30. Chemical-shift index of resonances of CaM complexed to the peptide C20W. Low-field shift are indicative for a-heUces and high field shifts for /3-strand. The chemical-shift index is correlating with the secondary structure found in the NMR-derived structure (arrows for /3-strands, boxes for a-helices).

NICS nucleus-independent chemical shift index... 

Aromaticity is associated with delocalization of bonds and like the concept of a chemical bond itself, it is ill-defined. Various properties of molecules can be used to construct measures of aromaticity [51-53], Rigorous approaches estimating energy of the resonance stabilization are computationally expensive. A magnetic-property-based criterion of aromaticity called nuclear-independent chemical shift index (NICS) is a widely accepted measure of local aromatic character of a molecule... 

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