Chemical shift deviation

Although most substituted coumarins give rise to signals which agree with those calculated by the normal additivity of substituent chemical shifts, deviations from the predicted values are observed for C-6 and C-7 and for the carbon atoms of the substituents in 6,7-dimethylcoumarin (8IT2021). These ortho proximity effects have been noted in aromatic... 

Si NMR offers a unique method for characterization of these complexes since their chemical shifts deviate greatly from the normal range found for most tetravalent silicon compounds. A number of different types of extracoordinate silicon complexes have been prepared and characterized (125) and their Si NMR chemical shifts determined. The chemical shift data are in Table XVIII. 

The Ala-Pro-Tyr peptide is modelled with the N- and C-termini blocked with acetyl and methylamide groups respectively (see Figure 2). The type a and b dsproline-aromatic interactions are respectively characterised by strong chemical shift deviations on the a proton of the alanine residue and on the yl proton of the proline residue that are mainly due to the ring current effect of the tyrosine residue. This property will be used to identify the different crsproline-aromatic interactions. 

Figure 5. Two families of conformations of Ala-cisPro-Tyr extracted from the PDB analysis [34,7] displaying the cwproline-aromatic interaction. The first family (left) corresponds to all the hits that display a chemical shift deviation (due to the ring current effect of the aromatic residue) < -0.25 ppm on the a proton of the alanine residue. The second conesponds to the family of conformations that display a chemical shift deviation < -0.25 ppm on the yl proton of the proline residue.

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.

In order to scrutinize this finding in more detail, we investigated the percentage maximum positive or negative H NMR chemical shift deviation A ax and A in, respectively, i.e., the difference between the experimentally determined values and the respective value of a Unear function connecting the shift values of the pure... 

Fig. 15 Maximum percentage H NMR chemical shift deviation on the example of binary mixtures of [C2mim][OAc] and [C2mim][BF4] at 20° C...

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