Torsion from chemical shifts

Torsion Angle Constraints from Chemical Shifts... 

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). 

The apparent relationship between torsion angle and 170 chemical shift has been explored further by examination of the data for several ortho alkyl-substituted nitrobenzenes and dinitrobenzenes (Table 23). The deshielding values of the shifts reflect increasing nitrogen-oxygen double bond character, which is expected as the nitro group is rotated from the plane of the aromatic ring. 

FIGURE 6. Plot of torsional angle between aromatic rings and nitro groups vs the 170 chemical shift (ppm) data. Reprinted with permission from D. J. Craik, G. C. Levy and R. T. C. Brownlee,. /. Org. Chem., 48, 1601 (1983). Copyright (1983) American Chemical Society... 

Shen Y, Bax A. Protein backbone chemical shifts predicted from 30. searching a database for torsion angle and sequence homology. 

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