Torsion Angle Constraints from Chemical Shifts

Torsion Angle Constraints from Chemical Shifts 

Chemical shifts are very sensitive probes of the molecular environment of a spin. However, in many cases their dependence on the structure is complicated and either not fully understood or too intricate to allow the derivation of reliable conformational constraints [37, 38]. An exception in this respect are the deviations of 13C (and, to some extent, 1 xf) chemical shifts from their random coil values that are correlated with the local 

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Torsion Angle Constraints from Chemical Shifts... 

A second approach is the semiquantitative use of chemical shifts, mostly from a-carbons, for restricting the range of backbone torsion angles in proteins. Statistical data show that deviations of these shifts from their values in unfolded polypeptides correlate with the presence of regular secondary structures. Thus, the resonances of a-carbons in a-helices are typically shifted down-field by about 3 ppm. those in P-sheets up-field by about 1.5 ppm. Measurements of a-carbon chemical shifts may therefore be converted to constraints on the backbone torsion angles 4> and of selected residues, and these constraints complement the information obtained from coupling constants. 

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