Amide proton chemical shift changes

Figure 1. Amide proton chemical shift changes in the trp repressor-mtr ODNA operator upon complex formation.

The effect of a change in temperature or solvent on chemical shift is a parameter which must be carefully studied. On the basis of several examples it was proposed that evidence for hydrogen bonding (36) could be derived from the failure of an amide-proton chemical shift to be sensitive to a change in temperature. Later observations from this laboratory have shown that the chemical shift of certain amide protons not hydrogen bonded also may not be sensitive to a change in temperature. 

Hybrid amide-thiourea hosts based on p-f-butylcalix[4]arene have been prepared with coloured p-nitrophenyl (11.37a) or fluorescent 1-napthyl substituents (11.37b). In DMSO the host binds strongly to dicarboxylates, particularly adipate (log/T= 2X10" M" ), accompanied by a colour change from yellow to red in the case of the nitrophenyl derivative. The colour change is reversible on addition of protic solvents such as methanol but is not observed for acetate or other basic, monovalent anions. While large chemical shift changes for the thiourea NH protons are observed for 11.37b (A5 up to ca. 3.5 ppm), the thiourea NH resonances for the more acidic 11.37a disappear due to deprotonation. The fluorescence of the napthyl substituent in 11.37b is significantly quenched in the absence of anions due to a PET process. Addition of dicarboxylates results in an increase of fluorescence emission intensity in a broad band from 410 - 600 nm. Addition of anion reduces the efficiency of the PET process. 

Fig. 45. Chemical-shift differences of the amide protons of a) CaM/C20W and b) CaM/MI3 complex with respect to free (Ca )4-CaM. For the CaM/C20W complex, major changes occur only in the C-terminal domain of CaM, indicating that the peptide C20W binds only to this domain. This is in contrast to the CaM/M 13 complex where both domains are involved in binding the peptide M13, and hence chemical-shift changes can be observed over the complete amino acid sequence of CaM.

The chemical shift may reveal the position of one atomic grouping with respect to another by anisotropic shielding. This is an effect likely to be sensitive to temperature and solvent changes as seen in the valine amide-proton resonance in Figure 3. 

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