Exchange rates, amide protons

The formation of an intramolecular H-bond is supported by the slower rate of amide proton exchange in pyridine/10% CD3OD. The influence of the stereochemistry on turn formation and turn geometry has been investigated. Seebach and coworkers have demonstrated that dipeptide sequences consisting... 

The number of NMR parameters available for measurement is rather small, consisting of the chemical shift, relaxation rates (/1 and lo), scalar (J) couplings, dipolar (D) couplings, cross-relaxation rates (the NOE), and hydrogen exchange rates. All of these have been quantified for many of the amide protons of A131 A, and most of the data suggest the presence of little persistent structure. 

Amide proton temperature coefficients and hydrogen exchange rates can provide information about hydrogen-bonding interactions and solvent sequestration in unfolded or partly folded proteins (Dyson and Wright, 1991). Abnormally low temperature coefficients, relative to random coil values, are a clear indication of local structure and interactions. 

Scheme 19), displays interesting CEST properties when its amide NH resonance is irradiated.192 As the exchange rate of the amide protons is base catalyzed, the complex proves to be an efficient pH-sensitive probe in the 5.5-8.2 pH range. 

Significantly reduced rates for proton-deuterium exchange prove that the corresponding amide proton is either involved in stable hydrogen bonds, shielded from solvent access or both. Because of the ambiguity in interpretation, additional information about the persistence of hydrogen bonds stemming from structure calculations or from relaxation data should be available. 

Whereas 15N longitudinal and transverse relaxation rates can be determined with sufficient precision, the determined values of the 15N 1H -NOE differ significantly from the true values for residues involved in fast amide proton exchange. A comparison between the values for the 15N 1H -NOE for NPY in solution in the presence and in the absence of DPC is displayed in the Fig. 5.9. The comparison reveals two striking differences. 

Significant improvements in the appearance of the NMR spectrum of molten globule states are, however, observed upon addition of small amounts of trifluoroethanol (TFE), less than 10 vol%. Amide proton exchange rates are reduced and resonances are sharpened although the chemical shift dispersion is also decreased [51]. The net result is an increased resolution that simphfies considerably the spectral assignment. The increased spectral resolution is in fact due to the fact that the peptides become more denatured rather than more structured and that, in order to obtain more structural information about the folded state, the peptide is partially unfolded ... 

Nonenzymatic cleavages 148 Exchange rates of amide protons... 

When the proton does not exchange rapidly as on the N-H of the amide group, normal coupling is observed. Since the rates of proton exchange are often critically dependent on the solution conditions, coupling to acidic protons is variable and thus may or may not be observed. 

---