Abstract
The amide hydrogen exchange rates of peptides (and other biomolecules) can be related to their secondary and tertiary structures. The exchange rates in the range ∼0.1 to ∼100 s<sup>−1</sup> can be conveniently measured in H<sub>2</sub>O by a double resonance technique involving a combination of two separate <sup>1</sup>H NMR experiments: (1) the transfer of solvent saturation and (2) the selective saturation recovery of the labile hydrogen. When a peptide exists in a dynamic equilibrium among several conformations, the interpretation of the above two experiments in terms of intrinsic exchange rates is not straightforward in general. The effect of a multistate conformational averaging is considered in the interpretation of the above two experiments, and analytical expressions have been presented for a few limiting cases of conformational and chemical equilibria. The amide hydrogen exchange rates of the pentapeptide Arg-Lys-Asp-Val-Tyr, the active fragment of the thymic hormone thymopoietin, have been measured in H<sub>2</sub>O through the transfer of solvent saturation and saturation recovery experiments. The results suggest that this fragment exists in a highly motile dynamic equilibrium among several conformations. In some of these, the Val<sup>4</sup> NH hydrogen is shielded significantly from the solvent.
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