Solution Conformation of Peptides through 1 H Nuclear Magnetic Double Resonance Measurement of Amide Hydrogen Exchange Rates: Study of the Active Pentapeptide Fragment of Thymopoietin

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⁻¹ can be conveniently measured in H₂O by a double resonance technique involving a combination of two separate ¹H 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 H₂O 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 Val⁴ NH hydrogen is shielded significantly from the solvent.