Conformation of the eosinophil chemotactic tetrapeptides and analogues in dimethyl sulfoxide. 1H n.m.r. studies.

Abstract

Proton nuclear magnetic resonance parameters are reported for DMSO-d6 solutions of the eosinophil chemotactic tetrapeptides, Val1-Gly2-Ser3-Glu4 and Ala1-Gly2-Ser3-Glu4, as well as three analogues of the Val1 tetrapeptide, D-Val1, Ala2 and Ala3. The synthesis of Val-(S)-[alpha-2 H1] Gly-Ala-Glu, in which the glycine has been stereospecifically deuterated in the H alpha 3 position, has allowed the assignment of the 1H resonances belonging to individual H alpha 2 and H alpha 3 glycine methylene protons. Simulation of the glycine ABX spin system yields two vicinal coupling constants which are consistent with a highly preferred conformation about the glycine HN-C alpha bond. The chemical shifts, coupling constants, temperature coefficients of amide proton chemical shifts and calculated side chain rotamer populations are reported for all peptides. The coupling constant analysis and temperature coefficients of amide proton chemical shifts together suggest that a type I beta-turn conformation is preferred by the Ala3 analogue. The 1H n.m.r. parameters of the other peptides suggest that these can also adopt a beta-turn conformation in DMSO. There are, however, considerable differences in the extent of conformational averaging undergone by the various peptides.