Conformational study of two substance P hexapeptides by two-dimensional NMR

The conformation of two substance P (SP) related hexapeptides. Glp-Phe-Phe-(L-Pro)-Leu-Met.NH₂(I) and Glp-Phe-Phe-(D-Pro)-Leu-Met.NH₂ (II), in two solvents, chloroform-d and trifluoroethanol(TFE)-d₃/H₂O, was studied by two-dimensional NMR methods, including COSY, TOCSY, ROESY and HMQC. The study shows that these two peptides exist predominantly in the extended form in TFE/H₂O, but in general exhibit a reverse-turn structure in chloroform. I is clearly less ordered than II in both solvents. Furthermore, extensive Phe³-Pro⁴cis?trans isomerization was found in I but not in II. The differences in the conformational behavior of these two peptides, which are selective agonists for neurokinin NK1 and NK2 receptors, respectively, are discussed.