Conformations of synthetic alamethicin fragments. Evidence for 3 helical folding from 270-MHz hydrogen-1 nuclear magnetic resonance and circular dichroism studies

Abstract

¹H NMR studies at 270 MHz on the synthetic alamethicin fragments Z-Aib-Pro-Aib-Ala-Aib-Ala-OMe (1-6), Boc-Gln-Aib-Val-Aib-Gly-Leu-Aib-OMe (7-13), Boc-Leu-Aib-Pro-Val-Aib-OMe (12-16), and Boc-Gly-Leu-Aib-Pro-Val-Aib-OMe (11-16) have been carried out in CDCl₃ and (CD₃)₂SO. The intramolecularly hydrogen bonded amide hydrogens in these peptides have been delineated by using solvent titration experiments and temperature coefficients of NH chemical shifts in (CD₃)₂SO. All the peptides adopt highly folded structures, characterized by intramolecular 4 → 1 hydrogen bonds. The 1-6 fragment adopts a 310 helical conformation with four hydrogen bonds, in agreement with earlier studies (Rao, Ch. P., Nagaraj, R., Rao, C. N. R., & Balaram, P. (1980) Biochemistry 19, 425-431]. The 7-13 fragment also appears to be folded in 3₁₀ helical fashion, although some intramolecular hydrogen bonds are loosened in (CD₃)₂-SO). The 11-16 fragment favors a structure with three intramolecular hydrogen bonds of the 4 → 1 and 5 → 1 types. CD studies in trifluoroethanol suggest a helical structure for the 1-13 and 1-17 fragments and alamethicin, while IR studies support a helical structure for the 1-13 peptide, stabilized by intramolecular hydrogen bonding. On the basis of fragment conformations and earlier studies of the stereochemistry of α-aminoisobutyric acid (Aib) containing peptides, a structure is suggested for the alamethicin backbone. A largely 3₁₀ helical folding pattern is postulated for the hydrophobic 1-17 segments, with a polar flexible C-terminal tripeptide.