Membrane channel forming polypeptides. 270-MHz hydrogen-1 nuclear magnetic resonance studies on the conformation of the 11-21 fragment of suzukacillin

Abstract

270-MHz ¹H NMR studies on the synthetic suzukacillin fragments Boc-Leu-Aib-Gly-Leu-Aib-OMe (13-17), Boc-Gln-Aib-Leu-Aib-Gly-Leu-Aib-OBz (11-17), Boc-Leu-Aib-Gly-Leu-Aib-Pro-Val-Aib-Aib-OMe (13-21), and Boc-Gln-Aib-Leu- Aib-Gly-Leu-Aib-Pro-Val-Aib-Aib-OMe (11-21) have been carried out in CDCl₃ and (CD₃)₂SO. The intramolecularly hydrogen-bonded amide hydrogens in these peptides have been identified by using solvent titration experiments and temperature coefficients of NH chemical shifts in (CD₃)₂,S0. The peptides are shown to favor conformations stabilized by intramolecular 4→1 hydrogen bonds. The 11-21 fragment adopts a highly folded, largely 3₁₀ helical conformation stabilized by seven intramolecular hydrogen bonds. An eighth NH group [Gly(5)] appears to be involved in a weaker interaction. Evidence for the possible participation of the Gln side-chain carboxamide group in hydrogen bonding to the peptide backbone is also presented.