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

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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) were carried out in CDCl3 and (CD3)2SO. The intramolecularly hydrogen-bonded amide hydrogens in these peptides were identified by using solvent titration experiments and temperature coefficients of NH chemical shifts in (CD3)2SO. The peptides favor conformations stabilized by intramolecular 4 .fwdarw. 1 hydrogen bonds. The 11-21 fragment adopts a highly folded, largely 310 helical conformation stabilized by 7 intramolecular hydrogen bonds. An 8th 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.