An Ornithine-Free Gramicidin S Analogue Using Norleucine, Cyclo(Val–Nle–Leu–D-Phe–Pro)<sub>2</sub>, Forms Helically Aligned β-Sheets

The structure of an ornithine (Orn)-free Gramicidin S (GS) analogue, cyclo(Val–Nle–Leu–D-Phe–Pro)2 (NGS), was studied. Its circular dichroism (CD) spectrum showed that NGS has a structure similar to GS, though the value of [θ] indicated smaller β-turn and sheet populations. This is probably because the Nle side chain could not form intramolecular hydrogen bonds stabilizing the sheet structure. The chemical shift perturbation of αH and JNH–αH...