Puckering effects of 4-hydroxy-<scp>L</scp>-proline isomers on the conformation of ornithine-free Gramicidin S

The cyclic peptide <i>cyclo</i>(Val-Leu-Leu-d-Phe-Pro)₂ (peptide <b>1</b>) was specifically designed for structural chemistry investigations, drawing inspiration from Gramicidin S (GS). Previous studies have shown that Pro residues within <b>1</b> adopt a down-puckering conformation of the pyrrolidine ring. By incorporating fluoride-Pro with 4-<i>trans</i>/<i>cis</i>-isomers into <b>1</b>, an up-puckering conformation was successfully induced. In the current investigation, introducing hy-droxy-prolines with 4-<i>trans</i>/<i>cis</i>-isomer configurations (tHyp/cHyp) into <b>1</b> gave <i>cyclo</i>(Val-Leu-Leu-d-Phe-tHyp)₂ methanol disolvate monohydrate, C₆₂H₉₄N₁₀O₁₂·2CH₄O·H₂O (<b>4</b>), and <i>cyclo</i>(Val-Leu-Leu-d-Phe-cHyp)₂ monohydrate, C₆₂H₉₄N₁₀O₁₂·H₂O (<b>5</b>), respectively. However, the puckering of <b>4</b> and <b>5</b> remained in the down conformation, regardless of the geometric position of the hydroxyl group. Although the backbone structure of <b>4</b> with <i>trans</i>-substitution was asymmetric, the asymmetric backbone of <b>5</b> with <i>cis</i>-substitution was unexpected. It is speculated that the anti-cipated influence of stress from the geometric positioning, which was expected to affect the puckering, may have been mitigated by inter-actions between the hydroxyl groups of hy-droxy-proline, the solvent mol-ecules, and peptides.