Exploring the conformational and biological versatility of β-turn-modified gramicidin S by using sugar amino acid homologues that vary in ring size.

Monobenzylated sugar amino acids (SAAs) that differ in ether ring size (containing an oxetane, furanoid, and pyranoid ring) were synthesized and incorporated in one of the β-turn regions of the cyclo-decapeptide gramicidin S (GS). CD, NMR spectroscopy, modeling, and X-ray diffraction reveal that the ring size of the incorporated SAA moieties determines the spatial positioning of their cis-oriented carboxyl and aminomethyl substituents, thereby subtly influencing the amide linkages with the adjacent amino acids in the sequence. Unlike GS itself, the conformational behavior of the SAA-containing peptides is solvent dependent. The derivative containing the pyranoid SAA is slightly less hydrophobic and displays a diminished haemolytic activity, but has similar antimicrobial properties as GS.