An Unusual Reverse Turn Structure Adopted by a Furanoid Sugar Amino Acid Incorporated in Gramicidin S

A new reverse turn, replacing one of the native type II' beta-turns in the cyclic peptide antibiotic gramicidin S, induced by a furanoid sugar amino acid is revealed. The C3-hydroxyl function plays a pivotal role by acting as a H-bond acceptor, consequently flipping the amide bond between residues i and i + 1, as was established by NMR and X-ray crystallographic analysis.