Dehydrophenylalanine (ΔPhe) was incorporated into an antibiotic peptide gramicidin S (GS) in place of D-Phe4,4′ to prepare an unsaturated analog. Conformational analysis with 1H-NMR indicated that the unsaturated analog has much the same backbone conformation as that of natural gramicidin S as shown by NOE experiments. Studies on temperature dependences and on the chemical shift differences showed that the hydrogen bonds between Val-NH and Leu-CO in the unsaturated analog are strengthened by the incorporation of ΔPhe4,4′. This resulted in the reinforcement of the β-sheet structure which is the most important structural element for GS bioactivity. [ΔPhe4,4′]gramicidin S exhibited indeed very strong antimicrobial activities against Gram-positive bacteria as well as the natural peptide.
