Gramicidin S

A remarkable increase of multidrug-resistant bacteria has become a serious threat to public health threat. The amphiphilic antibiotics that attack bacterial membranes are promising agents for combating bacterial pathogens. Among them, cyclic decapeptide gramicidin S (GS), cyclo(-Val-Orn-Leu-D-Phe-Pro)₂, are attractive targets for drug discovery. GS possesses the strong antibiotic activity against Gram-positive bacteria and weak antibiotic activity against Gram-negative bacteria. In addition, no resistance has been found for the antibiotics, because it requires significant alteration of the lipid composition of the cell membrane. However, the high antibacterial potency of GS is intimately associated with high hemolytic activity, preventing the direct use in combating the microbial resistance. The efforts to increase its therapeutic index, namely, minimizing its hemolytic activity while maintaining its high antibiotic activity, have been devoted.

The increasing number of pathogenic bacteria that are resistant to clinically applied antibiotics has led to a search for potent new antibiotics with alternative targets. Cationic antimicrobial peptides (CAPs) comprise a broad class of molecules that target the lipid bilayer of bacteria. Bacteria do not easily build up resistance against such peptides, and therefore CAPs are considered as promising leads for the development of new antibiotics. A prominent member of the CAP family is the cyclo-decapeptide gramicidin S (GS) isolated from Aneurinibacillus migulans. Its secondary structure consists of a short antiparallel β-sheet interconnected by two type II’ β-turns. The amino acid sequence and its secondary structure render the molecule amphipathic with the cationic side chains oriented on one face of the β-sheet and the hydrophobic side chains on the opposing face. The cationic amphipathic characteristics of the peptide are responsible for its membrane disrupting properties, and GS is active against both Gram-positive and to a lesser extent against Gram-negative bacteria. In combination with the antibiotic agent framycetin, which targets Gram-negative bacteria, GS has proven to be an effective antibiotic against external ear infections.

Based on the research, the author designed a series of compounds as the analogs of an antibiotic gramicidin S (GS). GS, cyclo(-Val1,1’-Orn2,2’-Leu3,3’-D-Phe4,4’-Pro5,5’-)₂, is a typical amphiphilic peptide known to take on intramolecular antiparallel β-sheet conformation with two β-turns at D-Phe-Pro sequences. The characteristic feature of this conformation is the orientation of side chains in which charged Orn side chains are on one side and the hydrophobicVal and Leu side chains are on the other side of the molecule. From the study of many synthetic analogs in which these three residues were substituted, it was concluded that thisorientation of side chains was essentialfor the activityof GS.