Drastic reduction in antimicrobial activity by replacement of Orn residues with Lys in cyclized amphiphilic beta-structural model peptides

Recent investigation have indicated that cyclic dodeca- and tetradecapeptides, cyclo(-Leu-Orn-Leu-Orn-D-Phe-Pro)2 (Orn-DLL-12) and cyclo(-Leu-Orn-Leu-Orn-Leu-D-Phe-Pro)2 (Orn-DLL-14), which are designed on the basis of a cyclic beta-structural antibiotic, gramicidin S (GS), inhibit the growth of Gram-positive and -negative bacteria with high potency [Ando, S., Nishikawa, H., Takiguchi, H., Lee, S. & Sugihara, G. (1993) Biochim. Biophys. Acta 1147, 42-49]. In this study we designed and synthesized two analogs, Lys-DLL-12 and Lys-DLL-14, in which four Orn residues in Orn-DLL-12 and Orn-DLL-14 were replaced by Lys residues, respectively, and investigated their interactions with model membranes in terms of CD and dye-leakage experiments, antimicrobial activity and lytic activity for human erythrocytes. Both peptides newly designed showed no antimicrobial activity and no lytic activity of erythrocytes. The present CD study showed that the presence of neutral liposomes and of acidic liposomes of natural or synthetic phospholipids results in no remarkable conformational difference between Orn-DLL-12/-14. The leakage experiment showed a clear relation between the antimicrobial activity and the leakage ability in acidic synthetic phospholipid liposomes but no correlation in acidic natural ones. The difference in hydrophobic and hydrophilic balance between Orn-DLL-12/14 and Lys-DLL-12/14 (derived from the increasing hydrophobicity due to an increase of four methylene units by the substitution of Lys for Orn) may be one of the important factors in the drastic decrease in activity.

Amino acids that specify structure through hydrophobic clustering and histidine-aromatic interactions lead to biologically active peptidomimetics

Acyclic beta-sheet structure can be nucleated in heptapeptides when the 4-(2-aminoethyl)-6-dibenzofuranpropanoic acid residue (1) is flanked in sequence by two His residues, a His residue and a hydrophobic residue or by two hydrophobic residues. Acyclic beta-sheet peptidomimetics having an appropriate sequence have sufficient structural integrity to exhibit antimicrobial activity equivalent to that of gramicidin S.

Antibacterial Activity of Acyclic Decapeptide Analogs of Gramicidin S

Three acyclic decapeptide analogs of gramicidin S, although found to possess antibacterial activity, apparently have modes of action which differ from that of the naturally occurring cyclic antibiotic. In contrast to the immediate action of gramicidin S, one of the decapeptides produced complete bacteriostasis only after several cell divisions had occurred. Furthermore, mixtures of gramicidin S with either of two of the acyclic peptides were synergistic. Some implications of these findings are discussed.