Properties of synthetic analogs of gramicidin S containing L-serine or L-glutamic acid residue in place of L-ornithine residue

Effects of single D-amino acid substitutions on disruption of beta-sheet structure and hydrophobicity in cyclic 14-residue antimicrobial peptide analogs related to gramicidin S

Gramicidin S (GS) is a 10-residue cyclic beta-sheet peptide with lytic activity against the membranes of both microbial and human cells, i.e. it possesses little to no biologic specificity for either cell type. Structure-activity studies of de novo-designed 14-residue cyclic peptides based on GS have previously shown that higher specificity against microbial membranes, i.e. a high therapeutic index (TI), can be achieved by the replacement of a single L-amino acid with its corresponding D-enantiomer [Kondejewski, L.H. et al. (1999) J. Biol. Chem. 274, 13181]. The diastereomer with a D-Lys substituted at position 4 caused the greatest improvement in specificity vs. other L to D substitutions within the cyclic 14-residue peptide GS14, through a combination of decreased peptide amphipathicity and disrupted beta-sheet structure in aqueous conditions [McInnes, C. et al. (2000) J. Biol. Chem. 275, 14287]. Based on this information, we have created a series of peptide diastereomers substituted only at position 4 by a D- or L-amino acid (Leu, Phe, Tyr, Asn, Lys, and achiral Gly). The amino acids chosen in this study represent a range of hydrophobicities/hydrophilicities as a subset of the 20 naturally occurring amino acids. While the D- and L-substitutions of Leu, Phe, and Tyr all resulted in strong hemolytic activity, the substitutions of hydrophilic D-amino acids D-Lys and D-Asn in GS14 at position 4 resulted in weaker hemolytic activity than in the L-diastereomers, which demonstrated strong hemolysis. All of the L-substitutions also resulted in poor antimicrobial activity and an extremely low TI, while the antimicrobial activity of the D-substituted peptides tended to improve based on the hydrophilicity of the residue. D-Lys was the most polar and most efficacious substitution, resulting in the highest TI. Interestingly, the hydrophobic D-amino acid substitutions had superior antimicrobial activity vs. the L-enantiomers although substitution of a hydrophobic D-amino acid increases the nonpolar face hydrophobicity. These results further support the role of hydrophobicity of the nonpolar face as a major influence on microbial specificity, but also highlights the importance of a disrupted beta-sheet structure on antimicrobial activity.

Preparation and properties of novel gramicidin S analogs possessing a tri-, tetra- or pentamethylene bridge between ornithine side chains

Gramicidin S (GS) analogs in which the Ndelta atoms of the two Orn side chains are linked by an oligomethylene bridge [-(CH2)n-; n=3-5] were prepared via the bis(p-nitrobenzenesulfonyl) derivative [Orn(NBS)2,2']GS. For comparison the nonbridged secondary amino group-containing analog [Orn(Me)2,2']GS was also prepared. 1H NMR and CD spectral analysis indicated that these analogs adopt the same beta-sheet conformation as GS. The antimicrobial activities of these analogs were very similar, but were slightly dependent on the bridge chain length, the trimethylene-bridged analog being the most potent.