Development of the structural basis for antimicrobial and hemolytic activities of peptides based on gramicidin S and design of novel analogs using NMR spectroscopy

The structures of 14-residue head-to-tail cyclic gramicidin S peptides have been investigated to develop the structural rationale for their antimicrob ial and hemolytic profiles. The basis for these studies is G514 (cyclo(VKLK VdYPLKVKLdYP)), designed as an extension of the naturally occurring antimic robial peptide. The structure of GS14 has been determined using NMR methods and was found to exist in a highly amphipathic antiparallel beta-sheet con formation. Systematic enantiomeric substitutions within the framework of th e GS14 peptide were found to decrease the amphipathicity of this molecule. These results indicated that there was a direct correlation between the hig h amphipathic character and potent hemolytic activity in the diastereomers, whereas an inverse correlation existed between amphipathicity and antimicr obial function. To define the structural consequences of changing the amphi pathic nature of GS14 analogs to maximize antimicrobial activity and to min imize hemolysis, NMR structures were determined in water and the membrane-m imetic solvent trifluoroethanol. The structures show that these attributes are the result of induction of the beta-sheet character in a membrane envir onment and the positioning of charged side chains on the hydrophobic face o f the cyclic framework, thus decreasing the amphipathicity and directed hyd rophobicity of these molecules. Implications for the design of more effecti ve antimicrobials are discussed.