A series of gramicidin S (GS) analogues have been synthesized where the Phe (i + 1) and Pro (i + 2) residues of the beta-turn have been swapped while the respective chiralities (D-, L-) at each position are preserved, and Phe is replaced by surrogates with aromatic side chains of diverse size, orientation, and flexibility. Although most analogues preserve the beta-sheet structure, as assessed by NMR, their antibiotic activities turn out to be highly dependent on the bulkiness and spatial arrangement of the aromatic side chain. Significant increases in microbicidal potency against both Gram-positive and Gram-negative pathogens are observed for several analogues, resulting in improved therapeutic profiles. Data indicate that seemingly minor replacements at the GS beta-turn can have significant impact on antibiotic activity, highlighting this region as a hot spot for modulating GS plasticity and activity.

This work was supported by Ministerio de Ciencia e Innovación (BIO2008-04487-CO3-02 to D.A., CTQ2008-00080/BQU to M.A.J., CTQ2007-62245 to C.C.), Fondo de Investigaciones Sanitarias (PI061125 and RD06/0021/0006 to L.R.), by the regional governments of Aragón (research group E40), Catalonia (SGR2008-492), and Madrid (COMBACT S-BIO-0260/2006). C.S. and C.M.S. thank Ministerio de Educación y Ciencia and Consejo Superior de Investigaciones Científicas-European Social Fund for FPU and I3P fellowships, respectively. This project has been funded in whole or in part with federal funds from the National Cancer Institute, National Institutes of Health, under contract number HHSN261200800001E. The content of this publication does not necessarily reflect the view of the policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organization imply endorsement by the U.S. Government. This research was supported (in part) by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research.