SYNTHESIS AND STUDY OF NORMAL, ENANTIO, RETRO, AND RETROENANTIO ISOMERS OF CECROPIN A-MELITTIN HYBRIDS, THEIR END GROUP EFFECTS AND SELECTIVE ENZYME INACTIVATION

In our effort to understand the structural requirements for the antimi crobial activity of cecropin A (CA) and melittin (M), we synthesized t he normal, enantio, retro and retroenantio hybrid analogs; we related activity to their sequence, chirality, amide bond direction (helix dip ole) and end group charges. To compare the effect of the end groups, e ach of these analogs was synthesized both with an acid and an amide C- terminus and also with and without an N-alpha-acetyl N-terminus. The a ll-L-and all-D-enantiomers of several cecropin-melittin hybrids were p reviously found to be equally potent against several bacterial species , and no chiral effect was observed. This general rule has now been co nfirmed and extended. However, two exceptions have been found. All-L-C A(1-13)M(1-13) acid was 5 times and 9 times less potent than the all-D -analog, respectively, toward Gram-positive Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa. All-L-CA(1-7)M(2-9) acid was 5 t imes and 14 times less active against S. aureus and P. aeruginosa, res pectively, than its all-D acid isomer. The corresponding D- and L-retr o analogs differed only marginally. A role for proteolytic enzymes has been implicated as a cause for these differences in the activities of L-and D-enantiomers. In all cases, blocking the alpha-amine by acetyl ation had no significant effect on potency. The retro and retroenantio analogs of CA(1-13)M(1-13) acid were as potent as their normal and en antio analogs against all the test bacteria. The C-terminal amides als o showed similar potency against four test bacteria. It should be note d that the negative end of the helix dipole of a normal peptide points toward the C-terminus, whereas it points away in the case of a retro derivative when viewed in the direction of the normal sequence. (C) Mu nksgaard 1997.