Antimicrobial peptides: synthesis and antibacterial activity of linear and cyclic drosocin and apidaecin 1b analogues

Drosocin and apidaecin Ib are two insect antimicrobial peptides showing a significant sequence homology and a common mechanism of action, which includes stereoselective elements but is devoid of any pore-forming activity. A substantial difference between the two peptides is the presence in the drosocin sequence of an O-glycosylated threonine residue, which is important for its antimicrobial activity. Through the synthesis of a series of differently glycosylated drosocin analogues, we have shown that the antimicrobial activity against several Gram-negative bacteria appears to be modulated by the sugar moiety (Gal vs GalNAc) and the type of glycosidic linkage (alpha-O-, beta-O-, or alpha-C-). The insertion of a glycosylated threonine residue in the apidaecin Ib sequence improves the sequence homology with drosocin but reduces the antimicrobial activity. To gain information on the possible bioactive conformation of these peptides, we synthesized an unglycosylated cyclic analogue of drosocin, containing an intrachain disulfide bond, and the head-to-tail cyclic analogues of drosocin and apidaecin, as well as their corresponding cyclic dimers. Only the large cyclic dimer of apidaecin partially retained the antimicrobial activity, suggesting that a bending of the peptide chain, in particular in the middle of the molecule, is not a structural element characteristic of the bioactive conformation of drosocin and apidaecin. Experiments aimed at testing the effect of selected drosocin and apidaecin peptides on biological membranes showed that some peptides display a moderate hemolytic activity and that a dissociation between antibacterial activity and cytotoxicity to eukaryotic cells can be achieved in differently glycosylated peptide analogues.