Engineered Salt-insensitive α-Defensins with End-to-end Circularized Structures

We designed a retro-isomer and seven circularized "beta-tile" peptide analogs of a typical rabbit alpha-defensin, NP-1. The analogs retained defensin-like architecture after the characteristic end-to-end, Cys(3,31) (C I:C VI), alpha-defensin disulfide bond was replaced by a backbone peptide bond. The retro-isomer of NP-1 was as active as the parent compound, suggesting that overall topology and amphipathicity governed its antimicrobial activity. A beta-tile design with or without a single cross-bracing disulfide bond sufficed for antimicrobial activity, and some of the analogs retained activity against Escherichia coli and Salmonella typhimurium in NaCl concentrations that rendered NP-1 inactive. The new molecules had clustered positive charges resembling those in protegrins and tachyplesins, but were less cytotoxic. Such simplified alpha-defensin analogs minimize problems encountered during the oxidative folding of three-disulfide defensins. In addition, they are readily accessible to a novel thia zip cyclization procedure applicable to large unprotected peptide precursors of 31 amino acids in aqueous solutions. Collectively, these findings provide new and improved methodology to create salt-insensitive defensin-like peptides for application against bacterial diseases.