Synthesis of the antibacterial peptide cecropin A(1-33)

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Abstract
Cecropin A(1-33) was synthesized by an improved stepwise solid-phase method. The synthesis was designed to give high coupling yields and minimal amounts of byproducts. All coupling steps were monitored for completion by a new ninhydrin procedure; the fully protected peptide-resin was analyzed for deletion peptides by the solid-phase Edman preview technique. Both methods indicated that the average coupling yield was > 99.8%. The unpurified peptide mixture resulting from HF cleavage and extraction into 10% acetic acid was analyzed by reverse-phase high-pressure liquid chromatography; 93% of the total product was shown to be the desired [Trp(For)2]cecropin A(1-33), indicating an average yield per synthetic cycle of 99.8%. Removal of the formyl group at pH 9, followed by ion-exchange chromatography, gave the purified product. Cecropin A(1-33) showed antibacterial activity against gram-positive and gram-negative bacteria. Against Escherichia coli, the activity was only slightly lower than that of the natural 37-residue cecropin A when tested over a 100-fold concentration range; the minimum inhibitory concentration was .apprx. 1 .mu.M. The formyl derivative was somewhat less effective in killing E. coli than the free 1-33 peptide. The antibacterial activity was discussed in terms of an amphipathic .alpha.-helix structure and the binding of the peptide to bacterial membranes.