Influence of preformed α‐helix and α‐helix induction on the activity of cationic antimicrobial peptides

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Abstract
One prominent class of cationic antibacterial peptides comprises the α-helical class, which is unstructured in free solution but folds into an amphipathic α-helix upon insertion into the membranes of target cells. To investigate the importance of α-helicity and its induction on interaction with membranes, a series of peptides was constructed based on a hybrid of moth cecropin (amino acids 1-8) and bee melittin (amino acids 1-18) peptides. The new peptides were predicted to have a high tendency to form α-helices or to have preformed α-helices by virtue of construction of a lactam bridge between glutamate and lysine side-chains at positions i and i+ 4 at various locations along the primary sequence. In two examples where the use of lactam bridge constraints induced and stabilized α-helical structure in benign (aqueous buffer) and/or hydrophobic medium, there was a decrease in antibacterial activity relative to the linear counterparts. Thus the preformation of α-helix in solution was not necessarily beneficial to antimicrobial activity. In the one case where the lactam bridge did result in increased antibacterial activity (lower minimal inhibitory concentration values) it did not increase α-helical content in benign or hydrophobic medium. Broadly speaking, good activity of the peptides against Pseudomonas aeruginosa correlated best (r2= 0.88) with a helican parameter which was calculated as the induction of α-helix in α membrane-mimicking environment divided by the α-helix formation under benign conditions. Interestingly, the activity of the lactam bridge peptide constructs correlated in part with alterations in bacterial outer or cytoplasmic membrane permeability.