The effect of d-amino acid substitution on the selectivity of temporin L towards target cells: Identification of a potent anti-Candida peptide

The frog skin peptide temporin L (TL, 13-residues long) has a wide and potent spectrum of antimicrobial activity, but it is also toxic on mammalian cells at its microbicidal concentrations. Previous studies have indicated that its analogue [Pro³]TL has a slightly reduced hemolytic activity and a stable helical conformation along residues 6–13. Here, to expand our knowledge on the relationship between the extent/position of α-helix in TL and its biological activities, we systematically replaced single amino acids within the α-helical domain of [Pro³]TL with the corresponding d isomers, known as helix breakers. Structure–activity relationship studies of these analogues, by means of CD and NMR spectroscopy analyses as well as antimicrobial and hemolytic assays were performed. Besides increasing our understanding on the structural elements that are responsible for cell selectivity of TL, this study revealed that a single l to d amino acid substitution can preserve strong anti-Candida activity of [Pro³]TL, without giving a toxic effect towards human cells.