A Helix-Stabilized Cell-Penetrating Peptide as an Intracellular Delivery Tool

Two types of cationic cyclic alpha,alpha-disubstituted a-amino acids: Api(C2NH2) (which possesses a lysine mimic side chain) and Api(C2Gu) (which possesses an arginine mimic side chain), were developed. These amino acids were incorporated into an arginine-based peptide sequence [(l-Arg-l-Arg-dAA)(3): dAA=Api(C2NH2) or Api(C2Gu)], and the relationship between the secondary structures of the resulting peptides and their ability to pass through cell membranes was investigated. The peptide containing Api(C2Gu) formed a stable a-helical structure and was more effective at penetrating cells than the nonhelical Arg nonapeptide (R-9). Furthermore, the peptide was able to deliver plasmid DNA into various types of cells in a highly efficient manner.