Effect of helicity and hydrophobicity on cell-penetrating ability of arginine-rich peptides

Arginine (Arg)-rich peptides are one of the typical cell-penetrating peptides (CPPs), which can deliver membrane-impermeable compounds into intracellular compartments. Guanidino groups in Arg-rich peptides are critical for their high cell-penetrating ability, although it remains unclear whether peptide secondary structures contribute to this ability. In the current study, we designed four Arg-rich peptides containing alpha,alpha-disubstituted alpha-amino acids (dAAs), which prefer to adopt a helical structure. The four dAA-containing peptides adopted slightly different peptide secondary structures, from a random structure to a helical structure, with different hydrophobicities. In these peptides, dipropylglycine-containing peptide exhibited the highest helicity and hydrophobicity, and showed the best cell-penetrating ability. These findings suggested that the helicity and hydrophobicity of Arg-rich peptides contributes to their high cell-penetrating ability.