Cell-penetrating peptides (CPPs) are an attractive tool for delivering membrane-impermeable compounds, including anionic biomacromolecules such as DNA and RNA, into living cells. Amphipathic helical peptides composed of hydrophobic amino acids and cationic amino acids are typical CPPs. In the current study, we designed amphipathic helical 12-mer peptides containing α,α-disubstituted α-amino acids (dAAs), which are known to stabilize peptide secondary structures. The dominant secondary structures of peptides in aqueous solution differed according to the introduced dAAs. Peptides containing hydrophobic dAAs and adopting a helical structure exhibited a good cell-penetrating ability. As an application of amphipathic helical peptides, small interfering RNA (siRNA) delivery into living human hepatoma cells was investigated. One of the peptides containing dAAs dipropylglycine formed stable complexes with siRNA at appropriate zeta-potential and size for intracellular siRNA delivery. This peptide showed effective RNA interference efficiency at short peptide length and low concentrations of peptide and siRNA. These findings will be helpful for the design of amphipathic helical CPPs as intracellular siRNA delivery.

细胞穿透肽（CPP）是一种吸引人的工具，可将不透膜的化合物（包括阴离子生物大分​​子，如DNA和RNA）输送到活细胞中。由疏水氨基酸和阳离子氨基酸组成的两亲性螺旋肽是典型的CPP。在当前的研究中，我们设计了包含α，α-二取代的α-氨基酸（dAAs）的两亲性螺旋12聚体肽，已知这些肽可以稳定肽的二级结构。水溶液中肽的主要二级结构根据引入的dAA的不同而不同。含有疏水性dAA并具有螺旋结构的肽表现出良好的细胞穿透能力。作为两亲性螺旋肽的一种应用，研究了将小干扰RNA（siRNA）递送到活人肝癌细胞中的方法。含有dAAs二丙基甘氨酸的一种肽与siRNA在适当的Zeta电位和大小下形成稳定的复合物，可用于细胞内siRNA递送。该肽在短肽长度和低浓度的肽和siRNA上显示出有效的RNA干扰效率。这些发现将有助于将两亲性螺旋CPP设计为细胞内siRNA递送。