Light-responsive helical polypeptides capable of reducing toxicity and unpacking DNA: toward nonviral gene delivery.

Nonviral gene delivery with synthetic cationic polymeric vectors is widely recognized as an attractive alternative to viral gene delivery, which suffers from inherent immunogenicity and various side effects. The transfection efficiency and chemotoxicity of these polymeric vectors are often closely related to the density of their cationic charge. Materials with low charge density usually show low toxicity but are often poor transfection agents. Polycations with high charge density could mediate effective gene transfer, which is however often associated with significant, charge-induced toxicity. When modified with various charge-reducing moieties, including saccharides, hydrocarbons, and poly(ethylene glycol) (PEG), polycations often benefit from improved safety profiles while in the meantime suffer from significantly reduced gene delivery capabilities. In addition to the charge-induced toxicity, excessive positive charges on polycations would also enhance the electrostatic attraction with the nucleic acids to restrict intracellular gene release. Therefore, it would be of great interest to develop a highly charged polycation that possesses full transfection capacity and membrane activity during gene transfer, but can be triggered to transform to a less charged or uncharged material with low membrane activity after transfection, such that intracellular DNA unpacking can be facilitated and toxicity can be reduced. Cell-penetrating peptides (CPPs), notable for their excellent membrane activities, have been developed and used in drug and gene delivery. Helical structures are often observed in CPPs or formed in CPPs during membrane transduction, and have been tied to their membrane activity. Mechanistically, the helical CPP presents a rigid amphiphilic structure that interacts with and destabilizes lipid bilayers, creating transient pathways to facilitate the passive diffusion of exogenous materials. Well-known examples of CPPs include oligoarginine, HIV-TAT, and penetratin. Despite their excellent membrane permeability, CPPs are often too short (10–25 peptide residues) and lack adequate cationic charge to efficiently condense and deliver genes by themselves. As such, CPPs often serve as membraneactive ligands incorporated or conjugated to delivery vehicles to improve delivery efficiencies. We recently developed high-molecular-weight (MW), cationic, cell-penetrating, a-helical polypeptides, termed PVBLG-8 (Scheme 1A). By maintaining a minimum sep-