Cell-penetrating-peptide-coated nanoribbons for intracellular nanocarriers.

The self-assembly of designed molecules is a powerful approach for the construction of novel supramolecular architectures. 2] Self-assembled nanostructures are finding growing use in biological applications, which include molecular detection, drug delivery, and gene delivery. The most important points that can be considered in developing selfassembled biomaterials are the precise control of nanostructures, effective functionalization to suit for the specific bioapplications, and the biocompatibility of the building blocks. Of the many types of molecular building blocks, peptide-based building blocks have the advantage that their constituent amino acids are biocompatible and structurally diverse. The a-helical, b-sheet, and hydrophobic interactions have been the main driving forces for the peptide assemblies and generally result in coiled-coil a-helical peptide bundles, b-sheet peptide ribbons or tubes, and cylindrical micelles. Besides the naturally occurring b-sheet peptides, such as bamyloid, many artificial b-sheet peptides have been designed. The design principle for most of the artificial bsheet peptide sequences is the alternating placement of positively charged, hydrophobic, and negatively charged amino acids. The combination of attraction between oppositely charged amino acids and solvophobic interactions between hydrophobic amino acids is the driving force for the proper b-sheet hydrogen-bonding arrangement in which the formation of a bilayered peptide ribbon is most favorable. The bilayered ribbon is stabilized by the interactions between hydrophobic surfaces of each b tape, which then generates a hydrophobic interface inside the ribbon. We envisioned that the hydrophobic interface inside the ribbon is a suitable place to encapsulate hydrophobic molecules and can therefore be potentially used for drug-delivery applications. Herein, we report the surface functionalization of nanostructures with cell-penetrating peptides (CPPs) and the successful encapsulation of hydrophobic molecules inside the peptide nanoribbon structure while preserving the ribbon morphology (Figure 1). The peptide TbP is designed for self-assembly and is composed of three functional blocks, a Tat CPP block (GRKKRRQRRRPPQ; Tat48–60), a flexible-linker block (GSGG), and a b-sheet assembly block (FKFEFKFEFKFE; Scheme 1). The CPPs consist of a short strand of amino acids that are capable of penetrating cell membranes. Many cationic CPPs, including Tat CPP from human immunodeficiency virus type-1 (HIV-1) Tat protein, have been shown to efficiently cross the cytoplasmic membrane and the nucleus pore complex (NPC) barriers. The flexible-linker block was designed to decouple the Tat CPP block from the b-sheet assembly block, thereby minimizing undesirable interactions between them. The (FKFE)n sequence has been shown to form b-sheet-mediated nanostructures in which the bilayered ribbon is the most stable structure. The bilayer is stabilized by hydrophobic and p–p-stacking interactions of phenylalanine residues on one face of the b tape (Figure 1). The CD spectrum of TbP in pure water showed a strong negative minimum at 201 nm and very weak minimum at 215 nm, indicating that random-coil structures are most prevalent and b-sheet formation is minimal (Figure 2a). These results indicate that both the Tat CPP and the b-sheet assembly blocks predominantly form random-coil structures in pure water. The Tat CPP is known to form a random-coil structure in solution. We hypothesized that the well-known b-sheet assembly block in TbP forms hardly any b sheets because of nonspecific electrostatic interactions between multiple positive charges at the Tat CPP block and multiple negative charges at the b-sheet assembly block, and the Figure 1. Representation of the nanoribbon formed by self-assembly of TbP and encapsulation of hydrophobic guest molecules.