Six surfactant-like peptides with the same amino acid composition but different primary sequences are designed, including G3A3V3I3K3, K3I3V3A3G3, I3V3A3G3K3, K3G3A3V3I3, V3G3I3A3K3, and K3A3I3G3V3. These peptides form antiparallel β-sheets during self-assembly. Because the constituent residues have different side chain size and hydrophobicity, sequence changes adjust group distribution and hydrophobicity on the two sides of a given β-sheet. This consequently tunes the binding energy of the side-to-side pairing conformations and leads to different self-assembled structures. G3A3V3I3K3 and K3I3V3A3G3 form short nanorods with diameters of 8.5 ± 1.0 nm and lengths <150 nm. I3V3A3G3K3 and K3G3A3V3I3 form nanosheets with heights of 4.0 ± 0.5 nm and limited lengths and widths. V3G3I3A3K3 and K3A3I3G3V3 form long fibrils with diameters of 7.0 ± 1.0 nm and lengths of micrometer scale. These nanostructures exhibit different capacity in encapsulating insoluble hydrophobic drug molecules and delivering them into the cells. The nanosheets of I3V3A3G3K3 and K3G3A3V3I3 can encapsulate both nile red and doxorubicin molecules to an extent of up to 17−23% in mole ratio. Moreover, the shape and size of the nanostructures affect the drug delivery into cells greatly, with the nanosheets and short rods exhibiting higher efficiency than the long fibrils. The study provides new insights into programmed peptide self-assembly toward specific functionalities.Keywords: drug delivery; encapsulation; nanostructures; peptide self-assembly; primary sequence; β-sheet pairing;

A new type of conjugated molecule including T′3–(AKAE)2, T′3–(IKIE)2, and A′3–(AKAE)2 was designed by linking short peptide nucleic acid (PNA) segments with short ionic self-complementary peptide (ISCP) sequences. These short conjugates showed high hybridization affinity and specificity for λ-DNA. They can induce efficient DNA condensation at low micromole concentrations via a specific mechanism that involves the base pair recognition between DNA and the PNA segment and the self-aggregation of the bound PNA–ISCP molecules. Atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements indicated that λ-DNA took an elongated conformation while it compacted into globules when interacting with the PNA–ISCP conjugates. The ethidium bromide displacement assay indicated that the PNA–ISCP conjugates induced DNA condensation in a way different from conventional cationic condensers such as polyethyleneimine (PEI) and hexadecyltrimethylammonium bromide (CTAB). The interaction between T′3–(AKAE)2 and a single chain oligonucleotide, d(A)36, was further studied and the results revealed that the PNA–ISCP conjugates bound with DNA mainly via base pairing recognition. The volume ratio of λ-DNA and the λ-DNA/PNA–ISCP globules was calculated based on AFM measurements, which was near 1 : 1, suggesting that the condensation was an intramolecular folding process for λ-DNA, which was prompted by the self-aggregation of the bound PNA–ISCP molecules.

A peptide nucleic acid (PNA)-peptide conjugated molecule, T′3(AKAE)2, was designed to have both a PNA segment for oligonucleotide binding and an ionic self-complementary peptide sequence for self-association. T′3(AKAE)2 could co-assemble with oligoadenines (d(A)x) to form virus-like supramolecular structures whose morphology showed dependence on the chain length and rigidity of the d(A)x molecules. Smaller nanospheres with diameters of 13.0±2.0 nm were produced in the case of d(A)6. Wormlike aggregates with lengths of 20–50 nm and diameters of 15.0±2.5 nm were found in the cases of d(A)12, d(A)18, d(A)24 and d(A)30. And larger spherical aggregates with diameters of 18±5 nm came into presence in the cases of d(A)36 and d(A)42. These nanostructures were suggested to be formed under a cooperative effect of base pair recognition and peptidic association. The study provides insights into the programmed assembly of a multi-components system as well as control of the size and shape of the co-assembled structures, which is of great significance in developing gene/drug delivery systems.

•Gemini peptides were produced by disulfide bond linkage of the single-chain ones.•The single-chain-to-gemini transition enhanced the self-assembly ability.•Gemini geometry introduced additional constraints in molecular conformation.•The intra- and intermolecular hydrogen bonding produced different structures.•Peptide self-assembly depended greatly on the side chain groups.By designing cysteine-containing single-chain peptides and then linking two such molecules with disulfide bond under oxidation, a series of amphiphilic gemini peptides were successfully synthesized. The gemini geometry introduced not only additional constraints in molecular conformations but also the differentiated intra- and intermolecular hydrogen bonding. These aspects result in specific transition of the self-assembly behavior. The single-chain peptides tended to form spherical aggregates, while the gemini molecules all self-assembled into fiber-like structures, especially that I3C–CI3 could form short thin fibers with highly ordered lateral alignments that are rarely found. Moreover, the self-assembly of both the single-chain and the gemini peptides showed great dependence on the side chain groups. With increasing the size of the side chain alkyl groups, the molecules gave decreased critical aggregation concentration (CAC) and were more ready to arrange into ordered assemblies. This should be ascribed to the enhanced hydrophobic interaction and the subsequent force balance shifts.The amphiphilic peptides self-assembled into different structures with variation of the side chain groups as well as the single-chain-to-gemini structural transition. The gemini geometry probably introduced additional constraints in molecular conformations and thus to result in specific molecular arrangements and self-assembled structures.