Polymerization-induced color changes of polydiacetylene-containing liposomes and peptide amphiphile fibers.

Polydiacetylenes have received much attention due to their intrinsic optical properties. Their inclination to change color in response to environmental factors has been extensively exploited in the sensing of analytes. In this study we functionalized diacetylene-containing peptide amphiphiles and phospholipids with α-bromo esters so that they could be used as initiators in atom transfer radical polymerization (ATRP) reactions. Subsequently, the supramolecular assemblies formed by these molecules upon their addition to water, namely peptide amphiphile fibers and liposomes, were stabilized by polymerizing the diacetylene moieties present in the molecules. As a result, highly colored, disassembly resistant, macro initiators were created. To investigate whether steric crowding on the surface of these assemblies could influence the color of the polydiacetylenes, we utilized the initiator functionality that had been introduced prior to assembly in ATRP. We found that the chromatic properties of the polydiacetylenes were directly related to the formation of polymer on the surface of peptide amphiphile fibers as well as liposomes. Furthermore, we were able to demonstrate that the progress of this color change could be monitored with UV-visible spectroscopy.

Effect of the diacetylene position on the chromatic properties of polydiacetylenes from self-assembled peptide amphiphiles.

Three diacetylene-containing peptide amphiphiles, with exactly the same structure, except for the position of the diacetylene moiety within the hydrophobic tail, were investigated on their assembly behavior, polymerization characteristics, and chromatic properties. After polymerization it was found that the color and, thus, the conjugation length of the polydiacetylenes was dependent both on mobility and on packing of the groups on both sides of the triple bonds. The polydiacetylene with the highest conjugation length and most stable structure was obtained when the diacetylenes were positioned in the middle of the hydrophobic tail. When the diacetylenes were close to the peptide, the mismatch between hydrogen bond and alkyl tail spacing resulted in a decreased conjugation length of the resulting polymer, while when positioned close to the end of the amphiphile the mobility of the terminal alkyl chains hampered the packing of the diacetylenes.

Patterns of diacetylene-containing peptide amphiphiles using polarization holography.

The polarization dependence of a diacetylene polymerization was studied. For this purpose, peptide amphiphile fibers with a diacetylene moiety, which could only polymerize in the direction of the fiber, were employed. If nonaligned samples were illuminated with polarized light, only the fibers parallel to the polarization direction of the light were polymerized. With magnetically aligned fibers, spatially selective polymerization was accomplished using polarization holography.

Self-assembly and polymerization of diacetylene-containing peptide amphiphiles in aqueous solution.

A systematic study was performed of the fiber forming properties and polymerization characteristics of two peptide amphiphiles containing a diacetylene functionality in the alkyl tail comprising 23 and 25 C atoms, respectively. Both diyne containing peptide amphiphiles were able to form stable beta-sheet fibers of micrometers length in an aqueous solution. However, there was a large difference between the stability of the two amphiphiles. This was shown by a large difference in assembly and disassembly temperature and by different behavior during polymerization. Because the monomers were preorganized with a tight molecular packing, the polymerization could be carried out using wavelengths up to 532 nm. For both amphiphiles, the fiber structure did not change when the polymerization was carried out at an elevated temperature. The degree of polymerization, however, barely decreased for the longer amphiphile (2) but showed a gradual decline for the shorter one (1) when the temperature was raised from room temperature to the melting temperature of the fibers. Furthermore, the pH did not influence the fiber assembly for 2, but hampered it for 1 at alkaline pH. The fiber structure was, for both of the amphiphiles, not dependent on the pH. After polymerization, the molecular packing of the amphiphiles was only slightly influenced by an increase in temperature, as indicated by the small color change of polymerized fibers, which was also reversible. Additionally, pH had no influence on the assembly structure, as indicated by the color of the polymer which was the same at all pH values. Thus, both fibers increased in stability upon polymerization. The large difference in assembly and polymerization behavior of the two similar-looking amphiphiles 1 and 2, with a 23 or 25 carbon tail, is indicative of the subtlety of the assembly and disassembly processes in these fibrous architectures.

Delivery of DNA and siRNA by novel gemini-like amphiphilic peptides.

We report the design, synthesis, and characterization of a novel type of cationic lipopeptide, gemini-like amphiphilic peptides or 'geminoids'. As an example, the SPKR peptide, inspired by biological nucleic acid binding motifs, was appended with unsaturated (oleoyl/oleyl) alkyl tails. The compound shows remarkable DNA and siRNA delivery, without lysogenic helper lipid, in a variety of cells, with a moderate cytotoxic effect. It aggregates to nanoparticles that combine with DNA to lipoplexes, which undergo a change from lamellar to the more lysogenic hexagonal packing upon lowering the pH. The versatility of the chemical approach allowed us to study peptides related to SPKR, and to establish that the Pro and at least one of the cationic (Lys, Arg) residues are essential for the biological activity.