Hendrik Frisch

pH-Switchable ampholytic supramolecular copolymers.

β-sheet-encoded anionic and cationic dendritic peptide amphiphiles form supramolecular copolymers when self-assembled in a 1:1 feed ratio of the monomers. These ampholytic materials have been designed for on-off polymerization in response to pH triggers. The cooperative supramolecular self-assembly process is switched on at a physiologically relevant pH value and can be switched off by increasing or decreasing the pH value.

pH-switchable self-assembled materials.

Self-assembled materials, which are able to respond to external stimuli, have been extensively studied over the last decades. A particularly exciting stimulus for a wide range of biomedical applications is the pH value of aqueous solutions, since deprotonation-protonation events are crucial for structural and functional properties of biopolymers. In living cells and tissues, intra- and extracellular pH values are stringently regulated, but can deviate from pH neutral as observed for example in tumorous, inflammatory sites, in endocytic pathways, and specific cellular compartments. By using a pH-switch as a stimulus, it is thereby possible to address specific targets in order to cause a programmed response of the supramolecular material. This strategy has not only been successfully applied in fundamental research but also in clinical studies. In this feature article, current strategies that have been used in order to design materials with pH-responsive properties are illustrated. This discussion only addresses selected examples from the last four years, the self-assembly of polymer-based building blocks, assemblies emerging from small molecules including surfactants or derived from biological macromolecules, and finally the controlled self-assembly of oligopeptides.

pH‐Regulated Selectivity in Supramolecular Polymerizations: Switching between Co‐ and Homopolymers

A strategy is presented to regulate the selectivity in aqueous supramolecular polymerizations by changes in pH. In neutral buffered conditions, oppositely charged phenylalanine-based dendritic peptide amphiphiles self-assemble into (A-B)n alternating copolymers of low polydispersity when mixed in a 1:1 comonomer feed ratio. Via pH switch of the glutamic acid and lysine side chains, attractive Coulomb interactions in the coassembled materials are screened and selective polymerization occurs to form (A)n homopolymers of the acidic comonomer at low pH and (B)n homopolymers of the basic comonomer at high pH, while the complementary comonomer is released during the transition. Reversible switching is demonstrated between these three different polymeric states, which were characterized by CD and fluorescence spectroscopy, using a peptide based minimalistic fluorophore/quencher pair, and transmission electron microscopy.

A soft supramolecular carrier with enhanced singlet oxygen photosensitizing properties

Herein we report the self-assembly of a supramolecular singlet oxygen photosensitizing system from an adamantane-functionalized, hexaanionic water-soluble zinc(II) phthalocyanine (PC) and β-cyclodextrin vesicles (CDV). Characterisation of the designed PC, which was synthesized by an asymmetric statistical condensation, was carried out by several analytical techniques such as MALDI-HRMS, NMR, IR, UV/vis as well as steady state and time resolved fluorescence spectroscopy. The influence of the docking of the PC to the CDVs on the PC photoluminescence as well as on the singlet oxygen photoproduction quantum yields was investigated. The results indicate that the host–guest interaction of the photosensitizer and the CDVs significantly prevents the formation of inactive aggregates, and enhances the photosensitizing ability of the PC. The supramolecular assembly constitutes a biocompatible photoactive platform for the design of phototherapeutic agents.

Kinetically Controlled Sequential Growth of Surface-Grafted Chiral Supramolecular Copolymers.

We report a facile strategy to grow supramolecular copolymers on Au surfaces by successively exposing a surface-anchored monomer to solutions of oppositely charged peptide comonomers. Charge regulation on the active chain end of the polymer sufficiently slows down the kinetics of the self-assembly process to produce kinetically trapped copolymers at near-neutral pH. We thereby achieve architectural control at three levels: The β-sheet sequences direct the polymerization away from the surface, the height of the supramolecular copolymer brushes is well-controlled by the stepwise nature of the alternating copolymer growth, and 2D spatial resolution is realized by using micropatterned initiating monomers. The programmable nature of the resulting architectures renders this concept attractive for the development of customized biomaterials or chiral interfaces for optoelectronics and sensor applications.

Tuneable pH-regulated supramolecular copolymerisation by mixing mismatched dendritic peptide comonomers

Charged phenylalanine-rich dendritic peptides form highly stable and pH-switchable rod-like supramolecular copolymers, when co-assembled with a matching oppositely charged dendritic comonomer. Here, we demonstrate that by mismatching a strong with a weak β-sheet encoded comonomer, both the stability and the pH-triggered disassembly of the copolymers shifts drastically from pH 4.2 to biologically relevant pH 5.8.

The synthesis of dendritic EDOT-peptide conjugates and their multistimuli-responsive self-assembly into supramolecular nanorods and fibers in water

We report the synthesis of amphiphilic dendritic EDOT-peptide conjugates and discuss their stimuli-responsive self-assembly into polyanionic nanorods in water. In order to expand the general concept of frustrated growth, whereby attractive supramolecular interactions within the enlarged π-system of the hydrophobic core of a dendritic peptide are hampered by repulsive interactions in the hydrophilic periphery, we show that changes in the pH and ionic strength are both able to independently trigger the self-assembly of the dendritic monomers into supramolecular nanorods and nanofibers. These transitions are analyzed using circular dichroism and fluorescence spectroscopic methods, and the resulting supramolecular polymers are characterized by transmission electron microscopy.