Sensing cell adhesion by means of a colourimetric response provides an intuitive measure of cell binding. In this study polydiacetylene-containing peptide amphiphiles fibres were designed to sense cell adhesion by means of a colour change. The diacetylene-containing peptide amphiphiles were functionalised with the cell-binding motif RGDS, and subsequently mixed with non-functionalised diacetylene-containing spacer amphiphiles. The diacetylenes in the backbone of these fibres were polymerised using UV-light to give dark blue fibre solutions. Subsequent cell adhesion induced a colour change from blue to pink. The propensity of the RGDS fibres to change colour upon cell adhesion could be tuned by varying the C-terminal amino acid of the spacer amphiphile. In addition to this, by varying the RGDS density we found that the optimum colourimetric response was obtained for fibres with a 6:1 ratio of non-RGDS to RGDS amphiphiles.

Activatable cell-penetrating peptides are of great interest in drug delivery because of their enhanced selectivity which can be controlled by the external stimuli that trigger their activation. The use of a specific enzymatic reaction to trigger uptake of an inert peptide offers a relevant targeting strategy because the activation process takes place in a short time and only in areas where the specific cell surface enzyme is present. To this aim, the lysine side chain of Tat peptides was modified with an enzyme-cleavable domain of minimal size. This yielded blocked Tat-peptides which were inactive but that could be activated by coincubation with the selected enzymes.

A systematic study was performed on the influence of charge and steric hindrance on the assembly into fibres of a series of pentameric peptides based on the well-known β-sheet forming sequence Gly-Ala-Gly-Ala-Gly, which were N-terminally acylated with pentacosadiynoic acid. To investigate the effect of steric hindrance and charge repulsion on the fibre structure, either the N-terminal or the C-terminal amino acid in the sequence was replaced by a glutamic acid or lysine residue. Furthermore, peptide amphiphiles (PAs) with an amide or a free acid group at the C-terminus were compared. Steric hindrance and charge repulsion were addressed individually by varying the pH during and after fibre preparation. The self-assembled structures were examined with circular dichroism (CD) spectroscopy and transmission electron microscopy (TEM). UV spectroscopy was used to probe the diacetylene packing in the hydrophobic tail, both by polymerisation behaviour and chromatic properties of the polymers. In brief, the assembly was hindered more if the modification was close to the alkyl tail, and glutamic acid brought about a larger effect than lysine. PAs with two charges yielded assemblies which after polymerisation were found to be the most susceptible towards changes in pH, behaving as a colour-based pH sensor. Typically, TEM and UV showed the same trends, indicating that a distorted morphology as observed with TEM is indicative of a poorer molecular packing of the peptide amphiphile fibres, probed via the changes in absorption of the polydiacetylene backbone.

Proteins and peptides are fundamental components of living systems where they play crucial roles at both functional and structural level. The versatile biological properties of these molecules make them interesting building blocks for the construction of bio-active and biocompatible materials. A variety of molecular tools can be used to fashion the peptides necessary for the assembly of these materials. In this tutorial review we shall describe five of the main techniques, namely solid phase peptide synthesis, native chemical ligation, Staudinger ligation, NCA polymerisation, and genetic engineering, that have been used to great effect for the construction of a host of peptide-based materials.

A small library of oligoarginine peptides equipped with terminal cysteines was studied with respect to their cell-penetrating properties. The peptides themselves were inactive but gained the ability to enter cells upon extension of their sequence through disulfide bridge formation.

Herein we report on the development of a novel method of constraining a cell-penetrating peptide, which can be used to trigger transport of liposomes into cells upon in this case radiation with UV-light. A cell-penetrating peptide, which was modified on both termini with an alkyl chain, was anchored to the liposomal surface in a constrained and deactivated form. Since one of the two alkyl chains was connected to the peptide via a UV-cleavable linker, disconnection of this alkyl chain upon irradiation led to the exposure of the cell-penetrating peptide, and mediated the transport of the entire liposome particle into cells.A novel method of constraining a cell-penetrating peptide, which can be used for triggered transport of liposomes into cells upon radiation with UV-light.

A shelf-stable and easily prepared diazotransfer reagent, imidazole-1-sulfonyl azide hydrochloride, was used to transform the N-terminus of a model peptide on solid phase into an azide moiety. It is demonstrated that this conversion was accomplished within 30 min with high efficiency under aqueous conditions on a NovaPEG resin or in DMF on polystyrene beads.

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.

In this study the thermal and mechanical stabilities of self-assembled fibres of palmitoyl functionalized GANPNAAG have been examined. The thermal stability was investigated with circular dichroism, dynamic light scattering and fluorescence measurements monitoring the disassembly upon heating and reassembly upon cooling. The determination of the temperature dependence of the critical aggregation concentration allowed us to estimate the thermodynamic parameters involved in the fibre formation process. The mechanical stability of the fibres was studied using flow-induced birefringence. The peptide based self-assembled structures were found to be thermally stable up to 70 °C, while being fractured by moderate shear flow.

In this tutorial review we give an introduction into the field of stimulus responsive peptide based materials illustrated by some recent and current developments. We have tried to categorize them according to the stimulus the materials are responsive to, being pH, temperature, metal ions, enzymes and light. Because we have focused on the structural changes that these stimuli effect we have further classified the topics according to the secondary structures that are involved. These changes in molecular structure in turn cause a change in the macroscopic properties of the material they constitute. It is believed that these materials, often referred to as smart materials, have a great potential being applicable in areas like drug delivery, tissue engineering and bio-sensors.

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.

Proteins and peptides are fundamental components of living systems where they play crucial roles at both functional and structural level. The versatile biological properties of these molecules make them interesting building blocks for the construction of bio-active and biocompatible materials. A variety of molecular tools can be used to fashion the peptides necessary for the assembly of these materials. In this tutorial review we shall describe five of the main techniques, namely solid phase peptide synthesis, native chemical ligation, Staudinger ligation, NCA polymerisation, and genetic engineering, that have been used to great effect for the construction of a host of peptide-based materials.

Sensing cell adhesion by means of a colourimetric response provides an intuitive measure of cell binding. In this study polydiacetylene-containing peptide amphiphiles fibres were designed to sense cell adhesion by means of a colour change. The diacetylene-containing peptide amphiphiles were functionalised with the cell-binding motif RGDS, and subsequently mixed with non-functionalised diacetylene-containing spacer amphiphiles. The diacetylenes in the backbone of these fibres were polymerised using UV-light to give dark blue fibre solutions. Subsequent cell adhesion induced a colour change from blue to pink. The propensity of the RGDS fibres to change colour upon cell adhesion could be tuned by varying the C-terminal amino acid of the spacer amphiphile. In addition to this, by varying the RGDS density we found that the optimum colourimetric response was obtained for fibres with a 6:1 ratio of non-RGDS to RGDS amphiphiles.

In this tutorial review we give an introduction into the field of stimulus responsive peptide based materials illustrated by some recent and current developments. We have tried to categorize them according to the stimulus the materials are responsive to, being pH, temperature, metal ions, enzymes and light. Because we have focused on the structural changes that these stimuli effect we have further classified the topics according to the secondary structures that are involved. These changes in molecular structure in turn cause a change in the macroscopic properties of the material they constitute. It is believed that these materials, often referred to as smart materials, have a great potential being applicable in areas like drug delivery, tissue engineering and bio-sensors.

A small library of oligoarginine peptides equipped with terminal cysteines was studied with respect to their cell-penetrating properties. The peptides themselves were inactive but gained the ability to enter cells upon extension of their sequence through disulfide bridge formation.