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.

biodegradable gels, since physical crosslinking based on electrostatic interactions is generally favored over the use of chemical crosslinking to achieve sustained drug release, [ 17 ] cell attachment, [ 18 ] or hydrogel formation. [ 19 ] Recently, oppositely charged dextran microspheres [ 20 ] or poly(lactic-co -glycolic acid) (PLGA) nanospheres [ 21 ] have been used to form moldable scaffolds, but only indirect proof for electrostatic self-assembly was provided based on rheological characterization, while the underlying gel formation mechanism was not elucidated. Moreover, disadvantages of these gels include: i) the necessity to derivatize dextran or PLGA by grafting charged groups onto the polymer backbone, which moreover induced cytotoxicity; [20b] ii) the release of harmful degradation byproducts, such as lactic and glycolic acid (in case of PLGA nanospheres), which can denature entrapped signaling proteins [ 22 ] and cause infl ammatory responses [ 23 ] of the host tissue; and iii) the absence of cell-adhesive peptide sequences required for the attachment of anchorage-dependent mesenchymal stem cells such as fi bro-and osteoblasts. [ 17 ]

Ethynylation of various tryptophan-containing peptides and a single model protein was achieved using Waser's reagent, 1-[(triisopropylsilyl)ethynyl]-1,2-benziodoxol-3(1 H)-one (TIPS-EBX), under gold(I) catalysis. It was demonstrated by NMR that the ethynylation occurred selectively at the C2-position of the indole ring of tryptophan. Further, MS/MS showed that the tryptophan residues could be modified selectively with ethynyl functionalities even when the tryptophan was present as a part of the protein. Finally, the terminal alkyne was used to label a model peptide with a fluorophore by means of copper-catalyzed click chemistry.

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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.

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.

Polymersomes composed of block copolymers of which the blocks are coupled via a hydrazone moiety are shown to exchange surface PEG chains with the environment via an aniline-catalyzed transimination under equilibrium conditions. This methodology is used to functionalize polymersomes with a different inner and outer moiety in a dynamic covalent way. Secondly, the exchange of surface properties is also demonstrated between differently functionalized polymersomes. These results, therefore, open new routes to the design of complex vesicular surfaces by dynamic exchange.

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