Acid-responsive fibrillation and urease-assisted defibrillation of phenylalanine: a transient supramolecular hydrogel

Self-Assembly, Bioactivity, and Nanomaterials Applications of Peptide Conjugates with Bulky Aromatic Terminal Groups

The self-assembly and structural and functional properties of peptide conjugates containing bulky terminal aromatic substituents are reviewed with a particular focus on bioactivity. Terminal moieties include Fmoc [fluorenylmethyloxycarbonyl], naphthalene, pyrene, naproxen, diimides of naphthalene or pyrene, and others. These provide a driving force for self-assembly due to π-stacking and hydrophobic interactions, in addition to the hydrogen bonding, electrostatic, and other forces between short peptides. The balance of these interactions leads to a propensity to self-assembly, even for conjugates to single amino acids. The hybrid molecules often form hydrogels built from a network of β-sheet fibrils. The properties of these as biomaterials to support cell culture, or in the development of molecules that can assemble in cells (in response to cellular enzymes, or otherwise) with a range of fascinating bioactivities such as anticancer or antimicrobial activity, are highlighted. In addition, applications of hydrogels as slow-release drug delivery systems and in catalysis and other applications are discussed. The aromatic nature of the substituents also provides a diversity of interesting optoelectronic properties that have been demonstrated in the literature, and an overview of this is also provided. Also discussed are coassembly and enzyme-instructed self-assembly which enable precise tuning and (stimulus-responsive) functionalization of peptide nanostructures.

pH-feedback systems to program autonomous self-assembly and material lifecycles

pH-responsive systems have gained importance for the development of smart materials and for biomedical applications because they can switch between different states by simple acid/base triggers. However, such equilibrium systems lack the autonomous behaviour that is so ubiquitous in living systems that self-regulate out of equilibrium. As a contribution to the emerging field of autonomous chemical systems, we have developed pH-feedback systems (pH-FS) based on the coupling of acid- and base-producing steps in chemical reaction networks. The resulting autonomous nonlinear pH curves can be coupled with a variety of pH-sensitive building blocks to program the lifecycles of the associated transient state at the level of self-assemblies and material systems. In this article, we discuss the different generations of such pH-feedback systems, the principles of their coupling to self-assemblies with lifecycles and highlight emerging concepts for the design of autonomous functional materials. The specificity, robustness, and flexible operation of such pH-FS can also be used to realize chemo-structural and chemo-mechanical feedbacks that extend the behaviour of such materials systems toward complex and functional life-like systems.

Insights into Chemically Fueled Supramolecular Polymers

Supramolecular polymerization can be controlled in space and time by chemical fuels. A nonassembled monomer is activated by the fuel and subsequently self-assembles into a polymer. Deactivation of the molecule either in solution or inside the polymer leads to disassembly. Whereas biology has already mastered this approach, fully artificial examples have only appeared in the past decade. Here, we map the available literature examples into four distinct regimes depending on their activation/deactivation rates and the equivalents of deactivating fuel. We present increasingly complex mathematical models, first considering only the chemical activation/deactivation rates (i.e., transient activation) and later including the full details of the isodesmic or cooperative supramolecular processes (i.e., transient self-assembly). We finish by showing that sustained oscillations are possible in chemically fueled cooperative supramolecular polymerization and provide mechanistic insights. We hope our models encourage the quantification of activation, deactivation, assembly, and disassembly kinetics in future studies.

Dissipative Acid-Fueled Reprogrammable Supramolecular Materials

Smart materials reversibly changing properties in response to a stimuli are promising for a broad array of applications. In this article, we report the use of trichloroacetic acid (TCA) as fuel to create new types of time-controlled materials switching reversibly from a gel to a solution (gel-sol-gel cycle). Applying various neutral amines as organogelators, TCA addition induces amine protonation, switching the system to a solution, while TCA decarboxylation over time enables a return to the initial gel state. Consequently, the newly obtained materials possess interesting time-dependent properties applied in the generation of remoldable objects, as an erasing ink, as chiroptical switches, or for the generation of new types of electrical systems.

Modulating defibrillation by tryptophan-mediated photo cleavage of disulfide bonds

Dibenzyloxycarbonyl-l-cystine forms amyloid-like fibers and a hydrogel; however, on UV irradiation, Trp-mediated disulfide bond cleavage occurs, leading to defibrillation and a gel to sol transition.

Photochromic and photothermal hydrogels derived from natural amino acids and heteropoly acids

A new class of supramolecular hydrogels have been designed and synthesized via the co-assembly of basic amino acids (AAs) and heteropoly acids (HPAs) under acidic conditions. The formation of gel-like samples is identified using an inverted tube method, rheology, and scanning and transmission electron microscopy. Fourier transform infrared spectroscopy reveals that the structural integrity of the HPAs is maintained during the gelation process. X-ray photoelectron spectroscopy (XPS) and proton nuclear magnetic resonance spectroscopy demonstrate that the anionic HPAs interact with both the protonated α-NH₂ and the protonated side groups of the basic amino acids, initiating the preferential growth of one-dimensional nanofibers. These nanofibers bundle and entangle with each other to form extended three-dimensional network structures. The resulting AA/HPA supramolecular hydrogels show clear stereoselectivity of the basic amino acids. With the decreasing enantiomeric excess of the basic amino acids, the gelation propensity of the AA/HPA complexes is found to be depressed. The co-assembled hydrogels show the UV-responsive photochromic behaviour because of the presence of HPAs. The corresponding XPS data confirm that the photochromism of the hydrogels is attributed to the intervalence charge-transfer transition resulting from the reduction of HPAs. Interestingly, the reduced HPAs within the hydrogel matrix can absorb the near-infrared (NIR) light and exhibit photo-thermal conversion properties, which elevates the bulk temperature of the AA/HPA hydrogels and induces the gel-to-sol transition. This study unveils that HPAs have unique capacity to promote the gelation of basic amino acids for the construction of supramolecular soft materials with functional features.

Double diffusion for the programmable spatiotemporal patterning of multi-domain supramolecular gels

To achieve spatial resolution of a multi-component gel, a double diffusion approach is used which enables the precise programming of self-assembled patterned domains with well-defined shapes and sizes. The low-molecular-weight gelators (LMWGs) used in this study are pH-responsive DBS-CO₂H and thermally-responsive DBS-CONHNH₂ (both based on 1,3:2,4-dibenzylidenesorbitol, DBS). A DBS-CONHNH₂ gel was initially assembled in a tray, and then loaded at carefully-selected positions with either basified DBS-CO₂H (i.e. DBS-carboxylate) or an acid. These soluble components subsequently diffuse through the pre-formed gel matrix, and in the domains when/where they mix, protonation of the DBS-carboxylate induces self-assembly of the DBS-CO₂H network, leading to a patterned gel-in-gel object with well-defined shape and dimensions. Using a strong acid achieves fast gelation kinetics, creating smaller, better-defined macroscale objects but with less nanoscale order. Using a weak acid source with slow kinetics, gives slightly larger objects, but on the nanoscale the DBS-CO₂H network formation is better controlled, giving more homogeneous nanoscale structures and stiffer objects. The patterned objects can be further reinforced by the presence of agarose polymer gelator. The shape of the patterning is programmed by both the shape of the central reservoir and the starting geometry in which the reservoirs are organised, with the balance between factors depending on assembly kinetics, as dictated by the choice of acid. This simple methodology therefore enables programming of patterned gels with spatiotemporal control and emergent patterning characteristics.

To achieve spatial resolution of a multi-component gel, a double diffusion approach is used which enables the precise programming of emergent self-assembled patterned domains with well-defined shapes and sizes.

Dissipative operation of pH-responsive DNA-based nanodevices

We demonstrate here the use of 2-(4-chlorophenyl)-2-cyanopropanoic acid (CPA) and nitroacetic acid (NAA) as convenient chemical fuels to drive the dissipative operation of DNA-based nanodevices. Addition of either of the fuel acids to a water solution initially causes a rapid transient pH decrease, which is then followed by a slower pH increase. We have employed such low-to-high pH cycles to control in a dissipative way the operation of two model DNA-based nanodevices: a DNA nanoswitch undergoing time-programmable open–close–open cycles of motion, and a DNA-based receptor able to release-uptake a DNA cargo strand. The kinetics of the transient operation of both systems can be easily modulated by varying the concentration of the acid fuel added to the solution and both acid fuels show an efficient reversibility which further supports their versatility.

We demonstrate here the use of 2-(4-chlorophenyl)-2-cyanopropanoic acid (CPA) and nitroacetic acid (NAA) as convenient chemical fuels to drive the dissipative operation of DNA-based nanodevices.

Chemically Fueled Three-State Chiroptical Switching Supramolecular Gel with Temporal Control

The recent discovery of temporally controlled gels opens broad perspectives to the field of smart functional materials. However, to obtain fully operative systems, the design of simple and robust gels displaying complex functions is desirable. Herein, we fuel dissipative gelating materials through iterative additions of trichloroacetic acid (TCA). This simple fuel enables to switch over time an acid/base-dependent commercially available amino acid gelator/DBU combination between three distinct states (anionic, cationic, and neutral), while liberating volatile CO₂ and CHCl₃ upon fuel consumption. Of interest, the anionic resting state of the system is obtained through trapping of 1 equiv of CO₂ through the formation of a carbamate. The system is tunable, robust, and resilient over time with over 25 consecutive sol-gel-sol cycles possible without significant loss of properties. Most importantly, because of the chiral nature of the amino acid gelator, the system features chiroptical switching properties moving reversibly between three distinct states as observed by ECD. The described system considerably enhances the potential of smart molecular devices for logic gates or data storage by adding a time dimension based on three states to the gelating materials. It is particularly simple in terms of chemical components involved, but it enables sophisticated functions.

Reaction-diffusion hydrogels from urease enzyme particles for patterned coatings

The reaction and diffusion of small molecules is used to initiate the formation of protective polymeric layers, or biofilms, that attach cells to surfaces. Here, inspired by biofilm formation, we present a general method for the growth of hydrogels from urease enzyme-particles by combining production of ammonia with a pH-regulated polymerization reaction in solution. We show through experiments and simulations how the propagating basic front and thiol-acrylate polymerization were continuously maintained by the localized urease reaction in the presence of urea, resulting in hydrogel layers around the enzyme particles at surfaces, interfaces or in motion. The hydrogels adhere the enzyme-particles to surfaces and have a tunable growth rate of the order of 10 µm min^-1 that depends on the size and spatial distribution of particles. This approach can be exploited to create enzyme-hydrogels or chemically patterned coatings for applications in biocatalytic flow reactors.

Urea-Urease Reaction in Controlling Properties of Supramolecular Hydrogels: Pros and Cons

Supramolecular hydrogels are useful in many areas such as cell culturing, catalysis, sensing, tissue engineering, drug delivery, environmental remediation and optoelectronics. The gels need specific properties for each application. The properties arise from a fibrous network that forms the matrix. A common method to prepare hydrogels is to use a pH change. Most methods result in a sudden pH jump and often lead to gels that are hard to reproduce and control. The urease-urea reaction can be used to control hydrogel properties by a uniform and controlled pH increase as well as to set up pH cycles. The reaction involves hydrolysis of urea by urease and production of ammonia which increases the pH. The rate of ammonia production can be controlled which can be used to prepare gels with differing properties. Herein, we show how the urease-urea reaction can be used for the construction of next generation functional materials.

Time-programmable pH: decarboxylation of nitroacetic acid allows the time-controlled rising of pH to a definite value

In this report it is shown that nitroacetic acid 1 (O2NCH2CO2H) can be conveniently used to control the pH of a water solution over time. Time-programmable sequences of the kind pH1(high)-pH2(low)-pH3(high) can be achieved, where both the extent of the initial pH jump (pH1(high)-pH2(low)) and the time required for the subsequent pH rising (pH2(low)-pH3(high)) can be predictably controlled by a judicious choice of the absolute and relative concentrations of the reagents (acid 1 and NaOH). Successive pH1(high)-pH2(low)-pH3(high) sequences can be obtained by subsequent additions of acid 1. As a proof of concept, the method is applied to control over time the pH-dependent host-guest interaction between alpha-cyclodextrin and p-aminobenzoic acid.

Supramolecular gelation controlled by an iodine clock

Programming supramolecular assembly in the time domain is a fundamental aspect of the design of biomimetic materials. We achieved the time-controlled sol-gel transition of a poly(vinyl alcohol)-iodine supramolecular complex by generating iodine in situ with a clock reaction. We demonstrate that both the gelation time and the mechanical properties of the resulting hydrogel can be tuned by properly selecting the clock parameters or through competitive iodine complexation.

Spatial and temporal diffusion-control of dynamic multi-domain self-assembled gels

The dynamic assembly of a pH-responsive low-molecular-weight gelator (LMWG) within the pre-formed matrix of a second LMWG has been achieved via diffusion of an acid from a reservoir cut into the gel. Self-assembly of the acid-triggered LMWG as it converts from micellar aggregates at basic pH into gel nanofibers at lower pH values can be both spatially and temporally controlled. The pH-responsive LMWG has an impact on the stiffness of the pre-formed gel in the domains in which it assembles. When low acid concentrations are used, LMWG assembly is transient – after the initial proton diffusion phase, the pH rises and disassembly occurs as the system equilibrates. Re-application of additional acid as ‘fuel’ can then re-assemble the LMWG network. Using glucono-δ-lactone (which slowly hydrolyses to gluconic acid) instead of HCl gives slower, more spatially-restricted assembly, and creates longer-lasting pH gradients within the gel. The presence of an agarose polymer gel network improves the mechanical strength of the gels and appears to slightly enhance the rate of proton diffusion. More sophisticated reservoir shapes can be cut into these more mechanically robust gels, enabling the creation of diffusion waves with different geometries, and hence different patterns of LMWG activation. Multiple reservoirs can be used to create overlapping proton diffusion waves, hence achieving differentiated pH patterns in the gel. Using acid diffusion in this way within gels is an intriguing and powerful way of dynamic patterning. The ability to temporally-evolve spatially-resolved patterns using biocompatible weak acids, and the change in rheological performance of the triggered domains, suggest potential future applications of this strategy in tissue engineering.

The assembly of a pH-sensitive LMWG within a pre-formed network of a second LMWG can be achieved by diffusing acids from pre-cut reservoirs, giving rise to patterned gels in which the rheological properties evolve with spatial and temporal control.

A transient non-covalent hydrogel by a supramolecular gelator with dynamic covalent bonds

In aqueous solution, equilibrium self-assembly and gelation occur at higher concentration but on addition of EDC non-equilibrium self-assembly and transient hydrogels are formed at low concentration, which dissolve upon anhydride hydrolysis.