Dissipative Self-Assembly of a Molecular Gelator by Using a Chemical Fuel

Active droplets through enzyme-free, dynamic phosphorylation

Life continuously transduces energy to perform critical functions using energy stored in reactive molecules like ATP or NADH. ATP dynamically phosphorylates active sites on proteins and thereby regulates their function. Inspired by such machinery, regulating supramolecular functions using energy stored in reactive molecules has gained traction. Enzyme-free, synthetic systems that use dynamic phosphorylation to regulate supramolecular processes have not yet been reported, to our knowledge. Here, we show an enzyme-free reaction cycle that consumes the phosphorylating agent monoamidophosphate by transiently phosphorylating histidine and histidine-containing peptides. The phosphorylated species are labile and deactivate through hydrolysis. The cycle exhibits versatility and tunability, allowing for the dynamic phosphorylation of multiple precursors with a tunable half-life. Notably, we show the resulting phosphorylated products can regulate the peptide's phase separation, leading to active droplets that require the continuous conversion of fuel to sustain. The reaction cycle will be valuable as a model for biological phosphorylation but can also offer insights into protocell formation.

Dynamic supramolecular hydrogels mediated by chemical reactions

Supramolecular self-assembly in a biological system is usually dominated by sophisticated metabolic processes (chemical reactions) such as catalysis of enzymes and consumption of high energy chemicals, leading to groups of biomolecules with unique dynamics and functions in an aqueous environment. In recent years, increasing efforts have been made to couple chemical reactions to molecular self-assembly, with the aim of creating supramolecular materials with lifelike properties and functions. In this feature article, after summarising the work of chemical reaction mediated supramolecular hydrogels, we first focus on a typical example where dynamic self-assembly of molecular hydrogels is activated by in situ formation of a hydrazone bond in water. We discuss how the formation of the hydrazone-based supramolecular hydrogels can be controlled in time and space. After that, we describe transient assembly of supramolecular hydrogels powered by out-of-equilibrium chemical reaction networks regulated by chemical fuels, which show unique properties such as finite lifetime, dynamic structures, and regenerative capabilities. Finally, we provide a perspective on the future investigations that need to be done urgently, which range from fundamental research to real-life applications of dynamic supramolecular hydrogels.

Chemically Fueled Dissipative Cross-Linking of Protein Hydrogels Mediated by Protein Unfolding

Cells assemble dynamic protein-based nanostructures far from equilibrium, such as microtubules, in a process referred to as dissipative assembly. Synthetic analogues have utilized chemical fuels and reaction networks to form transient hydrogels and molecular assemblies from small molecule or synthetic polymer building blocks. Here, we demonstrate dissipative cross-linking of transient protein hydrogels using a redox cycle, which exhibit protein unfolding-dependent lifetimes and mechanical properties. Fast oxidation of cysteine groups on bovine serum albumin by hydrogen peroxide, the chemical fuel, formed transient hydrogels with disulfide bond cross-links that degraded over hours by a slow reductive back reaction. Interestingly, despite increased cross-linking, the hydrogel lifetime decreased as a function of increasing denaturant concentration. Experiments showed that the solvent-accessible cysteine concentration increased with increasing denaturant concentration due to unfolding of secondary structures. The increased cysteine concentration consumed more fuel, which led to less direction oxidation of the reducing agent and affected a shorter hydrogel lifetime. Increased hydrogel stiffness, disulfide cross-linking density, and decreased oxidation of redox-sensitive fluorescent probes at a high denaturant concentration provided evidence supporting the unveiling of additional cysteine cross-linking sites and more rapid consumption of hydrogen peroxide at higher denaturant concentrations. Taken together, the results indicate that the protein secondary structure mediated the transient hydrogel lifetime and mechanical properties by mediating the redox reactions, a feature unique to biomacromolecules that exhibit a higher order structure. While prior works have focused on the effects of the fuel concentration on dissipative assembly of non-biological molecules, this work demonstrates that the protein structure, even in nearly fully denatured proteins, can exert similar control over reaction kinetics, lifetime, and resulting mechanical properties of transient hydrogels.

Transient and Dissipative Host-Guest Hydrogels Regulated by Consumption of a Reactive Chemical Fuel

The transient self-assembly of molecules under the direction of a consumable fuel source is fundamental to biological processes such as cellular organization and motility. Such biomolecular assemblies exist in an out-of-equilibrium state, requiring continuous consumption of high energy molecules. At the same time, the creation of bioinspired supramolecular hydrogels has traditionally focused on associations occurring at the thermodynamic equilibrium state. Here, hydrogels are prepared from cucurbit[7]uril host-guest supramolecular interactions through transient physical crosslinking driven by the consumption of a reactive chemical fuel. Upon action from this fuel, the affinity and dynamics of CB[7]-guest recognition are altered. In this way, the lifetime of transient hydrogel formation and the dynamic modulus obtained are governed by fuel consumption, rather than being directed by equilibrium complex formation.

Sugar-Fueled Dissipative Living Materials

Dissipative behaviors in biology are fuel-driven processes controlled by living cells, and they shape the structural and functional complexities in biological materials. This concept has inspired the development of various forms of synthetic dissipative materials controlled by time-dependent consumption of chemical or physical fuels, such as reactive chemical species, light, and electricity. To date, synthetic living materials featuring dissipative behaviors directly controlled by the fuel consumption of their constituent cells is unprecedented. In this paper, we report a chemical fuel-driven dissipative behavior of living materials comprising Staphylococcus epidermidis and telechelic block copolymers. The macroscopic phase transition is controlled by d-glucose which serves a dual role of a competitive disassembling agent and a biological fuel source for living cells. Our work is a significant step toward constructing a synthetic dissipative living system and provides a new tool and knowledge to design emergent living materials.

Bioinspired Self-Resettable Hydrogel Actuators Powered by a Chemical Fuel

The movements of soft living tissues, such as muscle, have sparked a strong interest in the design of hydrogel actuators; however, so far, typical manmade examples still lag behind their biological counterparts, which usually function under nonequilibrium conditions through the consumption of high-energy biomolecules and show highly autonomous behaviors. Here, we report on self-resettable hydrogel actuators that are powered by a chemical fuel and can spontaneously return to their original states over time once the fuels are depleted. Self-resettable actuation originates from a chemical fuel-mediated transient change in the hydrophilicity of the hydrogel networks. The actuation extent and duration can be programmed by the fuel levels, and the self-resettable actuation process is highly recyclable through refueling. Furthermore, various proof-of-concept autonomous soft robots are created, resembling the movements of soft-bodied creatures in nature. This work may serve as a starting point for the development of lifelike soft robots with autonomous behaviors.

Transient Host-Guest Complexation To Control Catalytic Activity

Signal transduction mechanisms are key to living systems. Cells respond to signals by changing catalytic activity of enzymes. This signal responsive catalysis is crucial in the regulation of (bio)chemical reaction networks (CRNs). Inspired by these networks, we report an artificial signal responsive system that shows signal-induced temporary catalyst activation. We use an unstable signal to temporarily activate an out of equilibrium CRN, generating transient host–guest complexes to control catalytic activity. Esters with favorable binding toward the cucurbit[7]uril (CB[7]) supramolecular host are used as temporary signals to form a transient complex with CB[7], replacing a CB[7]-bound guest. The esters are hydrolytically unstable, generating acids and alcohols, which do not bind to CB[7], leading to guest reuptake. We demonstrate the feasibility of the concept using signal-controlled temporary dye release and reuptake. The same signal controlled system was then used to tune the reaction rate of aniline catalyzed hydrazone formation. Varying the ester structure and concentration gave access to different catalyst liberation times and free catalyst concentration, regulating the overall reaction rate. With temporary signal controlled transient complex formation we can tune the kinetics of a second chemical reaction, in which the signal does not participate. This system shows promise for building more complex nonbiological networks, to ultimately arrive at signal transduction in organic materials.

Self-Regulating Colloidal Co-Assemblies That Accelerate Their Own Destruction via Chemo-Structural Feedback

Biological self‐assemblies self‐ and cross‐regulate each other via chemical reaction networks (CRNs) and feedback. Although artificial transient self‐assemblies have been realized via activation/deactivation CRNs, the transient structures themselves do mostly not engage in the CRN. We introduce a rational design approach for chemo‐structural feedback, and present a transient colloidal co‐assembly system, where the formed co‐assemblies accelerate their destruction autonomously. We achieve this by immobilizing enzymes of a deactivating acid‐producing enzymatic cascade on pH‐switchable microgels that can form co‐assemblies at high pH. Since the enzyme partners are immobilized on individual microgels, the co‐assembled state brings them close enough for enhanced acid generation. The amplified deactivator production (acid) leads to an almost two‐fold reduction in the lifetime of the transiently formed pH‐state. Our study thus introduces versatile mechanisms for chemo‐structural feedback.

Programmable Transient Supramolecular Chiral G-quadruplex Hydrogels via a Chemically Fueled Non-Equilibrium Self-assembly Strategy

The temporal and spatial control of natural systems has aroused great interest in the creation of synthetic mimics. Operating with boronic ester-based dynamic covalent chemistry and coupling it with an internal pH feedback system, herein, we developed a new chemically fueled reaction network to design non-equilibrium supramolecular chiral G-quadruplex hydrogels with programmable lifetime from minutes, to hours, to days, as well as high transparency and conductivity, excellent injectability and rapid self-healability. The cycle system can be controlled via in-situ kinetically-controlled formation and dissociation of dynamic boronic ester bonds between cis-diols of guanosine (G) and 5-fluorobenzoxaborole (B) under chemical fuels (KOH and 1,3-propanesultone), leading to the formation of a precipitate-solution-gel-precipitate cycle under non-equilibrium conditions. A combined experimental-computational approach revealed that the underlying mechanism of the non-equilibrium self-assembly involves aggregation and disaggregation of right-handed helical G-quadruplex superstructure. With consecutive cycles of fuel addition, the non-equilibrium system can be easily refueled at least 6 cycles without obvious loss in the rheological moduli of the transient hydrogels. The proposed dynamic boronic ester-based non-equilibrium self-assembly strategy offers a new option to design next-generation adaptive and interactive smart materials.

Gelation Kinetics-Structure Analysis of pH-triggered Low Molecular Weight Hydrogelators

Properties such as shear modulus, gelation time, structure of supramolecular hydrogels are strongly dependent on self-assembly, gelation triggering mechanism and processes used to form the gel. In our work we extend reported rheology analysis methodologies to pH-triggered supramolecular gels to understand structural insight using a model system based on N-N' Dibenzoyl-L-Cystine pH-triggered hydrogelator and Glucono-δ-Lactone as the trigger. We observed that Avrami growth model when applied to time-sweep rheological data of gels formed at lower trigger concentrations provide estimates of fractal dimension which agree well compared with visualization of the microstructure as seen via Confocal Laser Scanning Microscopy, for a range of gelator concentrations.

Triggering a transient organo-gelation system in a chemically active solvent

A transient organo-gelation system with spatiotemporal dynamic properties is described. Here, the solvent actively controls a complex set of equilibria that underpin the dynamic assembly event. The observed metastability is due to the in situ formation of a secondary solvent, acting as an antagonist against the primary solvent of the organogel.

Chemically Fueled Self-Assembly in Biology and Chemistry

Life is a non-equilibrium state of matter maintained at the expense of energy. Nature uses predominantly chemical energy stored in thermodynamically activated, but kinetically stable, molecules. These high-energy molecules are exploited for the synthesis of other biomolecules, for the activation of biological machinery such as pumps and motors, and for the maintenance of structural order. Knowledge of how chemical energy is transferred to biochemical processes is essential for the development of artificial systems with life-like processes. Here, we discuss how chemical energy can be used to control the structural organization of organic molecules. Four different strategies have been identified according to a distinguishable physical-organic basis. For each class, one example from biology and one from chemistry are discussed in detail to illustrate the practical implementation of each concept and the distinct opportunities they offer. Specific attention is paid to the discussion of chemically fueled non-equilibrium self-assembly. We discuss the meaning of non-equilibrium self-assembly, its kinetic origin, and strategies to develop synthetic non-equilibrium systems.

Pathway-Dependent Phase Transitions of Supramolecular Self-assemblies Containing Cationic Amphiphiles with Azobenzene and Disulfide Groups

There have been several attempts to construct supramolecular chemical systems that mimic the phase transitions in living systems. However, most of these phase transitions are one-to-one and induced by one stimulus or chemical; there have been few reports on the pathway-dependent phase transition of supramolecular self-assemblies in multi-step. To induce multistep phase transitions, molecular crystals were prepared that contained a cationic amphiphile bearing azobenzene and disulfide groups. A reducing agent caused the crystals to become vesicles, and adjacent, non-touching vesicles fused under UV and subsequent visible light. Adding a reducing agent to the worm-like aggregates that were generated after UV irradiation of the original crystals resulted in the growth of sheet-like aggregates. ¹ H NMR and fluorescence anisotropy measurements showed that a series of phase transitions was induced by changes in the phase structures from molecular conversions of the reactive amphiphiles. The multiple pathway-dependent phase transitions of supramolecular self-assemblies can provide a methodology for developing new stimuli-responsive materials that exhibit the desirable properties under specific circumstances from a systems chemistry viewpoint.

Chemical Fueling Enables Molecular Complexification of Self-Replicators*

Unravelling how the complexity of living systems can (have) emerge(d) from simple chemical reactions is one of the grand challenges in contemporary science. Evolving systems of self-replicating molecules may hold the key to this question. Here we show that, when a system of replicators is subjected to a regime where replication competes with replicator destruction, simple and fast replicators can give way to more complex and slower ones. The structurally more complex replicator was found to be functionally more proficient in the catalysis of a model reaction. These results show that chemical fueling can maintain systems of replicators out of equilibrium, populating more complex replicators that are otherwise not readily accessible. Such complexification represents an important requirement for achieving open-ended evolution as it should allow improved and ultimately also new functions to emerge.

Parasitic behavior in competing chemically fueled reaction cycles

Non-equilibrium, fuel-driven reaction cycles serve as model systems of the intricate reaction networks of life. Rich and dynamic behavior is observed when reaction cycles regulate assembly processes, such as phase separation. However, it remains unclear how the interplay between multiple reaction cycles affects the success of emergent assemblies. To tackle this question, we created a library of molecules that compete for a common fuel that transiently activates products. Often, the competition for fuel implies that a competitor decreases the lifetime of these products. However, in cases where the transient competitor product can phase-separate, such a competitor can increase the survival time of one product. Moreover, in the presence of oscillatory fueling, the same mechanism reduces variations in the product concentration while the concentration variations of the competitor product are enhanced. Like a parasite, the product benefits from the protection of the host against deactivation and increases its robustness against fuel variations at the expense of the robustness of the host. Such a parasitic behavior in multiple fuel-driven reaction cycles represents a lifelike trait, paving the way for the bottom-up design of synthetic life.

Chemically Fueled Volume Phase Transition of Polyacid Microgels

Microgels are soft colloids that show responsive behavior and are easy to functionalize for applications. They are considered key components for future smart colloidal material systems. However, so far microgel systems have almost exclusively been studied in classical responsive switching settings using external triggers, while internally organized, autonomous control mechanisms as found in supramolecular chemistry and DNA nanotechnology relying on fuel-driven out-of-equilibrium concepts have not been implemented into microgel systems. Here, we introduce chemically fueled transient volume phase transitions (VPTs) for poly(methacrylic acid) (PMAA) microgels, where the collapsed hydrophobic state can be programmed using the fuel concentration in a cyclic reaction network. We discuss details of the system behavior as a function of pH and fuel amount, unravel kinetically trapped regions and showcase transient encapsulation and time-programmed release as a first application.

The Transient Covalent Bond in Abiotic Nonequilibrium Systems

Biochemical systems accomplish many critical functions with by operating out-of-equilibrium using the energy of chemical fuels. The formation of a transient covalent bond is a simple but very effective tool in designing analogous reaction networks. This Minireview focuses on the fuel chemistries that have been used to generate transient bonds in recent demonstrations of abiotic nonequilibrium systems (i.e., systems that do not make use of biological components). Fuel reactions are divided into two fundamental classifications depending on whether the fuel contributes structural elements to the activated state, a distinction that dictates how they can be used. Reported systems are further categorized by overall fuel reaction (e.g., hydrolysis of alkylating agents, carbodiimide hydration) and illustrate how similar chemistry can be used to effect a wide range of nonequilibrium behavior, ranging from self-assembly to the operation of molecular machines.

Autonomous Transient pH Flips Shaped by Layered Compartmentalization of Antagonistic Enzymatic Reactions

Transient signaling orchestrates complex spatiotemporal behaviour in living organisms via (bio)chemical reaction networks (CRNs). Compartmentalization of signal processing is an important aspect for controlling such networks. However, artificial CRNs mostly focus on homogeneous solutions to program autonomous self-assembling systems, which limits their accessible behaviour and tuneability. Here, we introduce layered compartments housing antagonistic pH-modulating enzymes and demonstrate that transient pH signals in a supernatant solution can be programmed based on spatial delays. This overcomes limitations of activity mismatches of antagonistic enzymes in solution and allows to flexibly program acidic and alkaline pH lifecycles beyond the possibilities of homogeneous solutions. Lag time, lifetime, and the pH minima and maxima can be precisely programmed by adjusting spatial and kinetic conditions. We integrate these spatially controlled pH flips with switchable peptides, furnishing time-programmed self-assemblies and hydrogel material system.

Organocatalytic Control over a Fuel-Driven Transient-Esterification Network*

Signal transduction in living systems is the conversion of information into a chemical change, and is the principal process by which cells communicate. In nature, these functions are encoded in non-equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. However, man-made catalytically controlled networks are rare. We incorporated catalysis into an artificial fuel-driven out-of-equilibrium CRN, where the forward (ester formation) and backward (ester hydrolysis) reactions are controlled by varying the ratio of two organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. This fuel-driven strategy was expanded to a responsive polymer system, where transient polymer conformation and aggregation are controlled through fuel and catalyst levels. Altogether, we show that organocatalysis can be used to control a man-made fuel-driven system and induce a change in a macromolecular superstructure, as in natural non-equilibrium systems.

Coupled Metabolic Cycles Allow Out-of-Equilibrium Autopoietic Vesicle Replication

We report chemically fuelled out-of-equilibrium self-replicating vesicles based on surfactant formation. We studied the vesicles' autocatalytic formation using UPLC to determine monomer concentration and interferometric scattering microscopy at the nanoparticle level. Unlike related reports of chemically fuelled self-replicating micelles, our vesicular system was too stable to surfactant degradation to be maintained out of equilibrium. The introduction of a catalyst, which introduces a second catalytic cycle into the metabolic network, was used to close the first cycle. This shows how coupled catalytic cycles can create a metabolic network that allows the creation and perseverance of fuel-driven, out-of-equilibrium self-replicating vesicles.

Chemical-Fuel-Driven Assembly in Macromolecular Science: Recent Advances and Challenges

In the past decade, chemical-fuel-driven processes have been integrated with synthetic self-assembled systems, in which both the formation and properties can be carefully controlled. This strategy can drive systems far away from equilibrium, tailor the lifetime window of transient self-assembled systems, thus holding promise for future smart, adaptive, self-regulated, and life-like systems. By judging whether the building blocks or transient self-assembled systems participate in the fuel-to-waste conversion, the reported systems can be divided into two classes: dissipative self-assembly and self-assembly under dissipative conditions. Among these systems, the utilization of macromolecular building blocks to design non-equilibrium self-assemblied systems is becoming common. Macromolecular systems capable of dissipating energy with a programmed time domain have found widespread application, and have therefore been an active field of scientific inquiry. This Minireview aims to highlight the recent progress and opportunities of chemical-fuel-driven assembly in macromolecules. We envision that chemical-fuel-driven approach will play an increasingly important role in polymer science in the near future.

Nucleotide-Selective Templated Self-Assembly of Nanoreactors under Dissipative Conditions

Nature adopts complex chemical networks to finely tune biochemical processes. Indeed, small biomolecules play a key role in regulating the flux of metabolic pathways. Chemistry, which was traditionally focused on reactions in simple mixtures, is dedicating increasing attention to the network reactivity of highly complex synthetic systems, able to display new kinetic phenomena. Herein, we show that the addition of monophosphate nucleosides to a mixture of amphiphiles and reagents leads to the selective templated formation of self-assembled structures, which can accelerate a reaction between two hydrophobic reactants. The correct matching between nucleotide and the amphiphile head group is fundamental for the selective formation of the assemblies and for the consequent up-regulation of the chemical reaction. Transient stability of the nanoreactors is obtained under dissipative conditions, driven by enzymatic dephosphorylation of the templating nucleotides. These results show that small molecules can play a key role in modulating network reactivity, by selectively templating self-assembled structures that are able to up-regulate chemical reaction pathways.

Four-Dimensional Deoxyribonucleic Acid-Gold Nanoparticle Assemblies

Organization of gold nanoobjects by oligonucleotides has resulted in many three-dimensional colloidal assemblies with diverse size, shape, and complexity; nonetheless, autonomous and temporal control during formation remains challenging. In contrast, living systems temporally and spatially self-regulate formation of functional structures by internally orchestrating assembly and disassembly kinetics of dissipative biomacromolecular networks. We present a novel approach for fabricating four-dimensional gold nanostructures by adding an additional dimension: time. The dissipative character of our system is achieved using exonuclease III digestion of deoxyribonucleic acid (DNA) fuel as an energy-dissipating pathway. Temporal control over amorphous clusters composed of spherical gold nanoparticles (AuNPs) and well-defined core-satellite structures from gold nanorods (AuNRs) and AuNPs is demonstrated. Furthermore, the high specificity of DNA hybridization allowed us to demonstrate selective activation of the evolution of multiple architectures of higher complexity in a single mixture containing small and larger spherical AuNPs and AuNRs.

Multiple Light Control Mechanisms in ATP-Fueled Non-equilibrium DNA Systems

Fuel-driven self-assemblies are gaining ground for creating autonomous systems and materials, whose temporal behavior is preprogrammed by a reaction network. However, up to now there has been a lack of simple external control mechanisms of the transient behavior, at best using remote and benign light control. Even more challenging is to use different wavelengths to modulate the reactivity of different components of the system, for example, as fuel or building blocks. Success would enable such systems to navigate along different trajectories in a wavelength-dependent fashion. Herein, we introduce the first examples of light control in ATP-fueled, dynamic covalent DNA polymerization systems organized in an enzymatic reaction network of concurrent ATP-powered ligation and restriction. We demonstrate concepts for light activation and modulation by introducing caged ATP derivatives and caged DNA building blocks, making it possible to realize light-activated fueling, self-sorting in structure and behavior, and transition across different wavelength-dependent dynamic steady states.

Self-Assembly Propensity Dictates Lifetimes in Transient Naphthalimide-Dipeptide Nanofibers

Transient self-assembly of dipeptide nanofibers with lifetimes that are predictably variable through dipeptide sequence design are presented. This was achieved using 1,8-naphthalimide (NI) amino acid methyl-esters (Phe, Tyr, Leu) that are biocatalytically coupled to amino acid-amides (Phe, Tyr, Leu, Val, Ala, Ser) to form self-assembling NI-dipeptides. However, competing hydrolysis of the dipeptides results in disassembly. It was demonstrated that the kinetic parameters like lifetimes of these nanofibers can be predictably regulated by the thermodynamic parameter, namely the self-assembly propensity of the constituent dipeptide sequence. These lifetimes could vary from minutes, to hours, to permanent gels that do not degrade. Moreover, the in-built NI fluorophore was utilized to image the transient nanostructures in solution with stimulated emission depletion (STED) based super-resolution fluorescence microscopy.

Dissipative Assembly of Macrocycles Comprising Multiple Transient Bonds

Dissipative assembly has great potential for the creation of new adaptive chemical systems. However, while molecular assembly at equilibrium is routinely used to prepare complex architectures from polyfunctional monomers, species formed out of equilibrium have, to this point, been structurally very simple. In most examples the fuel simply effects the formation of a single short-lived covalent bond. Herein, we show that chemical fuels can assemble bifunctional components into macrocycles containing multiple transient bonds. Specifically, dicarboxylic acids give aqueous dianhydride macrocycles on treatment with a carbodiimide. The macrocycles are assembled efficiently as a consequence of both fuel-dependent and fuel-independent mechanisms; they undergo slower decomposition, building up as the fuel recycles the components, and are a favored product of the dynamic exchange of the anhydride bonds. These results create new possibilities for generating structurally sophisticated out-of-equilibrium species.