Nonequilibrium Catalytic Supramolecular Assemblies of Melamine- and Imidazole-Based Dynamic Building Blocks

Selective Aliphatic Aldimine Formation and Stabilization by a Hydrophobic Capsule in Water

Imines and enamines are useful intermediates in biochemical aldol-type reactions since their acid/base characteristics are accessible near neutrality. However, the generation of aldimines in water is a challenge as the equilibrium favors the components alkyl amines and aldehydes. Here, we report the generation, recognition, and protection of aldimines accomplished by a water-soluble capsule. The capsule provides a well-defined hydrophobic cavity isolating aldimines from the medium by a dry seam of Se-N chalcogen bonds instead of a wet seam of hydrogen bonds in water. A series of diverse aldimines are formed in situ and trapped rapidly (within 45 min at mM concentrations) in D2O with more than 90% yields. The encapsulated aldimines are stable for at least one month under ambient conditions. The aliphatic aldimines are selectively formed in the capsule even though they are less stable than aromatic imines in solution. The aldimines can be released from the capsule containers by competitive guests such as adamantane.

ATP-Regulated Formation of Transient Peptide Amphiphiles Superstructures

Self-assembly of biotic systems serves as inspiration for the preparation of synthetic supramolecular assemblies to mimic the structural, temporal, and functional aspects of living systems. Despite peptide amphiphiles (PAs) being widely studied in the context of biomimetic and bioactive functional nanomaterials, very little is currently known about the reversible and spatiotemporal control of their hierarchical self-assemblies. Here, it is shown that PA-based supramolecular nanofibers can transiently form superstructures, through binding with oppositely charged adenosine triphosphate (ATP), leading to charge screening and stabilization of bundled nanofibers. Enzymatic hydrolysis of ATP to adenosine monophosphate and phosphates causes the disassembly of the superstructures and recovery of individual nanofibers. The lifetime of superstructures can be controlled by adjusting the concentration of either ATP or enzyme. The role that the formation of bundled PA nanofibers has on chemical reactivity and catalysis is also evaluated. It is observed that superstructuration is responsible for downregulation in the PA activity, which can then be recovered by gradual disassembly of the bundles. These results demonstrate the potential of reversible and controlled hierarchical self-assembly to modulate the reactivity and catalysis of peptide nanostructures.

Allosteric Control of the Catalytic Properties of Dipeptide-Based Supramolecular Assemblies

Allostery, as seen in extant biology, governs the activity regulation of enzymes through the redistribution of conformational equilibria upon binding an effector. Herein, a minimal design is demonstrated where a dipeptide can exploit dynamic imine linkage to condense with simple aldehydes to access spherical aggregates as catalytically active states, which facilitates an orthogonal reaction due to the closer proximity of catalytic residues (imidazoles). The allosteric site (amine) of the minimal catalyst can concomitantly bind to an inhibitor via a dynamic exchange, which leads to the alternation of the energy landscape of the self-assembled state, resulting in downregulation of catalytic activity. Further, temporal control over allosteric regulation is realized via a feedback-controlled autonomous reaction network that utilizes the hydrolytic activity of the (in)active state as a function of time.

Key structural features to favour imines over hydrates in water: pyridoxal phosphate as a muse

Imination reactions in water represent a challenge not only because of the high propensity of imines to be hydrolysed but also as a result of the competing hydrate formation through H2O addition to the aldehyde. In the present work we report a successful approach that allows for favouring imitation reactions while silencing hydrate formation. Such remarkable reactivity and selectivity can be attained by fine-tuning the electronic and steric structural features of the ortho-substituents of the carbonyl groups. It resulted from studying the structure-reactivity relationships in a series of condensation reactions between different amines and aldehydes, comparing the results to the ones obtained in the presence of the biologically-relevant pyridoxal phosphate (PLP). The key role of negatively-charged and sterically-crowding units (i.e., sulfonate groups) in disfavouring hydrate formation was corroborated by DFT and steric-hindrance calculations. Furthermore, the best-performing aldehyde leads to higher imine yields, selectivity and stability than those of PLP itself, allowing for the inhibition of a PLP-dependent enzyme (transaminase) through dynamic aldimine exchange. These results will increase the applicability of imine-based dynamic covalent chemistry (DCvC) under physiological conditions and will pave the way for the design of new carbonyl derivatives that might be used in the dynamic modification of biomolecules.

Guanosine-based hydrogel as a supramolecular scaffold for template-assisted macrocyclization

The G-quartet-like supramolecular assembly present in guanosine hydrogel templates macrocyclization between bis-azide and bis-alkyne fragments. The resulting macrocycle enhances viscoelastic properties, and strengthens the hydrogel network. This approach holds potential for the in situ synthesis of drugs and their simultaneous delivery in a stimuli-responsive manner.

Temporal (Dis)Assembly of Peptide Nanostructures Dictated by Native Multistep Catalytic Transformations

Emergence of complex catalytic machinery via simple building blocks under non-equilibrium conditions can contribute toward the system level understanding of the extant biocatalytic reaction network that fuels metabolism. Herein, we report temporal (dis)assembly of peptide nanostructures in presence of a cofactor dictated by native multistep cascade transformations. The short peptide can form a dynamic covalent bond with the thermodynamically activated substrate and recruit cofactor hemin to access non-equilibrium catalytic nanostructures (positive feedback). The neighboring imidazole and hemin moieties in the assembled state rapidly converted the substrate to product(s) via a two-step cascade reaction (hydrolase-peroxidase like) that subsequently triggered the disassembly of the catalytic nanostructures (negative feedback). The feedback coupled reaction cycle involving intrinsic catalytic prowess of short peptides to realize the advanced trait of two-stage cascade degradation of a thermodynamically activated substrate foreshadows the complex non-equilibrium protometabolic networks that might have preceded the chemical emergence of life.

Orbiting Self-Organization of Filament-Tethered Surface-Active Droplets

Dissipative chemical systems hold the potential to enable life-like behavior in synthetic matter, such as self-organization, motility, and dynamic switching between different states. Here, out-of-equilibrium self-organization is demonstrated by interconnected source and drain droplets at an air-water interface, which display dynamic behavior due to a hydrolysis reaction that generates a concentration gradient around the drain droplets. This concentration gradient interferes with the adhesion of self-assembled amphiphile filaments that grow from a source droplet. The chemical gradient sustains a unique orbiting of the drain droplet, which is proposed to be driven by the selective adhesion of the filaments to the front of the moving droplet, while filaments approaching from behind are destabilized upon contact with the hydrolysis product in the trail of the droplet. Potential applications are foreseen in the transfer of chemical signals amongst communicating droplets in rearranging networks, and the implementation of chemical reactions to drive complex positioning routines in life-like systems.

A Carbodiimide-Fueled Reaction Cycle That Forms Transient 5(4H)-Oxazolones

In life, molecular architectures, like the cytoskeletal proteins or the nucleolus, catalyze the conversion of chemical fuels to perform their functions. For example, tubulin catalyzes the hydrolysis of GTP to form a dynamic cytoskeletal network. In contrast, myosin uses the energy obtained by catalyzing the hydrolysis of ATP to exert forces. Artificial examples of such beautiful architectures are scarce partly because synthetic chemically fueled reaction cycles are relatively rare. Here, we introduce a new chemical reaction cycle driven by the hydration of a carbodiimide. Unlike other carbodiimide-fueled reaction cycles, the proposed cycle forms a transient 5(4H)-oxazolone. The reaction cycle is efficient in forming the transient product and is robust to operate under a wide range of fuel inputs, pH, and temperatures. The versatility of the precursors is vast, and we demonstrate several molecular designs that yield chemically fueled droplets, fibers, and crystals. We anticipate that the reaction cycle can offer a range of other assemblies and, due to its versatility, can also be incorporated into molecular motors and machines.

Communication and Cross-Regulation between Chemically Fueled Sender and Receiver Reaction Networks

Nature connects multiple fuel-driven chemical/enzymatic reaction networks (CRNs/ERNs) via cross-regulation to hierarchically control biofunctions for a tailored adaption in complex sensory landscapes. Herein, we introduce a facile example of communication and cross-regulation among two fuel-driven DNA-based ERNs regulated by a concatenated RNA transcription regulator. ERN1 ("sender") is designed for the fuel-driven promoter formation for T7 RNA polymerase, which activates RNA transcription. The produced RNA can deactivate or activate DNA in ERN2 ("receiver") by toehold-mediated strand displacement, leading to a communication between two ERNs. The RNA from ERN1 can repress or promote the fuel-driven state of ERN2; ERN2 in turn feedbacks to regulate the lifetime of ERN1. Furthermore, the incorporation of RNase H allows for RNA degradation and enables the autonomous recovery of ERN2. We believe that concatenation of multiple CRNs/ERNs provides a basis for the design of more elaborate autonomous regulatory mechanisms in systems chemistry and synthetic biology.

Construction of transient supramolecular polymers controlled by mass transfer in biphasic systems

Inspired by life assembly systems, the construction of transient assembly systems with spatiotemporal control is crucial for developing intelligent materials. A widely adopted strategy is to couple the self-assembly with chemical reaction networks. However, orchestrating the kinetics of multiple reactions and assembly/disassembly processes without crosstalk in homogeneous solutions is not an easy task. To address this challenge, we propose a generic strategy by separating components into different phases, therefore, the evolution process of the system could be easily regulated by controlling the transport of components through different phases. Interference of multiple components that are troublesome in homogeneous systems could be diminished. Meanwhile, limited experimental parameters are involved in tuning the mass transfer instead of the complex kinetic matching and harsh reaction selectivity requirements. As a proof of concept, a transient metallo-supramolecular polymer (MSP) with dynamic luminescent color was constructed in an oil-water biphasic system by controlling the diffusion of the deactivator (water molecules) from the water phase into the oil phase. The lifetime of transient MSP could be precisely regulated not only by the content of chemical fuel, but also factors that affect the efficiency of mass transfer in between phases, such as the volume of the water phase, the stirring rate, and the temperature. We believe this strategy can be further extended to multi-compartment systems with passive diffusion or active transport of components, towards life-like complex assembly systems.

Evolution of Transient Luminescent Assemblies Regulated by Trace Water in Organic Solvents

Trace water in organic solvents can play a crucial role in the construction of supramolecular assemblies, which has not gained enough attention until very recent years. Herein, we demonstrate that residual water in organic solvents plays a decisive role in the regulation of the evolution of assembled structures and their functionality. By adding Mg(ClO₄)₂ into a multi-component organic solution containing terpyridine-based ligand 3Tpy and monodentate imidazole-based ligand M2, the system underwent an unexpected kinetic evolution. Metallo-supramolecular polymers (MSP) formed first by the coordination of 3Tpy and Mg²⁺, but they subsequently decomposed due to the interference of M2, resulting in a transient MSP system. Further investigation revealed that this occurred because residual water in the solvent and M2 cooperatively coordinated with Mg²⁺. This allowed M2 to capture Mg²⁺ from MSP, which led to depolymerization. However, owing to the slow reaction between trace water/M2/Mg²⁺, the formation of MSP still occurred first. Therefore, water regulated both the thermodynamics and kinetics of the system and was the key factor for constructing the transient MSP. Fine-tuning the water content and other assembly motifs regulated the assembly evolution pathway, tuned the MSP lifetime, and made the luminescent color of the system undergo intriguing transition processes over time.

Nonequilibrium Amyloid Polymers Exploit Dynamic Covalent Linkage to Temporally Control Charge-Selective Catalysis

Extant proteins exploit thermodynamically activated negatively charged coenzymes and hydrotropes to temporally access mechanistically important conformations that regulate vital biological functions, from metabolic reactions to expression modulation. Herein, we show that a short amyloid peptide can bind to a small molecular coenzyme by exploiting reversible covalent linkage to polymerize and access catalytically proficient nonequilibrium amyloid microphases. Subsequent hydrolysis of the activated coenzyme leads to depolymerization, realizing a variance of the surface charge of the assembly as a function of time. Such temporal change of surface charge dynamically modulates catalytic activities of the transient assemblies as observed in highly evolved modern-day biocatalysts.

Controllable Construction of Amino-Functionalized Dynamic Covalent Porous Polymers for High-Efficiency CO2 Capture from Flue Gas

The design of high-efficiency CO₂ adsorbents with low cost, high capacity, and easy desorption is of high significance for reducing carbon emissions, which yet remains a great challenge. This work proposes a facile construction strategy of amino-functional dynamic covalent materials for effective CO₂ capture from flue gas. Upon the dynamic imine assembly of N-site rich motif and aldehyde-based spacers, nanospheres and hollow nanotubes with spongy pores were constructed spontaneously at room temperature. A commercial amino-functional molecule tetraethylenepentamine could be facilely introduced into the dynamic covalent materials by virtue of the dynamic nature of imine assembly, thus inducing a high CO₂ capacity (1.27 mmol·g^-1) from simulated flue gas at 75 °C. This dynamic imine assembly strategy endowed the dynamic covalent materials with facile preparation, low cost, excellent CO₂ capacity, and outstanding cyclic stability, providing a mild and controllable approach for the development of competitive CO₂ adsorbents.

Biomacromolecule-Fueled Transient Volume Phase Transition of a Hydrogel

A metabolic cycle-inspired hydrogel which exhibits the biomacromolecule-fueled transient volume phase transition is reported. This hydrogel has the affinity and digestive capacity for a fuel α-poly-L-lysine by incorporating acrylic acid and trypsin. The hydrogel captured fuel and transiently shrank owing to the construction of electrostatic cross-linkages. This process was inherently connected with the digestion of these cross-linkages and the release of oligo-lysine as waste, which induced the reswelling of the hydrogel at equilibrium. The transient volume change of the hydrogel realized the fuel-stimulated transient release of a payload. This study provides a strategy for engineering materials with biomacromolecule-fueled dynamic functions under the out-of-equilibrium condition.