Carbon monoxide (CO) is an endogenously produced gasotransmitter with established anti-inflammatory and cytoprotective effects, yet its therapeutic exploration and application remain limited by challenges associated with controlled delivery. We recently demonstrated that diphenylcyclopropenone (DPCP) serves as a highly efficient, visible-light-triggered CO-generating motif when incorporated into polymeric micelles. Here, we investigate whether DPCP functions as a modular CO source by integrating it into chemically diverse crosslinked hydrogel networks. Hydrogels were selected as a representative solid biomaterial platform due to their widespread use in implants, tissue engineering, and cell culture. We demonstrate that DPCP-mediated CO generation is preserved across a range of hydrogel compositions, and that hydrogels allow for spatiotemporal control over CO release. However, CO release into solution depends on hydrogel network properties and chemistry. These results establish diarylcyclopropenones as a modular, tetherable, light-triggerable reservoir of CO gas and hydrogels as a tunable platform for its use.

Abstract Liquid-crystalline elastomers (LCEs) are soft, anisotropic materials exhibiting stimuli-responsive properties with potential utility in robotics, optics, and sensing. Historically, LCEs have been prepared via hydrosilylation reactions to form polysiloxane materials with either main-chain or side-chain liquid crystalline segments. Many recent examinations of LCEs utilize more accessible synthetic methods based on photopolymerization of acrylated monomers and compositions that contain heteroatom chain transfer or chain extension agents (e.g., thiols or amines) to enable network formation and alignment control. Here, we report the preparation of heteroatom-free (Si, S, and N) LCEs composed almost entirely of liquid crystalline precursors via catalytic chain transfer photopolymerization. Incorporation of a macrocyclic cobaloxime catalyst (CoBF) into formulations nearly entirely composed of a methacrylate liquid-crystal monomer (MeC6M) enables precise tuning of network mechanics and stimuli-responsive behavior through controlled chain transfer. The LCE maintains properties and stimuli response comparable to LCEs prepared by previously established methods with catalyst loading providing a direct handle over network mechanics and thermomechanical actuation.

Liquid crystalline elastomers (LCEs) are soft, anisotropic materials exhibiting stimuli-responsive properties with potential utility in robotics, optics, and sensing. Historically, LCEs have been prepared via hydrosilylation reactions to form polysilicon materials with either main chain or side-chain liquid crystalline segments. Many recent examinations of LCE utilize more accessible synthetic methods based on photopolymerization of acrylated monomers and compositions that contain heteroatom chain transfer or chain extension agents (e.g., thiols or amines) to enable network formation and alignment control. Here, we report the preparation of heteroatom-free (Si, S, N) LCE composed almost entirely of liquid crystalline precursors via catalytic chain transfer (CCT) photopolymerization. Incorporation of a macrocyclic cobaloxime catalyst (CoBF) into formulations nearly entirely composed of a methacrylate liquid crystal monomer (MeC6M) enables precise tuning of network mechanics and stimuli-responsive behavior through controlled chain transfer. The LCE maintain properties and stimuli-response comparable to LCEs prepared by previously established methods with catalyst loading providing a direct handle over network mechanics and thermomechanical actuation. The exceptionally high LC content afforded by this strategy may be particularly advantageous for dynamic optical application to maximize birefringence and minimize scatter.

Presented here is a detailed account of the development and implementation of macrocyclic cobaloxime complexes as sulfur-free, catalytic chain transfer agents (CTAs) in crosslinking photopoly-merizations. Although much of this review is dedicated to understanding the fundamentals of catalytic chain transfer (CCT) in photopolymerizations, its impact on network topology and resultant mechanical properties, future goals of applying this technology to multimaterial 3Dprinting are also discussed. It is our long-term ambition for catalytic, sulfur-free CTAs to supplant existing consumptive, sulfur-based agents to provide new, unexplored, and not currently possible to fabricate photopolymeric materials with a specific eye towards application in dentistry, additive manufacturing, and responsive materials.

Carbon monoxide (CO), along with nitric oxide and hydrogen sulfide, is one of a trinity of known gasotransmitters, or endogenously produced gaseous molecules that signal and regulate a panoply of physiological functions. CO releasing molecules (CORMs) are chemical tools that enable the study and application of this ephemeral gas, that, ideally, release CO on-demand when externally stimulated. Surveying the available triggers, photolysis is potentially advantageous: It is contactless and grants practitioners unparalleled spatial and temporal control. However, current phototriggered CORMs are capricious and do not meet current needs. Presented here is a highly efficient platform for the visible light triggered release of CO gas. This platform is built on a unique CO containing functionality, the cyclopropenone, which undergoes facile decarbonylation through visible light (470 nm) mediated photoredox catalysis. Due to the exothermic strain-release that occurs upon formation of CO, this photoreaction is rapid, quantitative, and has tunable release rates. To render this photo-CORM water-soluble, deliverable, and to keep reactants in proximity, necessary components were polymerized into block copolymers that self-assemble into CO releasing micelles (CORMIs). This platform was compared directly to other state-of-the-art CORMs, showing significantly improved CO production efficiency, lower toxicity, tunable release rates, and consistent efficacy in ex vivo and in vitro settings.

Thermosetting materials generated by photopolymerization are inherently highly crosslinked and suffer from significant shrinkage stress, are often brittle, and have a limited range of mechanical properties. Various classes of chain transfer agents (CTAs) have been investigated and developed to reduce the crosslinking density of photopolymers by terminating kinetic chains and initiating new chains in situ. Although CTAs are successful in manipulating the mechanical properties of photopolymers, they are traditionally consumed during the polymerization and are therefore required in high loadings (up to 20 wt% of total formulation). Moreover, traditional CTAs frequently contain sulfur which is malodorous and can create unstable formulations. Presented here is a catalytic, sulfur-free CTA that can be added in ppm quantities to exist-ing commercial monomer feedstocks to create photopolymers similar to those prepared using traditional CTAs, but at 10,000x lower loadings. These catalysts, which are based on macrocyclic cobaloximes, were found to tunably reduce the molecular weight of the kinetic chain proportional to catalyst loading. It was shown, using only commercial monomers, that this catalyst could reduce the glass transition temperature (Tg), rubbery modulus (E’rubbery), and stiffness.

The electron paramagnetic resonance (EPR) spectra of lanthanide(III) ions besides Gd3+, bound to small-molecule and protein chelators, are uncharacterized. Here, the EPR properties of 7 lanthanide(III) ions bound to the natural lanthanide-binding protein, lanmodulin (LanM), and the synthetic small-molecule chelator, 3,4,3-LI(1,2-HOPO) ("HOPO"), were systematically investigated. Echo-detected pulsed EPR spectra reveal intense signals from ions for which the normal continuous-wave first-derivative spectra are negligibly different from zero. Spectra of Kramers lanthanide ions Ce3+, Nd3+, Sm3+, Er3+, and Yb3+, and non-Kramers Tb3+ and Tm3+, bound to LanM are more similar to the ions in dilute aqueous:ethanol solution than to those coordinated with HOPO. Lanmodulins from two bacteria, with distinct metal-binding sites, had similar spectra for Tb3+ but different spectra for Nd3+. Spin echo dephasing rates (1/T-m) are faster for lanthanides than for most transition metals and limited detection of echoes to temperatures below similar to 6 to 12 K. Dephasing rates were environment dependent and decreased in the order water:ethanol>LanM>HOPO, which is attributed to decreasing librational motion. These results demonstrate that the EPR spectra and relaxation times of lanthanide(III) ions are sensitive to coordination environment, motivating wider application of these methods for characterization of both small-molecule and biomolecule interactions with lanthanides.

Binding of the neurotransmitter acetylcholine to its receptors on muscle fibers depolarizes the membrane and thereby triggers muscle contraction. We sought to understand at the level of three-dimensional structure how agonists and antagonists alter nicotinic acetylcholine receptor conformation. We used the muscle-type receptor from the Torpedo ray to first define the structure of the receptor in a resting, activatable state. We then determined the receptor structure bound to the agonist carbachol, which stabilizes an asymmetric, closed channel desensitized state. We find conformational changes in a peripheral membrane helix are tied to recovery from desensitization. To probe mechanisms of antagonism, we obtained receptor structures with the active component of curare, a poison arrow toxin and precursor to modern muscle relaxants. d-Tubocurarine stabilizes the receptor in a desensitized-like state in the presence and absence of agonist. These findings define the transitions between resting and desensitized states and reveal divergent means by which antagonists block channel activity of the muscle-type nicotinic receptor. Here the authors reveal the structural basis of how the nicotinic acetylcholine receptor type found on skeletal muscle and in fish electric organs desensitizes in response to agonist and how the arrow poison curare antagonizes the channel by stabilizing a desensitized state.

Nicotinic acetylcholine receptors are pentameric neurotransmitter-gated ion channels and are the archetypal members of the Cys-loop receptor superfamily. The nicotinic receptor subtype present in muscle fibers mediates the fast communication between motor neurons and skeletal muscles. This subtype is an important therapeutic target for myasthenic disorders and for neuromuscular blockers used during clinical anesthesia. However, our understanding of how this receptor is modulated is limited by a lack of high-resolution structural information.

Nicotinic acetylcholine receptors are members of the Cys-loop superfamily of pentameric ligand-gated ion channels. The electric organ of the Torpedo ray is extraordinarily rich in an acetylcholine receptor that is homologous to the human nicotinic receptor found at the neuromuscular junction. Due to this abundant natural source in the fish and the relatively accessible preparation of the neuromuscular junction (compared to a central synapse), this muscle-type receptor and specifically the fish receptors have long been used as the prototype for study of nicotinic receptors. However, an absence of structural detail at high resolution has limited the chemical interpretation of this archetypal nicotinic receptor. One of the main concerns in preparing receptor for high resolution structural analysis was its documented sensitivity to particular detergents and requirements for specific lipids in order to maintain function after reconstitution in a membrane. Here, we present methods for purifying native nicotinic receptor from Torpedo electric tissue that maintains functionality after reconstitution and that is amenable to high resolution structural analysis. The specific developments we describe include detergent exchange during purification, inclusion of specific lipids during purification and for nanodisc reconstitution, and synthesis of a new affinity reagent for rapid isolation of receptors.

Semicrystalline polymeric materials possessing extraordinary mechanical properties were rapidly fabricated using light from low viscosity liquids at room temperature.

The nicotinic acetylcholine receptor, a pentameric ligand-gated ion channel, converts the free energy of binding of the neurotransmitter acetylcholine into opening of its central pore. Here we present the first high-resolution structure of the receptor type found in muscle-endplate membrane and in the muscle-derived electric tissues of fish. The native receptor was purified from Torpedo electric tissue and functionally reconstituted in lipids optimal for cryo-electron microscopy. The receptor was stabilized in a closed state by the binding of α-bungarotoxin. The structure reveals the binding of a toxin molecule at each of two subunit interfaces in a manner that would block the binding of acetylcholine. It also reveals a closed gate in the ion-conducting pore, formed by hydrophobic amino acid side chains, located ∼60 Å from the toxin binding sites. The structure provides a framework for understanding gating in ligand-gated channels and how mutations in the acetylcholine receptor cause congenital myasthenic syndromes.

The ability to behave in a fluidlike manner fundamentally separates thermoset and thermoplastic polymers. Bridging this divide, covalent adaptable networks (CANs) structurally resemble thermosets with permanent covalent crosslinks but are able to flow in a manner that resembles thermoplastic behavior only when a dynamic chemical reaction is active. As a consequence, the rheological behavior of CANs becomes intrinsically tied to the dynamic reaction kinetics and the stimuli that are used to trigger those, including temperature, light, and chemical stimuli, providing unprecedented control over viscoelastic properties. CANs represent a highly capable material that serves as a powerful tool to improve mechanical properties and processing in a wide variety of polymer applications, including composites, hydrogels, and shape-memory polymers. This review aims to highlight the enabling material properties of CANs and the applied fields where the CAN concept has been embraced.

Thermally transformable/responsive (meth)acrylate photopolymer networks were constructed from commercial (meth)acrylate esters and synthetic di- and mono(meth)acylate monomers bearing thioester functionalities. The thermal responsiveness, here self-limited exchange, relied on the catalytic metamorphosis of thioesters into esters with the concomitant depletion of hydroxyls and subsequent generation of free thiols. The thioester-hydroxyl crossexchange was demonstrated in network systems with interchain thioesters as well as in networks with side-chain pendant thioacetyls. The interchain metamorphosis resulted in close to 80% conversion of thioesters into esters when 2 equiv of hydroxyl groups was initially present. In practical terms, such an outcome enabled efficient stress relaxation (60%) and good shape adaptation (90% shape fixity) in 1 h at 105 degrees C. On the other hand, side-chain S -> O acyl transfer reactions were found to vary in efficiency depending on the vicinity of thioesters and hydroxyls. When in close vicinity, efficient noncatalytic exchange was observed in materials with 1-to-1 thioester-to-hydroxyl ratios nearing 60%. The benefits of generating free thiols postpolymerization were further explored in enhanced two-stage curing systems where a subsequent thiol-ene photopolymerization was demonstrated at ambient as well as at elevated temperatures.

Truly recyclable and repolymerizable photopolymers were achieved by utilizing thiol–ene polymerization and thiol–thioester exchange reactions.

Muscle cells sense the mechanical properties of their microenvironment, and these properties can change in response to injury or disease. Hydrogels with dynamic material properties can be used to study the effect of such varying mechanical signals. Here, we report the ability of azadibenzocyclooctyne to undergo a cytocompatible, photoinitiated crosslinking reaction. This reaction is exploited as a strategy for on-demand stiffening of three-dimensional cell scaffolds formed through an initial strain-promoted azide-alkyne cycloaddition. Myoblasts encapsulated in these networks respond to increased matrix stiffness through decreased cell spreading and nuclear localization of Yes-associated protein 1 (YAP). However, when the photocrosslinking reaction is delayed to allow cell spreading, elongated myoblasts display increased YAP nuclear localization.

The synthesis of thiolactone monomers that mimic natural nucleosides and engage in robust ring opening polymerizations (ROP) is herein described. As each repeat unit contains a thioester functional group, dynamic rearrangement of the polymer is feasible via thiol-thioester exchange, demonstrated here by depolymerization of the polymers and coalescing of two polymers of different molecular weight or chemical composition. This approach constitutes the first step toward a platform that enables for the routine synthesis of sequence controlled polymers via dynamic template directed synthesis.

The extracellular matrix (ECM) constitutes a viscoelastic environment for cells. A growing body of evidence suggests that the behavior of cells cultured in naturally-derived or synthetic ECM mimics is influenced by the viscoelastic properties of these substrates. Adaptable crosslinking strategies provide a means to capture the viscoelasticity found in native soft tissues. In this work, we present a covalent adaptable hydrogel based on thioester exchange as a biomaterial for the in vitro culture of human mesenchymal stem cells. Through control of pH, gel stoichiometry, and crosslinker structure, viscoelastic properties in these crosslinked networks can be modulated across several orders of magnitude. We also propose a strategy to alter these properties in existing networks by the photo-uncaging of the catalyst 4-mercaptophenylacetic acid. Mesenchymal stem cells encapsulated in thioester hydrogels are able to elongate in 3D and display increased proliferation relative to those in static networks.

The exchange of thiolates and thiols has long been held as a nearly ideal reaction in dynamic covalent chemistry. The ability for the reaction to proceed smoothly in neutral aqueous media has propelled its widespread use in biochemistry, however, far fewer applications and studies have been directed towards its use in material science which primarily is performed in organic media. Herein, we present the exploration of this dynamic exchange in both small molecule and polymer settings with a wide sampling of thiols, thioesters, organic bases, and nucleophilic catalysts in various organic solvents. Effects of the character of the thiol and thioester, pK(a) or nucleophilicity of the catalyst, and heat on the reaction were investigated. The mechanism regarding the previously unexplored effectiveness of nucelophilic catalysts, such as quinuclidine or DABCO, to affect the thiol-thioester exchange was also explored. Finally, the use of the thiol-thioester exchange in a network polymer to reduce applied stresses or change shape of the material following polymerization was shown and the ability of basic and nucleophilic catalysts to promote these effects were benchmarked. The influence of polarity in these networks was also explored, with the rate of exchange shown to be easily tuned by the addition of diluents with varying polarities. Presented here is a so-called user's guide to the thiol-thioester exchange; we hope that this guide is instructive to practitioners in the field of material science which seek to utilize the thiol-thioester exchange in both linear and network polymers.