Aniline Catalysed Hydrazone Formation Reactions Show a Large Variation in Reaction Rates and Catalytic Effects

Transient Metallo-Lipidoid Assemblies Amplify Covalent Catalysis of Aqueous and Non-Aqueous Reactions

Dissipative supramolecular assemblies are hallmarks of living systems, contributing to their complex, dynamic structures and emerging functions. Living cells can spatiotemporally control diverse biochemical reactions in membrane compartments and condensates, regulating metabolite levels, signal transduction or remodeling of the cytoskeleton. Herein, we constructed membranous compartments using self-assembly of lipid-like amphiphiles (lipidoid) in aqueous medium. The new double-tailed lipidoid features Cu(II) coordinated with a tetravalent chelator that dictates the binding of two amphiphilic ligands in cis-orientation. Hydrophobic interactions between the lipidoids coupled with intermolecular hydrogen bonding led to a well-defined bilayer vesicle structure. Oil-soluble SNAr reaction is efficiently upregulated in the hydrophobic cavity, acting as a catalytic crucible. The modular system allows easy incorporation of exposed primary amine groups, which augments the catalysis of retro aldol and C-N bond formation reactions. Moreover, a higher-affinity chelator enables consumption of the Cu(II) template leveraging the differential thermodynamic stability, which allows a controllable lifetime of the vesicular assemblies. Concomitant temporal upregulation of the catalytic reactions could be tuned by the metal ion concentration. This work offers new possibilities for metal ion-mediated dynamic supramolecular systems, opening up a massive repertoire of functionally active dynamic "life-like" materials.

Switched "On" Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

We report the synthesis and operation of a molecular energy ratchet that transports a crown ether from solution onto a thread, along the axle, over a fluorophore, and off the other end of the thread back into bulk solution, all in response to a single pulse of a chemical fuel (CCl3CO2H). The fluorophore is a pyrene residue whose fluorescence is normally prevented by photoinduced electron transfer (PET) to a nearby N-methyltriazolium group. However, crown ether binding to the N-methyltriazolium site inhibits the PET, switching on pyrene fluorescence under UV irradiation. Each pulse of fuel results in a single ratchet cycle of transient fluorescence (encompassing threading, transport to the N-methyltriazolium site, and then dethreading), with the onset of the fluorescent time period determined by the amount of fuel in each pulse and the end-point determined by the concentration of the reagents for the disulfide exchange reaction. The system provides a potential alternative signaling approach for artificial molecular machines that read symbols from sequence-encoded molecular tapes.

Co(II) Acetate-Assisted Direct Synthesis of Acyl Hydrazones from Acyl Hydrazides under Mild Conditions

Acyl hydrazones are a class of synthetically important organic compounds that are recurrently in high demand for synthesis and use in various fields of chemistry and biology. We report the first Co(II) catalyzed one-component one-pot sustainable synthesis of acyl hydrazones only from acyl hydrazides under mild reaction conditions. Traditional and contemporary methodologies use two components (usually acyl hydrazides and aldehydes/ketones/alcohols/styrene) as the coupling partners. Our protocol, on the other hand, involves the in situ generation of aldehyde intermediate (detected by gas chromatography) from the acyl hydrazide, which then undergoes condensation with another molecule of the same acyl hydrazide in the same pot to yield acyl hydrazones in presence of mild base K2 CO3 and low-cost Co(OAc)2  ⋅ 4H2 O as catalyst. This method shows good functional group tolerance with good to excellent yield of products. Furthermore, some of the resulting acyl hydrazones have been used as synthetic precursors and explored in various post-synthetic modifications to afford N-heterocyclic compounds. Furthermore, photoswitchable properties of few synthesized acyl hydrazones are also explored using their E/Z isomerization around the C=N bond, as realized by high-pressure liquid chromatography (HPLC) and UV-vis spectroscopic studies.

On-Demand Release of Secondary Amine Bases for the Activation of Catalysts and Crosslinkers

Dynamic covalent (DCv) ureas have been used abundantly to design self-healing materials. We demonstrate that apart from self-healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile-N-oxides from chlorooximes, which react with acrylate-terminated polymers to form an isoxazoline polymer gel.

A Tale of Two Bioconjugations: pH Controlled Divergent Reactivity of Protein α-oxo-Aldehydes in Competing α-oxo-Mannich and Catalyst-Free Aldol Ligations

Site-selective chemical methods for protein bioconjugation have revolutionized the fields of cell and chemical biology through the development of novel protein/enzyme probes bearing fluorescent, spectroscopic, or even toxic cargos. Herein, we report two new methods for the bioconjugation of α-oxo aldehyde handles within proteins using small molecule aniline and/or phenol probes. The "α-oxo-Mannich" and "catalyst-free aldol" ligations both compete for the electrophilic α-oxo aldehyde, which displays pH divergent reactivity proceeding through the "Mannich" pathway at acidic pH to afford bifunctionalized bioconjugates, and the "catalyst-free aldol" pathway at neutral pH to afford monofunctionalized bioconjugates. We explore the substrate scope and utility of both of these bioconjugations in the construction of neoglycoproteins, in the process formulating a mechanistic rationale for how both pathways intersect with each other at different reaction pH's.

Click-functionalized hydrogel design for mechanobiology investigations

The advancement of click-functionalized hydrogels in recent years has coincided with rapid growth in the fields of mechanobiology, tissue engineering, and regenerative medicine. Click chemistries represent a group of reactions that possess high reactivity and specificity, are cytocompatible, and generally proceed under physiologic conditions. Most notably, the high level of tunability afforded by these reactions enables the design of user-controlled and tissue-mimicking hydrogels in which the influence of important physical and biochemical cues on normal and aberrant cellular behaviors can be independently assessed. Several critical tissue properties, including stiffness, viscoelasticity, and biomolecule presentation, are known to regulate cell mechanobiology in the context of development, wound repair, and disease. However, many questions still remain about how the individual and combined effects of these instructive properties regulate the cellular and molecular mechanisms governing physiologic and pathologic processes. In this review, we discuss several click chemistries that have been adopted to design dynamic and instructive hydrogels for mechanobiology investigations. We also chart a path forward for how click hydrogels can help reveal important insights about complex tissue microenvironments.

Tuneable Control of Organocatalytic Activity through Host-Guest Chemistry

Dynamic regulation of chemical reactivity is important in many complex chemical reaction networks, such as cascade reactions and signal transduction processes. Signal responsive catalysts could play a crucial role in regulating these reaction pathways. Recently, supramolecular encapsulation was reported to regulate the activities of artificial catalysts. We present a host-guest chemistry strategy to modulate the activity of commercially available synthetic organocatalysts. The molecular container cucurbit[7]uril was successfully applied to change the activity of four different organocatalysts and one initiator, enabling up- or down-regulation of the reaction rates of four different classes of chemical reactions. In most cases CB[7] encapsulation results in catalyst inhibition, however in one case catalyst activation by binding to CB[7] was observed. The mechanism behind this unexpected behavior was explored by NMR binding studies and pKa measurements. The catalytic activity can be instantaneously switched during operation, by addition of either supramolecular host or competitive binding molecules, and the reaction rate can be predicted with a kinetic model. Overall, this signal responsive system proves a promising tool to control catalytic activity.

Indoline Catalyzed Acylhydrazone/Oxime Condensation under Neutral Aqueous Conditions

Acylhydrazones formation has been widely applied in materials science and biolabeling. However, their sluggish condensation rate under neutral conditions limits its application. Herein, indolines with electron-donating groups are reported as a new catalyst scaffold, which can catalyze acylhydrazone, hydrazone, and oxime formation via an iminium ion intermediate. This new type of catalyst showed up to 15-fold rate enhancement over the traditional aniline-catalyzed reaction at neutral conditions. The identified indoline catalyst was successfully applied in hydrogel formation.

Bioconjugation with Aminoalkylhydrazine for Efficient Mass Spectrometry-Based Detection of Small Carbonyl Compounds

Bioconjugation through oxime or hydrazone formation is a versatile strategy for covalent labeling of biomolecules in vitro and in vivo. In this work, a mass spectrometry-based method was developed for the bioconjugation of small carbonyl compounds (CCs) with an aminoalkylhydrazine to form stable hydrazone conjugates that are readily detectable with electrospray ionization mass spectrometry (ESI-MS). Out of all hydrazine reagents tested, 2-(dimethylamino)ethylhydrazine (DMAEH) was selected for further analysis due to the fastest reaction rates observed. A thorough study of the reaction kinetics between structurally varied short-chain CCs and DMAEH was performed with the second-order reaction rate constants spanning in the range of 0.23-208 M-1 s-1. In general, small aldehydes reacted faster than the corresponding ketones. Moreover, a successful reaction monitoring of a deoxyribose-5-phosphate aldolase-catalyzed reversible retro-aldol cleavage of deoxyribose was demonstrated. Thus, the developed method shows potential also for ESI-MS-based enzyme kinetics studies.

Selective activation of organocatalysts by specific signals

Activation of the responsive organocatalyst proline by three different signals allows temporal control over chemical reaction kinetics.

Reminiscent of signal transduction in biological systems, artificial catalysts whose activity can be controlled by physical or chemical signals would be of high interest in the design of chemical systems that can respond to their environment. Self-immolative chemistry offers a generic method for the development of catalysts that can be activated by different signals. To demonstrate the versatility of that concept, we synthesized organocatalysts that can be activated by three different signals and that can be used to control two different reactions. In this way the organocatalyst proline is designed as a pro-catalyst that is activated either by the chemical signal H₂O₂, by light or by the enzyme penicillin acylase. The pro-catalysts were used to exert temporal control over the rate of an aldol reaction and a Michael reaction.