Switching imidazole reactivity by dynamic control of tautomer state in an allosteric foldamer

Photoswitchable Cascades for Allosteric and Bidirectional Control over Covalent Bonds and Assemblies

Studies of complex systems and emerging properties to mimic biosystems are at the forefront of chemical research. Dynamic multistep cascades, especially those exhibiting allosteric regulation, are challenging. Herein, we demonstrate a versatile platform of photoswitchable covalent cascades toward remote and bidirectional control of reversible covalent bonds and ensuing assemblies. The relay of a photochromic switch, keto-enol equilibrium, and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The accompanying distinct reactivity further enables photoswitchable dynamic covalent bonding and release of substrates bidirectionally through alternating two wavelengths of light, essentially realizing light-mediated signaling cycles. The downfall of energy by covalent bond formation/scission upon photochemical reactions offers the driving force for the controlled direction of the cascade. To show the molecular diversity, photoswitchable on-demand assembly/disassembly of covalent polymers, including structurally reconfigurable polymers, was realized. This work achieves photoswitchable allosteric regulation of covalent architectures within dynamic multistep cascades, which has rarely been reported before. The results resemble allosteric control within biological signaling networks and should set the stage for many endeavors, such as dynamic assemblies, molecular motors, responsive polymers, and intelligent materials.

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Stimulus-Induced Relief of Intentionally Incorporated Frustration Drives Refolding of a Water-Soluble Biomimetic Foldamer

Frustrated, or nonoptimal, interactions have been proposed to be essential to a protein's ability to display responsive behavior such as allostery, conformational signaling, and signal transduction. However, the intentional incorporation of frustrated noncovalent interactions has not been explored as a design element in the field of dynamic foldamers. Here, we report the design, synthesis, characterization, and molecular dynamics simulations of the first dynamic water-soluble foldamer that, in response to a stimulus, exploits relief of frustration in its noncovalent network to structurally rearrange from a pleated to an intercalated columnar structure. Thus, relief of frustration provides the energetic driving force for structural rearrangement. This work represents a previously unexplored design element for the development of stimulus-responsive systems that has potential application to materials chemistry, synthetic biology, and molecular machines.

Anion-Dependent Hydrogen-Bond Polarity Switching in Ethylene-bridged Urea Oligomers

The reversible coordination of anions to an N,N'-disubstituted 3,5-bis(trifluoromethyl)phenylurea located at a terminus of a linear chain of ethylene-bridged hydrogen-bonded ureas triggers a cascade of conformational changes. A series of hydrogen-bond polarity reversals propagates along the oligomer, leading to a global switch of its hydrogen-bond directionality. The induced polarity switch, transmitted through four reversible urea groups, results in a change in emission and excitation wavelengths of a fluorophore located at the opposite terminus of the oligomer. The molecule thus behaves as a chemical sensor with a relayed remote spectroscopic response to variations in anion concentration. The polarity switch induced by anion concentration constitutes an artificial communication mechanism for conveying information through oligomeric structures.

Chemically Fueled Communication Along a Scaffolded Nanoscale Array of Squaramides

Relaying conformational change over several nanometers is central to the function of allosterically regulated proteins. Replicating this mechanism artificially would provide important communication tools, but requires nanometer-sized molecules that reversibly switch between defined shapes in response to signaling molecules. In this work, 1.8 nm long rigid rod oligo(phenylene-ethynylene)s are scaffolds for switchable multi-squaramide hydrogen-bond relays. Each relay can adopt either a parallel or an antiparallel orientation relative to the scaffold; the preferred orientation is dictated by a director group at one end. An amine director responded to proton signals, with acid-base cycles producing multiple reversible changes in relay orientation that were reported by a terminal NH, which is 1.8 nm distant. Moreover, a chemical fuel acted as a dissipative signal. As the fuel was consumed, the relay reverted to its original orientation, illustrating how information from out-of-equilibrium molecular signals can be communicated to a distant site.

Dynamic and Persistent Cyclochirality in Hydrogen-Bonded Derivatives of Medium-Ring Triamines

Cyclic triureas derived from 1,4,7-triazacyclononane (TACN) were synthesized; X-ray crystallography showed a chiral bowl-like conformation with each urea hydrogen-bonded to its neighbor with uniform directionality, forming a "cyclochiral" closed loop of hydrogen bonds. Variable-temperature 1H NMR, 1H-1H exchange spectroscopy, Eyring analysis, computational modeling, and studies in various solvents revealed that cyclochirality is dynamic (ΔG‡25°C = 63-71 kJ mol-1 in noncoordinating solvents), exchanging between enantiomers by two mechanisms: bowl inversion and directionality reversal, with the former subject to a slightly smaller enantiomerization barrier. The enantiomerization rate substantially increased in the presence of hydrogen-bonding solvents. Population of only one of the two cyclochiral hydrogen-bond directionalities could be induced by annulating one ethylene bridge with a trans-cyclohexane. Alternatively, enantiomerization could be inhibited by annulating one ethylene bridge with a cis-cyclohexane (preventing bowl inversion) and replacing one urea function with a formamide (preventing directionality reversal). Combining these structural modifications resulted in an enantiomerization barrier of ΔG‡25°C = 93 kJ mol-1, furnishing a planar-chiral, atropisomeric bowl-shaped structure whose stereochemical stability arises solely from its hydrogen-bonding network.

Non-Covalent Interactions Enforce Conformation in Switchable and Water-Soluble Diketopiperazine-Pyridine Foldamers

To reach their potential as mimics of the dynamic molecules present in biological systems, foldamers must be designed to display stimulus-responsive behavior. Here we report such a foldamer architecture based on alternating pyridine-diketopiperazine linkers. Epimerization is conveniently prevented through a copper-catalyzed coupling protocol. The compounds' native unswitched conformation is first discovered in the solid and solution state. The foldamers can be solubilized in DMSO and pH 9.5 buffer, retaining conformational control to a large degree. Lastly, dynamic switching is demonstrated through treatment with acid, leading to behaviour we describe as stimulus-responsive sidechain reconfiguration.