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1. Impact of gelation method on thixotropic properties of phenylalanine-derived supramolecular hydrogels

   https://doi.org/10.1039/d0sm01217c  

   Quigley, Elena; Johnson, Jade; Liyanage, Wathsala; Nilsson, Bradley L. (November 2020, Soft Matter)

   null (Ed.)

   Supramolecular hydrogels formed by noncovalent self-assembly of low molecular weight (LMW) agents are promising next-generation biomaterials. Thixotropic shear response and mechanical stability are two emergent properties of hydrogels that are critical for biomedical applications including drug delivery and tissue engineering in which injection of the hydrogel will be necessary. Herein, we demonstrate that the emergent thixotropic properties of supramolecular phenylalanine-derived hydrogels are dependent on the conditions in which they are formulated. Specifically, hydrogels formed from fluorenylmethoxycarbonyl (Fmoc) modified phenylalanine derivatives, 3-fluorophenylalanine (Fmoc-3F-Phe) and pentafluorophenylalanine (Fmoc-F5-Phe), were characterized as a function of gelation conditions to examine how shear response and mechanical stability properties correlate to mode of gelation. Two distinct methods of gelation were compared. First, spontaneous self-assembly and gelation was triggered by a solvent exchange method in which a concentrated solution of the gelator in dimethylsulfoxide was diluted in water. Second, gelation was promoted by dissolution of the gelator in water at basic pH followed by gradual pH adjustment from basic to mildly acidic by the hydrolysis of glucono-delta-lactone. Hydrogels formed under solvent exchange conditions were mechanically unstable and poorly shear-responsive whereas hydrogels formed by gradual acidification were temporally stable and had highly shear-responsive viscoelastic character. These studies confirm that gelation environment and mechanism have a significant influence on the emergent properties of supramolecular hydrogels and offer insight into how gelation conditions can be used to tune hydrogel properties for specific applications. 

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2. Dynamic‐Covalent Crosslinking of Benzenetricarboxamide–Phenylboronate Conjugates

   https://doi.org/10.1002/mabi.202300001  

   VandenBerg, Michael A.; Xian, Sijie; Xiang, Yuanhui; Webber, Matthew J. (February 2023, Macromolecular Bioscience)

   Abstract In an effort to augment the function of supramolecular biomaterials, recent efforts have explored the creation of hybrid materials that couple supramolecular and covalent components. Here, the benzenetricarboxamide (BTA) supramolecular polymer motif is modified to present a phenylboronic acid (PBA) in order to promote the crosslinking of 1D BTA stacks by PBA–diol dynamic‐covalent bonds through the addition of a multi‐arm diol‐bearing crosslinker. Interestingly, the combination of these two motifs serves to frustrate the resulting assembly process, yielding hydrogels with worse mechanical properties than those prepared without the multi‐arm diol crosslinker. Both systems with and without the crosslinker do, however, respond to the presence of a physiological level of glucose with a reduction in their mechanical integrity; repulsive electrostatic interactions in the BTA stacks occur in both cases upon glucose binding, with added competition from glucose with PBA–diol bonds amplifying glucose response in the hybrid material. Accordingly, the present results point to an unexpected outcome of reduced hydrogel mechanics, yet increased glucose response, when two disparate dynamic motifs of BTA supramolecular polymerization and PBA–diol crosslinking are combined, offering a vision for future preparation of glucose‐responsive supramolecular biomaterials. 

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3. Implications of Supramolecular Crosslinking on Hydrogel Toughening by Directional Freeze‐Casting and Salting‐Out

   https://doi.org/10.1002/adfm.202402613  

   Ye, Zhou; Chi, Teng; Evans, Connor_J; Liu, Dongping; Addonizio, Christopher_J; Su, Bo; Pramudya, Irawan; Xiang, Yuanhui; Roeder, Ryan_K; Webber, Matthew_J (April 2024, Advanced Functional Materials)

   Abstract Dynamic hydrogel crosslinking captures network reorganization and self‐healing of natural materials, yet is often accompanied by reduced mechanical properties compared to covalent analogs. Toughening is possible in certain materials with processing by directional freeze‐casting and salting‐out, producing hierarchically organized networks with directionally enhanced mechanical properties. The implications of including dynamic supramolecular crosslinking alongside such processes are unclear. Here, a supramolecular hydrogel prepared from homoternary crosslinking by pendant guests with a free macrocycle is subsequently processed by directional freeze‐casting and salting‐out. The resulting hydrogels tolerate multiple cycles of compression. Excitingly, supramolecular affinity dictates the mechanical properties of the bulk hydrogels, with higher affinity interactions producing materials with higher Young's modulus and enhanced toughness under compression. The importance of supramolecular crosslinking is emphasized with a supramolecular complex that is converted in situ into a covalent crosslink. While supramolecular hydrogels do not fracture and spontaneously self‐heal when cut, their covalent analogs fracture under moderate strain and do not self‐heal. This work shows a molecular‐scale origin of bulk hydrogel toughening attributed to affinity and dynamics of supramolecular crosslinking, offering synergy in combination with bulk post‐processing techniques to yield materials with enhanced mechanical properties tunable at the molecular scale for the needs of specific applications. 

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4. Collision cross‐section analysis of self‐assembled metallomacrocycle isomers and isobars via ion mobility mass spectrometry

   https://doi.org/10.1002/rcm.8717  

   Endres, Kevin_J; Barthelmes, Kevin; Winter, Andreas; Antolovich, Robert; Schubert, Ulrich_S; Wesdemiotis, Chrys (February 2020, Rapid Communications in Mass Spectrometry)

   RationaleCoordinatively driven self‐assembly of transition metal ions and bidentate ligands gives rise to organometallic complexes that usually contain superimposed isobars, isomers, and conformers. In this study, the double dispersion ability of ion mobility mass spectrometry (IM‐MS) was used to provide a comprehensive structural characterization of the self‐assembled supramolecular complexes by their mass and charge, revealed by the MS event, and their shape and collision cross‐section (Ω), revealed by the IM event. MethodsSelf‐assembled complexes were synthesized by reacting a bis(terpyridine) ligand exhibiting a 60^odihedral angle between the two ligating terpyridine sites (T) with divalent Zn, Ni, Cd, or Fe. The products were isolated as (Metal²⁺[T])_n(PF₆)_2nsalts and analyzed using IM‐MS after electrospray ionization (ESI) which produced several charge states from eachn‐mer, depending on the number of PF₆ˉ anions lost upon ESI. Experimental Ω data, derived using IM‐MS, and computational Ω predictions were used to elucidate the size and architecture of the complexes. ResultsOnly macrocyclic dimers, trimers, and tetramers were observed with Cd²⁺, whereas Zn²⁺formed the same plus hexameric complexes. These two metals led to the simplest product distributions and no linear isomers. In sharp contrast, Ni²⁺and Fe²⁺formed all possible ring sizes from dimer to hexamer as well as various linear isomers. The experimental and theoretical Ω data indicated rather planar macrocyclic geometries for the dimers and trimers, twisted 3D architectures for the larger rings, and substantially larger sizes with spiral conformation for the linear congeners. Adding PF₆ˉ to the same complex was found to mainly cause size contraction due to new stabilizing anion–cation interactions. ConclusionsComplete structural identification could be accomplished using ESI‐IM‐MS. Our results affirm that self‐assembly with Cd²⁺and Zn²⁺proceeds through reversible equilibria that generate the thermodynamically most stable structures, encompassing exclusively macrocyclic architectures that readily accommodate the 60^oligand used. In contrast, complexation with Ni²⁺and Fe²⁺, which form stronger coordinative bonds, proceeds through kinetic control, leading to more complex mixtures and kinetically trapped less stable architectures, such as macrocyclic pentamers and linear isomers. 

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5. Transient and Dissipative Host–Guest Hydrogels Regulated by Consumption of a Reactive Chemical Fuel

   https://doi.org/10.1002/ange.202216537  

   Su, Bo; Chi, Teng; Ye, Zhou; Xiang, Yuanhui; Dong, Ping; Liu, Dongping; Addonizio, Christopher J.; Webber, Matthew J. (January 2023, Angewandte Chemie)

   Abstract 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. 

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