Beyond Covalent Crosslinks: Applications of Supramolecular Gels

Traditionally, gels have been defined by their covalently cross-linked polymer networks. Supramolecular gels challenge this framework by relying on non-covalent interactions for self-organization into hierarchical structures. This class of materials offers a variety of novel and exciting potential applications. This review draws together recent advances in supramolecular gels with an emphasis on their proposed uses as optoelectronic, energy, biomedical, and biological materials. Additional special topics reviewed include environmental remediation, participation in synthesis procedures, and other industrial uses. The examples presented here demonstrate unique benefits of supramolecular gels, including tunability, processability, and self-healing capability, enabling a new approach to solve engineering challenges. Keywords: supramolecular gel; self-assembly; gels; applied soft matte

Emerging investigator series: aramid amphiphile nanoribbons for the remediation of lead from contaminated water

Aramid amphiphiles with chelating head groups spontaneously form robust nanoribbons in water with surfaces capable of heavy metal remediation.</p

Dynamics in supramolecular nanomaterials

The self-assembly of amphiphilic small molecules in water leads to nanostructures with customizable structure-property relationships arising from their tunable chemistries. Characterization of these assemblies is generally limited to their static...</p

Domain-selective thermal decomposition within supramolecular nanoribbons

AbstractSelf-assembly of small molecules in water provides a powerful route to nanostructures with pristine molecular organization and small dimensions (&lt;10 nm). Such assemblies represent emerging high surface area nanomaterials, customizable for biomedical and energy applications. However, to exploit self-assembly, the constituent molecules must be sufficiently amphiphilic and satisfy prescribed packing criteria, dramatically limiting the range of surface chemistries achievable. Here, we design supramolecular nanoribbons that contain: (1) inert and stable internal domains, and (2) sacrificial surface groups that are thermally labile, and we demonstrate complete thermal decomposition of the nanoribbon surfaces. After heating, the remainder of each constituent molecule is kinetically trapped, nanoribbon morphology and internal organization are maintained, and the nanoribbons are fully hydrophobic. This approach represents a pathway to form nanostructures that circumvent amphiphilicity and packing parameter constraints and generates structures that are not achievable by self-assembly alone, nor top-down approaches, broadening the utility of molecular nanomaterials for new targets.</jats:p

Asymmetric Collapse in Biomimetic Complex Coacervates Revealed by Local Polymer and Water Dynamics

Complex coacervation is a phenomenon characterized by the association of oppositely charged polyelectrolytes into micrometer-scale liquid condensates. This process is the purported first step in the formation of underwater adhesives by sessile marine organisms, as well as the process harnessed for the formation of new synthetic and protein-based contemporary materials. Efforts to understand the physical nature of complex coacervates are important for developing robust adhesives, injectable materials, or novel drug delivery vehicles for biomedical applications; however, their internal fluidity necessitates the use of in situ characterization strategies of their local dynamic properties, capabilities not offered by conventional techniques such as X-ray scattering, microscopy, or bulk rheological measurements. Herein, we employ the novel magnetic resonance technique Overhauser dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP), together with electron paramagnetic resonance (EPR) line shape analysis, to concurrently quantify local molecular and hydration dynamics, with species- and site-specificity. We observe striking differences in the structure and dynamics of the protein-based biomimetic complex coacervates from their synthetic analogues, which is an asymmetric collapse of the polyelectrolyte constituents. From this study we suggest charge heterogeneity within a given polyelectrolyte chain to be an important parameter by which the internal structure of complex coacervates may be tuned. Acquiring molecular-level insight to the internal structure and dynamics of dynamic polymer complexes in water through the in situ characterization of site- and species-specific local polymer and hydration dynamics should be a promising general approach that has not been widely employed for materials characterization.X111918sciescopu

Morphological Transitions of a Photoswitchable Aramid Amphiphile Nanostructure

Self-assembly of small amphiphilic molecules in water can lead to nanostructures of varying geometries with pristine internal molecular organization. Here we introduce a photoswitchable aramid amphiphile (AA), designed to exhibit extensive hydrogen bonding and robust mechanical properties upon self-assembly, while containing a vinylnitrile group for photoinduced cis-trans isomerization. We demonstrate spontaneous self-assembly of the vinylnitrile-containing AA in water to form nanoribbons. Upon UV irradiation, trans-to-cis isomerizations occur concomitantly with a morphological transition from nanoribbons to nanotubes. The nanotube structure persists in water for over six months, stabilized by strong and collective intermolecular interactions. We demonstrate that the nanoribbon-to-nanotube transition is reversible upon heating and that switching between states can be achieved repeatedly. Finally, we use electron microscopy to capture the transition and propose mechanisms for nanoribbon-to-nanotube rearrangement and vice versa. The stability and switchability of photoresponsive AA nanostructures make them viable for a range of future applications

Effects of molecular flexibility and head group repulsion on aramid amphiphile self-assembly

Strongly interacting amphiphilic molecules self-assemble in water. The flexibility of the amphiphiles and their head group repulsion mediate their nanostructure geometry

Quantifying residue-specific conformational dynamics of a highly reactive 29-mer peptide

© 2020, The Author(s). Understanding structural transitions within macromolecules remains an important challenge in biochemistry, with important implications for drug development and medicine. Insight into molecular behavior often requires residue-specific dynamics measurement at micromolar concentrations. We studied MP01-Gen4, a library peptide selected to rapidly undergo bioconjugation, by using electron paramagnetic resonance (EPR) to measure conformational dynamics. We mapped the dynamics of MP01-Gen4 with residue-specificity and identified the regions involved in a structural transformation related to the conjugation reaction. Upon reaction, the conformational dynamics of residues near the termini slow significantly more than central residues, indicating that the reaction induces a structural transition far from the reaction site. Arrhenius analysis demonstrates a nearly threefold decrease in the activation energy of conformational diffusion upon reaction (8.0 kBT to 3.4 kBT), which occurs across the entire peptide, independently of residue position. This novel approach to EPR spectral analysis provides insight into the positional extent of disorder and the nature of the energy landscape of a highly reactive, intrinsically disordered library peptide before and after conjugation

Domain-Specific Phase Transitions in a Supramolecular Nanostructure

Understanding thermal phase behavior within nanomaterials can inform their rational design for medical technologies like drug delivery systems and vaccines, as well as for energy technologies and catalysis. This study resolves thermal phases of discrete domains within a supramolecular aramid amphiphile (AA) nanoribbon. Dynamics are characterized by X-band EPR spectroscopy of spin labels positioned at specific sites through the nanoribbon cross-section. The fitting of the electron paramagnetic resonance (EPR) line shapes reveals distinct conformational dynamics, with fastest dynamics at the surface water layer, intermediate dynamics within the flexible cationic head group domain, and slowest dynamics in the interior aramid domain. Measurement of conformational mobility as a function of temperature reveals first- and second-order phase transitions, with melting transitions observed in the surface and head group domains and a temperature-insensitive crystalline phase in the aramid domain. Arrhenius analysis yields activation energies of diffusion at each site. This work demonstrates that distinct thermal phase behaviors between adjacent nanodomains within a supramolecular nanostructure may be resolved and illustrates the utility of EPR spectroscopy for thermal phase characterization of nanostructures

Asymmetric Collapse in Biomimetic Complex Coacervates Revealed by Local Polymer and Water Dynamics

Complex coacervation is a phenomenon characterized by the association of oppositely charged polyelectrolytes into micrometer-scale liquid condensates. This process is the purported first step in the formation of underwater adhesives by sessile marine organisms, as well as the process harnessed for the formation of new synthetic and protein-based contemporary materials. Efforts to understand the physical nature of complex coacervates are important for developing robust adhesives, injectable materials, or novel drug delivery vehicles for biomedical applications; however, their internal fluidity necessitates the use of in situ characterization strategies of their local dynamic properties, capabilities not offered by conventional techniques such as X-ray scattering, microscopy, or bulk rheological measurements. Herein, we employ the novel magnetic resonance technique Overhauser dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP), together with electron paramagnetic resonance (EPR) line shape analysis, to concurrently quantify local molecular and hydration dynamics, with species- and site-specificity. We observe striking differences in the structure and dynamics of the protein-based biomimetic complex coacervates from their synthetic analogues, which is an asymmetric collapse of the polyelectrolyte constituents. From this study we suggest charge heterogeneity within a given polyelectrolyte chain to be an important parameter by which the internal structure of complex coacervates may be tuned. Acquiring molecular-level insight to the internal structure and dynamics of dynamic polymer complexes in water through the in situ characterization of site- and species-specific local polymer and hydration dynamics should be a promising general approach that has not been widely employed for materials characterization