Elisha Krieg

Supramolecular Polymers in Aqueous Media

This review discusses one-dimensional supramolecular polymers that form in aqueous media. First, naturally occurring supramolecular polymers are described, in particular, amyloid fibrils, actin filaments, and microtubules. Their structural, thermodynamic, kinetic, and nanomechanical properties are highlighted, as well as their importance for the advancement of biologically inspired supramolecular polymer materials. Second, five classes of synthetic supramolecular polymers are described: systems based on (1) hydrogen-bond motifs, (2) large π-conjugated surfaces, (3) host-guest interactions, (4) peptides, and (5) DNA. We focus on recent studies that address key challenges in the field, providing mechanistic understanding, rational polymer design, important functionality, robustness, or unusual thermodynamic and kinetic properties.

Supramolecular gel based on a perylene diimide dye: multiple stimuli responsiveness, robustness, and photofunction.

Design of an extensive supramolecular three-dimensional network that is both robust and adaptive represents a significant challenge. The molecular system PP2b based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups self-assembles in aqueous media into extended supramolecular fibers that form a robust three-dimensional network resulting in gelation. The self-assembled systems were characterized by cryo-TEM, cryo-SEM, and rheological measurements. The gel possesses exceptional robustness and multiple stimuli-responsiveness. Reversible charging of PP2b allows for switching between the gel state and fluid solution that is accompanied by switching on and off the materials birefringence. Temperature triggered deswelling of the gel leads to the (reversible) expulsion of a large fraction of the aqueous solvent. The dual sensibility toward chemical reduction and temperature with a distinct and interrelated response to each of these stimuli is pertinent to applications in the area of adaptive functional materials. The gel also shows strong absorption of visible light and good exciton mobility (elucidated using femtosecond transient absorption), representing an advantageous light harvesting system.

A recyclable supramolecular membrane for size-selective separation of nanoparticles

Most practical materials are held together by covalent bonds, which are irreversible. Materials based on noncovalent interactions can undergo reversible self-assembly, which offers advantages in terms of fabrication, processing and recyclability1, but the majority of noncovalent systems are too fragile to be competitive with covalent materials for practical applications, despite significant attempts to develop robust noncovalent arrays1,2,3,4. Here, we report nanostructured supramolecular membranes prepared from fibrous assemblies5 in water. The membranes are robust due to strong hydrophobic interactions6,7, allowing their application in the size-selective separation of both metal and semiconductor nanoparticles. A thin (12 µm) membrane is used for filtration (∼5 nm cutoff), and a thicker (45 µm) membrane allows for size-selective chromatography in the sub-5 nm domain. Unlike conventional membranes, our supramolecular membranes can be disassembled using organic solvent, cleaned, reassembled and reused multiple times. Supramolecular membranes prepared from fibrous assemblies in water can be disassembled in organic solvent after use and then cleaned, reassembled and reused numerous times.

Noncovalent Water‐Based Materials: Robust yet Adaptive

The adaptive properties of noncovalent materials allow easy processing, facile recycling, self-healing, and stimuli responsiveness. However, the poor robustness of noncovalent systems has hampered their use in real-life applications. In this Concept Article we discuss the possibility of creating robust noncovalent arrays by utilizing strong hydrophobic interactions. We describe examples from our work on aqueous assemblies based on aromatic amphiphiles with extended hydrophobic cores. These arrays exhibit fascinating properties, including robustness, multiple stimuli-responsiveness, and pathway-dependent self-assembly. We have shown that this can lead to functional materials (filtration membranes) rivaling covalent systems. We anticipate that water-based noncovalent materials have the potential to replace or complement conventional polymer materials in various fields, and to promote novel applications that require the combination of robustness and adaptivity.

Understanding the effect of fluorocarbons in aqueous supramolecular polymerization: ultrastrong noncovalent binding and cooperativity.

Achieving supramolecular polymerization based on strong yet reversible bonds represents a significant challenge. A solution may be offered by perfluoroalkyl groups, which have remarkable hydrophobicity. We tested the idea that a perfluorooctyl chain attached to a perylene diimide amphiphile can dramatically enhance the strength of supramolecular bonding in aqueous environments. Supramolecular structures and polymerization thermodynamics of this fluorinated compound (1-F) were studied in comparison to its non-fluorinated analogue (1-H). Depending on the amount of organic cosolvent, 1-F undergoes cooperative or isodesmic aggregation. The switching between two polymerization mechanisms results from a change in polymer structure, as observed by cryogenic electron microscopy. 1-F showed exceptionally strong noncovalent binding, with the largest directly measured association constant of 1.7 × 10(9) M(-1) in 75:25 water/THF mixture (v/v). In pure water, the association constant of 1-F is estimated to be at least in the order of 10(15) M(-1) (based on extrapolation), 3 orders of magnitude larger than that of 1-H. The difference in aggregation strength between 1-F and 1-H can be explained solely on the basis of the larger surface area of the fluorocarbon group, rather than a unique nature of fluorocarbon hydrophobicity. However, differences in aggregation mechanism and cooperativity exhibited by 1-F appear to result from specific fluorocarbon conformational rigidity.

Separation, Immobilization, and Biocatalytic Utilization of Proteins by a Supramolecular Membrane

Membrane separation of biomolecules and their application in biocatalysis is becoming increasingly important for biotechnology, demanding the development of new biocompatible materials with novel properties. In the present study, an entirely noncovalent water-based material is used as a membrane for size-selective separation, immobilization, and biocatalytic utilization of proteins. The membrane shows stable performance under physiological conditions, allowing filtration of protein mixtures with a 150 kDa molecular weight cutoff (∼8 nm hydrodynamic diameter cutoff). Due to the biocompatibility of the membrane, filtered proteins stay functionally active and retained proteins can be partially recovered. Upon filtration, large enzymes become immobilized within the membrane. They exhibit stable activity when subjected to a constant flux of substrates for prolonged periods of time, which can be used to carry out heterogeneous biocatalysis. The noncovalent membrane material can be easily disassembled, purified, reassembled, and reused, showing reproducible performance after recycling. The robustness, recyclability, versatility, and biocompatibility of the supramolecular membrane may open new avenues for manipulating biological systems.

Self-assembly of perylenediimide-single strand DNA conjugates: Employing hydrophobic interactions and DNA base pairing to create a diverse structural space

The self-assembly behavior of DNA conjugates possessing a perylenediimide (PDI) head group and an N-oligonucleotide tail has been investigated using a combination of optical spectroscopy and cryogenic transmission electron microscopy (cryo-TEM) imaging. To obtain insight into the interplay between PDI hydrophobic interactions and DNA base-pairing we employed systematic variation in the length and composition of the oligo tails. Conjugates with short (TA)n or (CG)n oligo tails (n≤3) form helical or nonhelical fibers constructed from π-stacked PDI head groups with pendent oligo tails in aqueous solution. Conjugates with longer (TA)n oligo tails also form stacks of PDI head groups, which are further aggregated by base-pairing between their oligo tails, leading to fiber bundling and formation of bilayers. The longer (CG)n conjugates form PDI end-capped duplexes, which further assemble into PDI-stacked arrays of duplexes leading to large scale ordered assemblies. Cryo-TEM imaging reveals that (CG)3 gives rise to both fibers and large assemblies, whereas (CG)5 assembles preferentially into large ordered structures.