Cross-linked supramolecular polymer gels constructed from discrete multi-pillar[5]arene metallacycles and their multiple stimuli-responsive behavior.

A new family of discrete hexakis-pillar[5]arene metallacycles with different sizes have been successfully prepared via coordination-driven self-assembly, which presented very few successful examples of preparation of discrete multiple pillar[n]arene derivatives. These newly designed hexakis-pillar[5]arene metallacycles were well characterized with one-dimensional (1-D) multinuclear NMR ((1)H and (31) P NMR), two-dimensional (2-D) (1)H-(1)H COSY and NOESY, ESI-TOF-MS, elemental analysis, and PM6 semiempirical molecular orbital methods. Furthermore, the host-guest complexation of such hexakis-pillar[5]arene hosts with a series of different neutral ditopic guests G1-6 were well investigated. Through host-guest interactions of hexakis-pillar[5]arene metallacycles H2 or H3 with the neutral dinitrile guest G5, the cross-linked supramolecular polymers H2⊃(G5)3 or H3⊃(G5)3 were successfully constructed at the high-concentration region, respectively. Interestingly, these cross-linked supramolecular polymers transformed into the stable supramolecular gels upon increasing the concentrations to a relatively high level. More importantly, by taking advantage of the dynamic nature of metal-ligand bonds and host-guest interactions, the reversible multiple stimuli-responsive gel-sol phase transitions of such polymer gels were successfully realized under different stimuli, such as temperature, halide, and competitive guest, etc. The mechanism of such multiple stimuli-responsive processes was well illustrated by in situ multinuclear NMR investigation. This research not only provides a highly efficient approach to the preparation of discrete multiple pillar[n]arene derivatives but also presents a new family of multiple stimuli-responsive "smart" soft matters.

Responsive supramolecular polymer metallogel constructed by orthogonal coordination-driven self-assembly and host/guest interactions.

An emerging strategy for the fabrication of advanced supramolecular materials is the use of hierarchical self-assembly techniques wherein multiple orthogonal interactions between molecular precursors can produce new species with attractive properties. Herein, we unify the spontaneous formation of metal-ligand bonds with the host/guest chemistry of crown ethers to deliver a 3D supramolecular polymer network (SPN). Specifically, we have prepared a highly directional dipyridyl donor decorated with a benzo-21-crown-7 moiety that undergoes coordination-driven self-assembly with a complementary organoplatinum acceptor to furnish hexagonal metallacycles. These hexagons subsequently polymerize into a supramolecular network upon the addition of a bisammonium salt due to the formation of [2]pseudorotaxane linkages between the crown ether and ammonium moieties. At high concentrations, the resulting 3D SPN becomes a gel comprising many cross-linked metallohexagons. Notably, thermo- and cation-induced gel-sol transitions are found to be completely reversible, reflecting the dynamic and tunable nature of such supramolecular materials. As such, these results demonstrate the structural complexity that can be obtained when carefully controlling multiple interactions in a hierarchical fashion, in this case coordination and host/guest chemistry, and the interesting dynamic properties associated with the materials thus obtained.

pH-responsive supramolecular polymerization in aqueous media driven by electrostatic attraction-enhanced crown ether-based molecular recognition.

All the previously reported supramolecular polymers based on crown ether-based molecular recognition have been prepared in anhydrous organic solvents. This is mainly due to the weakness of crown ether-based molecular recognition in the presence of water. Here we report a linear supramolecular polymer constructed from a heteroditopic monomer in an aqueous medium driven by crown ether-based molecular recognition through the introduction of electrostatic attraction. In addition, the reversible transition between the linear supramolecular polymer and oligomers is achieved by adding acid and base. This study realizes the breakthrough of the solvent for supramolecular polymerization driven by crown ether-based molecular recognition from anhydrous organic solvents to aqueous media. It is helpful for achieving supramolecular polymerization driven by crown ether-based molecular recognition in a completely aqueous medium.

Peptide-based viscoelastic matrices for drug delivery and tissue repair.

Molecular self-assembly has paved the way to create novel, supramolecular, functional biomaterials. Peptide-based biomaterials are gaining interest as a result of their programmability, biodegradability, and bioresorbability. Further, unlike polymeric materials, peptides can be made monodisperse with precise control over sequence, chain length, and stereochemistry. Peptide-based viscoelastic matrices have been designed and characterized for various biomedical applications, such as tissue engineering scaffolds or drug delivery vehicles. The 'holy grail' in designing an ideal tissue engineering scaffold lies in mimicking the cues of the tissue's natural extracellular matrix (ECM). Some of the key elements of ECM that are incorporated into these peptide scaffolds include cell-adhesive and protease-sensitive sequences for enhanced cell-cell and cell-biomaterial interactions. Peptide-based viscoelastic matrices can also be engineered with drug carrying protease-sensitive sequences for controlled and site-specific drug delivery. Molecular-level engineering of simple oligopeptide modules can be used to control the position and density of the bio-mimetic functionalities in the supramolecular structures, which demonstrates the power of the 'bottom-up' approach in self-assembly.

A multiresponsive, shape-persistent, and elastic supramolecular polymer network gel constructed by orthogonal self-assembly.

A cross-linked supramolecular polymer network gel is designed and prepared, which shows reversible gel-sol transitions induced by changes in pH, temperature, cation concentration, and metal co-ordination. The gel pore size is controlled by the amount of cross-linker added to the system, and the material can be molded into shape-persistent, free-standing objects with elastic behavior. These features are all due to the dynamically reversible host-guest complexation and good mechanical properties of the cross-linked polymer network. No single organogel has previously been reported to possess all of these features, making this supramolecular gel an unprecedentedly intelligent soft material.

Understanding the binding interactions between dendrimer and 18 common amino acids by NMR techniques.

In the present study, we focus on the interactions between poly(propylene imine) (PPI) dendrimer and 18 of the 20 common amino acids by several NMR techniques, including NMR titrations and NOESY analysis. Surface ionic interactions and interior encapsulations were observed, and the binding behavior of amino acids with PPI dendrimer depends much on the side-chain properties of the amino acid, such as charge and hydrophobic/hydrophilic properties. The (1)H NMR titration results show that the formation of PPI dendrimer-amino acid complexes are driven mainly by ionic interactions for all the amino acids except tryptophan, which is involved in strong hydrophobic interactions with the interior pockets of PPI. The hydrophobic encapsulation of tryptophan in PPI pockets is confirmed by NOESY analysis. Amino acids with negatively charged residues much more easily saturate the surface charges on PPI than amino acids with uncharged residues, whereas amino acids with positively charged residues are the most difficult to bind with the surface amine groups on the PPI dendrimer. A simultaneous occurrence of interior encapsulation (hydrophobic, hydrogen bond, or ionic interactions) and surface binding (ionic interactions) was observed for tryptophan, phenylalanine, arginine, lysine, histidine, cysteine, and asparagine, and a preferential surface ionic binding on the PPI surface rather than encapsulations in the interior was obtained for the other amino acids.

Supramolecular polymer nanofibers via electrospinning of a heteroditopic monomer.

Driven by the benzo-21-crown-7/secondary ammonium salt recognition motif, a linear supramolecular polymer was formed from self-organization of a low-molecular-weight self-complementary monomer in chloroform. From this supramolecular polymer, nanofibers were obtained successfully via electrospinning.

Removal of the fermentation by-product succinyl L-tyrosine from the beta-lactamase inhibitor clavulanic acid using a molecularly imprinted polymer.

Clavulanic acid is a beta-lactamase inhibitor used in therapeutic combinations with the penicillin-type antibiotics. During the fermentation leading to clavulanic acid, a succinyl L-tyrosine by-product is unavoidably formed. Occasionally, the amount of this by-product is found to be as high as 2% of the product even after standard purification operations. To further remove this impurity, we prepared a highly specific adsorbent for succinyl L-tyrosine with the molecular imprinting technique. This was performed by simultaneously using vinylbenzyl trimethylammonium chloride and methacrylic acid as the functional monomers. The imprinted polymer selectively bound succinyl L-tyrosine, and could be successfully used to remove this impurity at concentrations of less than 2% in the presence of clavulanic acid.