Tuning glycerol plasticization of chitosan with boric acid.

Chitosan-based bioplastics are attractive biodegradable alternatives to petroleum-derived plastics. However, optimizing the properties of chitosan materials to fit a particular application or obtain a desired property is not a trivial feat. Here, we report the tunability of glycerol-plasticized chitosan films with the addition of boric acid. In combination, glycerol and boric acid form neutral complexes that alter the hydrogen-bonding face of the plasticizer and ultimately limit glycerol's ability to plasticize chitosan. Thus, we found that chitosan films containing glycerol-boric acid complexes were less flexible, had increased thermal transition temperatures, and showed more uniform morphologies. Structural, thermal, mechanical and morphological characterization was performed using ATR-FTIR, TGA and DSC, DMA, and SEM respectively. Molecular-level interactions of the neutral boron complexes and D-glucosamine, the repeat unit of chitosan, were also investigated used NMR and ATR-FTIR. The results of this work demonstrate the necessity of specific hydrogen-bonding interactions between the plasticizer and the polymer for effective plasticization, an important insight into the plasticization mechanism of chitosan films. Furthermore, the formation of complexes with glycerol is a novel and convenient method for tuning the physical properties of chitosan films.

Cooperative crosslinking in polyvinyl alcohol organogels.

Manipulating and optimizing the properties of gels is important for practical applications but can be both synthetically difficult and expensive. In this work, we report an easily tunable polyvinyl alcohol (PVA) organogel formed with boric acid (BA) and 1,4-benzenediboronic acid (1,4-BDBA) as crosslinkers. While PVA and BA alone form weak aggregations in DMSO, adding small amounts of 1,4-BDBA dramatically improves the material properties and gelation. PVA organogels made with mixtures of BA and 1,4-BDBA have improved thermal properties, lower CGCs, and higher G' than those with either crosslinker alone. We propose that these enhanced material properties are the result of cooperative PVA crosslinking between 1,4-BDBA and BA. As the properties of this system can be improved by simply varying the ratio of crosslinkers, these organogels are highly adjustable and are a practical alternative to PVA hydrogels.

Understanding the Molecular-Level Interactions of Glucosamine-Glycerol Assemblies: A Model System for Chitosan Plasticization.

Glycerol is the most widely used plasticizer for the biopolymer chitosan. However, there remains a lack of understanding of the molecular-level interactions between chitosan and glycerol. Here, we describe an in-depth spectroscopic study of the intermolecular interactions between the monomeric repeating unit of chitosan, glucosamine, and the plasticizer glycerol. Infrared and nuclear magnetic resonance spectroscopy were used to probe glucosamine assembly at high and low concentrations to establish diagnostic signals for intra- and intermolecular glucosamine interactions. Systematic addition of glycerol was found to disrupt intramolecular glucosamine hydrogen bonds and promote glucosamine self-assembly. Furthermore, we observed a significant preference for glycerol binding to the amine functionality of glucosamine. These findings indicate that the plasticization of chitosan with glycerol requires a specific binding motif and likely occurs via the gel theory mechanism.

Controlling Structure Beyond the Initial Coordination Sphere: Complexation-Induced Reversed Micelle Formation in Calix[4]pyrrole-Containing Diblock Copolymers.

A diblock copolymer containing a strapped calix[4]pyrrole-based ion pair recognition subunit has been synthesized via RAFT polymerization. As prepared, the polymer is hydrophobic and devoid of any particular morphological form. However, upon ion pair complexation, the copolymer self-assembles to generate reverse micelles in organic media. The reverse micelles formed in this way may be used to extract alkali cation and cesium halide anion salts from an aqueous source into an organic receiving phase. The polymer proved more effective as an extractant than the corresponding free ion pair receptor.

Polyvinyl alcohol-boronate gel for sodium hydroxide extraction.

Gels formed from commercially available polyvinyl alcohol (PVA) and 1,4-benzene diboronic acid (BdBA) in DMSO absorb NaOH efficiently from a bulk aqueous solution decreasing its pH.

Recognition and Extraction of Cesium Hydroxide and Carbonate by Using a Neutral Multitopic Ion-Pair Receptor.

Current approaches to lowering the pH of basic media rely on the addition of a proton source. An alternative approach is described herein that involves the liquid-liquid extraction-based removal of cesium salts, specifically CsOH and Cs2 CO3 , from highly basic media. A multitopic ion-pair receptor (2) is used that can recognize and extract the hydroxide and carbonate anions as their cesium salts, as confirmed by 1 H NMR spectroscopic titrations, ICP-MS, single-crystal structural analyses, and theoretical calculations. A sharp increase in the pH and cesium concentrations in the receiving phase is observed when receptor 2 is employed as a carrier in U-tube experiments involving the transport of CsOH through an intervening chloroform layer. The pH of the source phase likewise decreases.

Molecular Recognition Under Interfacial Conditions: Calix[4]pyrrole-Based Cross-linkable Micelles for Ion Pair Extraction.

An anthracene-functionalized, long-tailed calix[4]pyrrole 1, containing both an anion-recognition site and cation-recognition functionality, has been synthesized and fully characterized. Upon ion pair complexation with FeF2, receptor 1 self-assembles into multimicelles in aqueous media. This aggregation process is ascribed to a change in polarity from nonpolar to amphiphilic induced upon concurrent anion and cation complexation and permits molecular recognition-based control over chemical morphology under interfacial conditions. Photoirradiation of the micelles serves to cross-link the anthracene units thus stabilizing the aggregates. The combination of ion pair recognition, micelle formation, and cross-linking can be used to extract FeF2 ion pairs from bulk aqueous solutions. The present work helps illustrate how molecular recognition and self-assembly may be used to control the chemistry of extractants at interfaces.

Supramolecular gels made from nucleobase, nucleoside and nucleotide analogs.

Supramolecular or molecular gels are attractive for various applications, including diagnostics, tissue scaffolding and targeted drug release. Gelators derived from natural products are of particular interest for biomedical purposes, as they are generally biocompatible and stimuli-responsive. The building blocks of nucleic acids (i.e. nucleobases, nucleosides, and nucleotides) are desirable candidates for supramolecular gelation as they readily engage in reversible, noncovalent interactions. In this review, we describe a number of organo- and hydrogels formed through the assembly of nucleosides, nucleotides, and their derivatives. While natural nucleosides and nucleotides generally require derivatization to induce gelation, guanosine and its corresponding nucleotides are well known gelators. This unique gelating ability is due to propensity of the guanine nucleobase to self-associate into stable higher-order assemblies, such as G-ribbons, G4-quartets, and G-quadruplexes.

A Dual-Responsive Bola-Type Supra-amphiphile Constructed from a Water-Soluble Calix[4]pyrrole and a Tetraphenylethene-Containing Pyridine Bis-N-oxide.

Complexation between a water-soluble calix[4]pyrrole and a ditopic pyridine N-oxide derivative in aqueous media produces a bola-type supra-amphiphile that self-assembles to produce higher order morphologies, including multilamellar vesicles and micelles depending on the pH. The present bola-type supra-amphiphile exhibits strong fluorescence due to structural changes and aggregation induced by host-guest complexation. The resulting structures may be used to recognize, encapsulate, and release non-fluorescent, water-soluble small molecules.

A Molecular Chaperone for G4-Quartet Hydrogels.

Thioflavin T (ThT) functions as a molecular chaperone for gelation of water by guanosine and lithium borate. Substoichiometric ThT (1 mol % relative to hydrogelator) results in faster hydrogelation as monitored by (1)H NMR and visual comparison. Vial-inversion tests and rheology show that ThT increases the stiffness of the Li(+) guanosine-borate (GB) hydrogel. In addition, the dye promotes relatively rapid and complete repair of a Li(+) GB hydrogel destroyed by shearing. We used rheology to show that other planar aromatics, some cationic and one neutral dye (methylene violet), also stiffened the Li(+) GB hydrogel. Data from powder X-ray diffraction, UV, and circular dichroism spectroscopy and ThT fluorescence indicate that G4 quartets are formed by the Li(+) GB system. We observed a species in solution by (1)H NMR that was intermediate in size between monomeric gelator and NMR-invisible hydrogel. The concentration of this intermediate decreased much faster when ThT was present in solution, again showing that the dye can accelerate hydrogel formation. We propose that ThT functions as a molecular chaperone by end stacking on terminal G4-quartets and promoting the assembly of these smaller fragments into longer G4-based structures that can then provide more cross-linking sites needed for hydrogelation.

G4-quartet·M(+) borate hydrogels.

The ability to modulate the physical properties of a supramolecular hydrogel may be beneficial for biomaterial and biomedical applications. We find that guanosine (G 1), when combined with 0.5 equiv of potassium borate, forms a strong, self-supporting hydrogel with elastic moduli >10 kPa. The countercation in the borate salt (MB(OH)4) significantly alters the physical properties of the hydrogel. The gelator combination of G 1 and KB(OH)4 formed the strongest hydrogel, while the weakest system was obtained with LiB(OH)4, as judged by (1)H NMR and rheology. Data from powder XRD, (1)H double-quantum solid-state magic-angle spinning (MAS) NMR and small-angle neutron scattering (SANS) were consistent with a structural model that involves formation of borate dimers and G4·K(+) quartets by G 1 and KB(OH)4. Stacking of these G4·M(+) quartets into G4-nanowires gives a hydrogel. We found that the M(+) cation helps stabilize the anionic guanosine-borate (GB) diesters, as well as the G4-quartets. Supplementing the standard gelator mixture of G 1 and 0.5 equiv of KB(OH)4 with additional KCl or KNO3 increased the strength of the hydrogel. We found that thioflavin T fluoresces in the presence of G4·M(+) precursor structures. This fluorescence response for thioflavin T was the greatest for the K(+) GB system, presumably due to the enhanced interaction of the dye with the more stable G4·K(+) quartets. The fluorescence of thioflavin T increased as a function of gelator concentration with an increase that correlated with the system's gel point, as measured by solution viscosity.

A G4·K⁺ hydrogel stabilized by an anion.

Supramolecular hydrogels derived from natural products have promising applications in diagnostics, drug delivery, and tissue engineering. We studied the formation of a long-lived hydrogel made by mixing guanosine (G, 1) with 0.5 equiv of KB(OH)4. This ratio of borate anion to ligand is crucial for gelation as it links two molecules of 1, which facilitates cation-templated assembly of G4·K(+) quartets. The guanosine-borate (GB) hydrogel, which was characterized by cryogenic transmission electron microscopy and circular dichroism and (11)B magic-angle-spinning NMR spectroscopy, is stable in water that contains physiologically relevant concentrations of K(+). Furthermore, non-covalent interactions, such as electrostatics, π-stacking, and hydrogen bonding, enable the incorporation of a cationic dye and nucleosides into the GB hydrogel.

Unexpected hydroxylamine-induced ring-closure reactions of meso-tetraphenylsecochlorin bisaldehyde.

Reaction of meso-tetraphenylsecochlorin bisaldehyde with hydroxylamine results in the formation of the known meso-tetraphenyl-2-nitroporphyrin and the novel meso-tetraphenylimidazolophorphyrin. The products are the result of two diverging pathways of the presumed intermediate monooxime monoaldehyde that are unusual and surprising, but fully rationalized. The structures of both products were confirmed by spectroscopic techniques and single crystal X-ray diffractometry. This reaction represents the first reaction in which a pyrrole in a porphyrin was formally replaced by an imidazole moiety. The optical properties of the free base and metalloimidazoloporphyrin under neutral and acidic conditions are discussed. Further, an alternative synthesis of the imidazoloporphyrin Ni(II) based on meso-tetraphenyl-1-formyl-chlorophin is presented.