Cationic hydrogels prepared from regioselectively azidated (1→3)-α-d-glucan via crosslinking and amination: Physical and adsorption properties

Bottom-Up Synthesized Glucan Materials: Opportunities from Applied Biocatalysis

Linear d-glucans are natural polysaccharides of simple chemical structure. They are comprised of d-glucosyl units linked by a single type of glycosidic bond. Noncovalent interactions within, and between, the d-glucan chains give rise to a broad variety of macromolecular nanostructures that can assemble into crystalline-organized materials of tunable morphology. Structure design and functionalization of d-glucans for diverse material applications largely relies on top-down processing and chemical derivatization of naturally derived starting materials. The top-down approach encounters critical limitations in efficiency, selectivity, and flexibility. Bottom-up approaches of d-glucan synthesis offer different, and often more precise, ways of polymer structure control and provide means of functional diversification widely inaccessible to top-down routes of polysaccharide material processing. Here the natural and engineered enzymes (glycosyltransferases, glycoside hydrolases and phosphorylases, glycosynthases) for d-glucan polymerization are described and the use of applied biocatalysis for the bottom-up assembly of specific d-glucan structures is shown. Advanced material applications of the resulting polymeric products are further shown and their important role in the development of sustainable macromolecular materials in a bio-based circular economy is discussed.

Polyglucuronic acids prepared from α-(1 → 3)-glucan by TEMPO-catalytic oxidation

2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO)-catalytic oxidation was applied to a water-insoluble α-(1 → 3)-glucan in water at pH 10 and room temperature (∼24 °C), with solid NaOCl·5H2O as the primary oxidant. Oxidation with NaOCl at 15 mmol/g gave a water-soluble TEMPO-oxidized product at a mass recovery ratio of 97 %. The carboxy content of the TEMPO-oxidized product was 5.3 mmol/g, which corresponds to a degree of C6-oxidation (DO) of 93 %. A new water-soluble α-(1 → 3)-polyglucuronic acid with a nearly homogeneous chemical structure was therefore quantitatively obtained. X-ray diffraction and solid-state 13C NMR spectroscopic analyses showed that the original α-(1 → 3)-glucan and its TEMPO-oxidized product with a carboxy content of 5.3 mmol/g had crystalline structures, whereas the oxidized products with DOs of 50 % and 66 % had almost disordered structures. The carboxy groups in the oxidized products were regioselectively methyl esterified with trimethylsilyl diazomethane, and analyzed by using size-exclusion chromatography with multi-angle laser-light scattering and refractive index detections. The results show that the original α-(1 → 3)-glucan and its oxidized products with DOs of 50 %, 66 %, and 93 % had weight-average degrees of polymerization of 671, 288, 54, and 45, respectively. Substantial depolymerization of the α-(1 → 3)-glucan molecules therefore occurred during catalytic oxidation, irrespective of the oxidation pH.

Facile fabrication of a broad-spectrum starch/poly(α-l-lysine) hydrogel adsorbent with thermal/pH-sensitive IPN structure through simultaneous dual-click strategy

A thermal/pH-sensitive interpenetrating network (IPN) hydrogel was prepared facilely from starch and poly(α-l-lysine) through amino-anhydride and azide-alkyne double-click reactions in one pot. The synthesized polymers and hydrogels were systematically characterized using different analytical techniques such as Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and rheometer. The preparation conditions of the IPN hydrogel were optimized via one-factor experiments. Experimental results indicated the IPN hydrogel possessed pH and temperature sensitivity. Effect of different parameters (pH, contact time, adsorbent dosage, initial concentration, ionic strength, and temperature) on adsorption behavior were investigated in monocomponent system with cationic methylene blue (MB) and anionic Eosin Y (EY) as model pollutants. The results indicated that the adsorption process of the IPN hydrogel for MB and EY followed pseudo-second-order kinetics. The adsorption data for MB and EY fitted well with the Langmuir isotherm model, indicating monolayer chemisorption. The good adsorption performance was due to various active functional groups (-COOH, -OH, -NH₂, etc.) in the IPN hydrogel. The strategy described here opens up a new way for preparing IPN hydrogel. The as-prepared hydrogel exhibits potential application and bright prospects as an adsorbent in wastewater treatment.

Freeze-thaw and solvent-exchange strategy to generate physically cross-linked organogels and hydrogels of curdlan with tunable mechanical properties

Physical gels from natural polysaccharides present the advantage of no toxic cross-linking agents and no chemical modification during preparation. Herein, novel physical gels, transparent organogels and opaque hydrogels from the microorganism-derived (1,3)-β-D-glucan of curdlan were prepared in dimethyl sulfoxide (DMSO) using the freeze-thaw technique, followed by a solvent-exchange strategy with water. The mechanical and structural properties of these gels were investigated by rheology, scanning electron microscopy, attenuated total reflection infrared spectroscopy, wide-angle X-ray diffraction and small-angle X-ray scattering. Gelation mechanisms and intermolecular interaction models have also been proposed. The good solvent DMSO serves as both a crosslinker and a pore-foaming agent in organogels. The reversible macromolecular conformation changes and phase separation of curdlan endow the gels with reversible transparency, volume change and tunable mechanical strength. The new design strategy of facile preparation and performance tuning provides a platform for developing new organogels and sterile hydrogels of curdlan.

In Vitro Synthesis of Branchless Linear (1 → 6)-α-d-Glucan by Glucosyltransferase K: Mechanical and Swelling Properties of Its Hydrogels Crosslinked with Diglycidyl Ethers

A hydrogel was prepared from a polysaccharide, enzymatically synthesized through a one-pot reaction in aqueous solution, and its properties as a functional material were evaluated. Enzymatic synthesis using glucosyltransferase K obtained from Streptococcus salivarius ATCC 25975 was performed with sucrose as a substrate. The synthetic product was unbranched linear (1 → 6)-α-d-glucan with a high molecular weight, M _w: 1.0-3.0 × 10⁵. The synthesized (1 → 6)-α-d-glucan was insoluble in water and crystallized in a monoclinic unit cell, which is consistent with the hydrated form of dextran. Transparent and highly swellable (1 → 6)-α-d-glucan hydrogels were obtained by crosslinking with diglycidyl ethers. The hydrogels showed no syneresis and no volume change during compression, resulting in the retention of shape under repeated compression. The elastic moduli of these hydrogels (<60 kPa) are smaller than those of other polysaccharide-based hydrogels having the same solid contents. The oven-dried gels could be restored to the hydrogel state with the original transparency and a recovery ratio greater than 98%. The mechanism of water diffusion into the hydrogel was investigated using the kinetic equation of Peppas. The properties of the hydrogel are impressive relative to those of other polysaccharide-based hydrogels, suggesting its potential as a functional biomaterial.