Chemoenzymatic synthesis and pH-responsive properties of amphoteric block polysaccharides

Here, we investigated the chemoenzymatic synthesis of alpha(1 -> 4)-linked amphoteric block polysaccharides. Amylouronic acid as an acidic block was first synthesized by 2,2,6,6-tetramethylpiperidine 1-oxyl-mediated oxidation of a water-soluble amylose (chemical reaction). A short maltooligosaccharide chain, serving as an initiating site for the following enzymatic polymerization, was then introduced at the non-reducing end of the product by thermostable alpha-glucan phosphorylase-catalyzed enzymatic oligomerization of alpha-D-glucose 1-phosphate. Finally, thermostable alpha-glucan phosphorylase-catalyzed enzymatic polymerization of alpha-D-glucosamine 1-phosphate from the produced primer provided a basic block at the nonreducing end, leading to the desired amphoteric block polysaccharides. The structures of the products at each step were determined by H-1 NMR analysis. Furthermore, amphoteric products exhibited specific inherent isoelectric points (pIs). When the pH-responsive properties in aqueous solutions were evaluated using a divalent acid and base, similar hierarchical assembling/disassembling processes were observed by shifting the pH values from the pI to both the acidic and basic pH.