Chemoenzymatic synthesis of carboxylate-terminated maltooligosaccharides and their use for cross-linking of chitin

In this paper, we report chemoenzymatic synthesis of maltooligosaccharides having carboxylate groups at both ends (carboxylate-terminated maltooligosaccharides, GlcA-Glc_n-GlcCOONa). The products were further used as cross-linker for water-soluble chitin (WSCh) to obtain network chitins. The synthesis of GlcA-Glc_n-GlcCOONa was achieved by thermostable phosphorylase-catalyzed enzymatic α-glucuronylation using α-d-glucuronic acid 1-phosphate with a carboxylated maltooligosaccharide, which was prepared by chemical oxidation at the reducing end of maltoheptaose with sodium hypoiodite. The structures of GlcA-Glc_n-GlcCOONa were evaluated by ¹H NMR and MALDI-TOF mass spectra. The obtained GlcA-Glc_n-GlcCOONa were used as cross-linker for WSCh by condensation in the presence of condensing agent. The reaction mixtures totally turned into hydrogel form in most cases. Morphologies of lyophilized samples (cryogels) from the hydrogels were evaluated by SEM measurement. The hydrogels could be converted into films by pressing. Furthermore, mechanical properties of the hydrogels and films were investigated by compression and tensile tests, respectively.