Effect of ionic liquid on the enzymatic synthesis of cello-oligosaccharides and their assembly into cellulose materials

Abstract

Imidazolium-based ionic liquids are important solvents for the processing of natural cellulose. Little is known about their use in synthesizing cellulose via bottom-up polymerization of β-1,4-d-glucosyl chains in solution. Here, we analyzed cellodextrin phosphorylase-catalyzed synthesis of cello-oligosaccharides, and the subsequent spontaneous self-assembly of the chains, in the presence of cellulose-dissolving ionic liquid, 1,3-dimethylimidazolium dimethyl phosphate ([Dmim]DMP) or 1-ethyl-3-methylimidazolium acetate ([Emim]OAc). The average chain length dropped from ~7.4 in buffer to ~6.4 in ionic liquid (30 vol%). The synthetic cellulose exhibited allomorph II crystal structure and showed nanosheet morphology of 4–5 nm thickness and several μm length. Its suspensions were hydrogels with viscoelastic properties dependent on solvent conditions used. Reactions in 10 vol% [Dmim]DMP or [Emim]OAc gave a hydrogel with elastic modulus of ~13 kPa and loss factor of ~0.18. Collectively, interactions of the ionic liquid with enzyme and cello-oligosaccharides delimit the polymerization and tune the assembly into cellulose networks.