The ability to chemically synthesize desired molecules followed by their in situ self-assembly in reaction solution has attracted much attention as a simple and environmentally friendly method to produce self-assembled nanostructures. In this study, α-d-glucose 1-phosphate monomers and cellobiose primers were subjected to cellodextrin phosphorylase-catalyzed reverse phosphorolysis reactions in aqueous solution in order to synthesize cellulose oligomers, which were then in situ self-assembled into crystalline nanoribbon network structures. The average degree-of-polymerization (DP) values of the cellulose oligomers were estimated to be approximately 7-8 with a certain degree of DP distribution. The cellulose oligomers crystallized with the cellulose II allomorph appeared to align perpendicularly to the base plane of the nanoribbons in an antiparallel manner. Detailed analyses of reaction time dependence suggested that the production of nanoribbon network structures was kinetically controlled by the amount of water-insoluble cellulose oligomers produced.

Crystalline poly- and oligosaccharides such as cellulose can form extremely robust assemblies, whereas the construction of self-assembled materials from such molecules is generally difficult due to their complicated chemical synthesis and low solubility in solvents. Enzyme-catalyzed oligomerization-induced self-assembly has been shown to be promising for creating nanoarchitectured crystalline oligosaccharide materials. However, the controlled self-assembly into organized hierarchical structures based on a simple method is still challenging. Herein, we demonstrate that the use of organic solvents as small-molecule additives allows for control of the oligomerization-induced self-assembly of cellulose oligomers into hierarchical nanoribbon network structures. In this study, we dealt with the cellodextrin phosphorylase-catalyzed oligomerization of phosphorylated glucose monomers from ᴅ-glucose primers, which produce precipitates of nanosheet-shaped crystals in aqueous solution. The addition of appropriate organic solvents to the oligomerization system was found to result in well-grown nanoribbon networks. The organic solvents appeared to prevent irregular aggregation and subsequent precipitation of the nanosheets via solvation for further growth into the well-grown higher-order structures. This finding indicates that small-molecule additives provide control over the self-assembly of crystalline oligosaccharides for the creation of hierarchically structured materials with high robustness in a simple manner.

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