Langmuir. 2016 Oct 4;32(39):10120-10125.

Multidimensional Self-Assembled Structures of Alkylated Cellulose Oligomers Synthesized via in Vitro Enzymatic Reactions.

Pubmed

 

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Cellulose is a structural polysaccharide typically used in the cell walls of plants, and is believed to be the most abundant organic molecule in the biosphere. Recently, nanocelluloses, such as cellulose nanofiber and cellulose nanocrystal, extracted from natural sources have gained considerable attention as relatively new crystalline nanomaterials due to their mechanical stiffness, chemical/thermal stability, lightweight properties, biocompatibility, etc [1]. However, the morphologies of natural nanocelluloses are limited to naturally driven fibrous or rod-like structures. In addition, it is frequently hard to control the chemical reactions of nanocelluloses for functional modification due to poor reactivities of hydroxyl groups.

The in vitro enzymatic synthesis of cellulose oligomers under water-based mild conditions has been demonstrated to develop freely designed artificial nanocelluloses with unique structural and morphological properties different from natural nanocelluloses. For example, the cellulase-catalyzed oligomerization of β-D-cellobiosyl fluoride monomers [2] and the cellodextrin phosphorylase (CDP)-catalyzed oligomerization of α-D-glucose 1-phosphate (αG1P) monomers from D-glucose primers [3] have been investigated. In the CDP-catalyzed synthetic system, the cellulose oligomers with an average degree of polymerization of ~10 are self-assembled into rectangular sheet-like nanocelluloses with a width of several hundred nanometers, a length of less than several micrometers, and a thickness of ~5 nm, in which the cellulose molecules are aligned perpendicular to the base plane to form a lamella crystal with the anti-parallel cellulose II allomorph (Figure 1).

 

 

Figure 1. (a) Reaction scheme for CDP-catalyzed synthesis of cellulose oligomers using αG1P monomers and D-glucose primers. (b) Transmission electron microscopy image of rectangular sheet-like nanocelluloses.

 

The notable feature of the CDP-catalyzed synthetic system is that not only D-glucose but also β-D-glucose derivatives with anomeric substitutes can be utilized as primers to obtain terminus-functionalized cellulose oligomer derivatives due to the poor substrate specificity of CDP [4,5]. In this paper, multidimensional self-assembled structures of alkyl β-cellulosides were developed by the CDP-catalyzed oligomerization of αG1P monomers against alkyl β-D-glucoside primers with different alkyl chain lengths in a single step in buffer solutions. The alkyl chain length significantly determined the morphologies of assemblies, such as nanoribbons (for hydrogels), helical nanorods, and distorted nanosheets, and the crystal structures of the cellulose moieties (cellulose I vs. II allomorphs) (Figures 2 and 3).

 

 

Figure 2. Reaction scheme for CDP-catalyzed synthesis of alkyl β-cellulosides using αG1P monomers and alkyl β-D-glucoside primers with different alkyl chain lengths.

 

 

Figure 3. Self-assembly of alkyl β-cellulosides with different alkyl chain lengths.

 

Inspired by the self-assembly of biomolecules into highly ordered structures, biomolecular building units, such as hybridized nucleic acids, α-helix bundles and β-sheet of peptides, and lipid bilayers, have been used for the construction of self-assembled soft materials. Nevertheless, crystalline polysaccharides, such as cellulose and chitin, with unique physicochemical properties have rarely been used as the building units because of difficulty in their precise organic synthesis, solubilization in ordinary solvents, and control of crystal structures. To the best of our knowledge, this paper is the first report on a new potential of crystalline oligosaccharides for structural components of molecular assemblies with diverse morphologies and crystal structures. This paper will facilitate the fundamental science and engineering of crystalline oligosaccharide-based molecular assemblies for a novel class of soft materials, which can be used in biomedical fields.

 

[1] Klemm, D.; Kramer, F.; Moritz, S.; Linsdström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. Angew. Chem. Int. Ed. 2011, 50, 5438-5466.

[2] Kobayashi, S.; Sakamoto, J.; Kimura, S. In Vitro Synthesis of Cellulose and Related Polysaccharides. Prog. Polym. Sci. 2001, 26, 1525-1560.

[3] Serizawa, T.; Kato, M.; Okura, H.; Sawada, T.; Wada, M. Hydrolytic Activities of Artificial Nanocellulose Synthesized Via Phosphorylase-Catalyzed Enzymatic Reactions. Polym. J. 2016, 48, 539-544.

[4] O’Neill, E. C.; Field, R. A. Enzymatic Synthesis Using Glycoside Phosphorylases. Carbohydr. Res. 2015, 403, 23-37.

[5] Yataka, Y.; Sawada, T.; Serizawa, T. Enzymatic Synthesis and Post-Functionalization of Two-Dimensional Crystalline Cellulose Oligomers with Surface-Reactive Groups. Chem. Commun. 2015, 51, 12525-12528.