Structure and crystallization of sub-elementary fibrils of bacterial cellulose isolated by using a fluorescent brightening agent

• Published in: Cellulose (London) Vol. 19; no. 3; pp. 713 - 727
• Main Authors: Suzuki, Shinji, Suzuki, Furitsu, Kanie, Yasumasa, Tsujitani, Koji, Hirai, Asako, Kaji, Hironori, Horii, Fumitaka
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer-Verlag 01.06.2012; Springer Netherlands; Springer Nature B.V
• Subjects: Aqueous solutions; Bioorganic Chemistry; Brightening; Brightening agents; Buffer solutions; buffers; Cellulose; Cellulose fibers; Ceramics; Chain dynamics; Chains; chemical structure; Chemistry; Chemistry and Materials Science; Composites; Crystal structure; Crystallization; crystals; Ethanol; Fluorescence; Glass; Gluconacetobacter xylinus; High resolution; Hydrogen bonding; Liquid crystals; Molecular motion; Molecular structure; Natural Materials; NMR; Nuclear magnetic resonance; nuclear magnetic resonance spectroscopy; Organic Chemistry; Original Paper; Periodic structures; Physical Chemistry; Polymer Sciences; Sodium dodecyl sulfate; Sodium hydroxide; Solid state; Sustainable Development; Variations; Washing; Bacterial cellulose; Sub-elementary fibrils; Structure formation; Crystallization
• DOI: 10.1007/s10570-012-9678-4
• ISSN: 0969-0239

The structure and crystallization of carefully isolated sub-elementary fibrils (SEFs) of bacterial cellulose have been investigated using TEM, WAXD, and high-resolution solid-state 13C NMR. The addition of a suitable amount of fluorescent brightener (FB) to the incubation medium of Acetobacter xylinum effectively suppressed the aggregation of the SEFs into the microfibrils, as previously reported. However, this study confirmed for the first time that serious structural change in the SEFs occurs during the removal of excess FB by washing with buffer solutions having pH values higher than 6 or with the alkaline aqueous solution that was frequently used in previous studies. In contrast, the isolation of unmodified SEFs was successfully performed by utilizing a washing protocol employing pH 7 citrate–phosphate buffer solution containing 1% sodium dodecyl sulfate. High-resolution solid-state 13C NMR and WAXD measurements revealed that the SEFs thus isolated are in the noncrystalline state in which the pyranose rings of the almost parallel cellulose chains appear to be stacked on each other. The respective CH2OH groups of the SEFs adopt the gt conformation instead of the tg conformation found in cellulose I α and I β crystals, and undergo significantly enhanced molecular motion in the absence of intermolecular hydrogen bonding associated with these groups. The main chains are also subject to rapid motional fluctuations while maintaining the parallel orientation of the respective chains, indicating that the SEFs have a liquid crystal-like structure with high molecular mobility. Moreover, the SEFs crystallize into cellulose I β when the FB molecules that may adhere to the surface of the SEFs are removed by extraction with boiling 70 v/v% ethanol and 0.1N NaOH aqueous solution. On the basis of these results, the crystallization of the SEFs into the I α and I β forms is discussed, including the possible formation of the crystalline-noncrystalline periodic structure in native cellulose.