Studies of microstructure in native celluloses using solid-state carbon-13 NMR

Abstract

High-resolution solid-state 13C NMR spectra have been taken on several native cellulosic materials as well as on a regenerated, low-DP cellulose I material. Resonance multiplicities are observed for several carbon positions in the anhydroglucose units. The C4 resonance is particularly notable in that both a broad resonance and a narrow-line multiplet can be distinguished. Arguments are presented for the assignment of the broad resonance feature to surface chains on crystallites as well as chains in three-dimensionally disordered regions. The narrow-line multiplets, which are assigned to chains in the interior of crystallites, show significant variations in relative multiplet intensities, implying that native celluloses exhibit heterogeneous crystal structures. On the basis of these spectra it is proposed that all native celluloses are a mixture of two crystalline modifications, cellulose I" and 9. All native celluloses examined represent mixtures of these two structures in various proportions. Similarities noted by other investigators, between Valonia ventricosa and Acetobacter xylinum celluloses on the one hand and cotton and ramie on the other hand, are reinforced by the NMR results. There is no indication in these samples that each elementary fibril must contain the mixture of the two forms typical of the bulk sample. Therefore, the possibility that native celluloses are biosynthetically tailored composites certainly exists. Finally, although the exact chain conformation of the I" and Is conformations cannot be determined by the NMR data, multiplicities can be interpreted as inequivalences within the unit cell. The cellulose I", Ifl, and II NMR spectra are consistent with ideas, based on other spectroscopic evidence, that cellulose II and cellulose I5 (the major constituent in cotton and ramie) represent different chain conformations and have inequivalent successive glycosidic linkages along each chain.

The Future Prospects of Microbial Cellulose in Biomedical Applications

“…Normally, A. xylinum cellulose synthesized in the form of organized, twisting ribbons, is a highly crystalline I Rrich cellulose. 45 However, it is known that the cellulose crystallization process can be interrupted by addition of fluorescent brightening agents or cellulose derivatives to the media, which interact with nascent cellulose. [46][47][48][49] The structure of cellulose composites formed by the addition of different cellwall polysaccharides and reagents, like gluco-and galactomannans, xyloglucan, and pectin, were recently investigated using X-ray diffraction, 13 C CP/MAS NMR, and electron microscopy techniques.…”

In vitro assembly of cellulose/xyloglucan networks: ultrastructural and molecular aspects

“…All native celluloses are composites of two crystalline forms, I~ and 16 VanderHart and Atalla, 1984). The Ic~ form can be annealed into the 16 form suggesting that the latter is the more stable (Debzi et al, 1991).…”

When a proton attacks cellobiose in the gas phase: ab initio molecular dynamics simulations

“…Nevertheless, the enhancement of cis over trans in the isolated cellobiose, by protonation, is of considerable interest in exploring the balance between the two species also in different environs. While it is known experimentally as well as theoretically [40][41][42][43][44][45][46][47][48][49][50] that cis is more stable than trans CB in the isolated species and also in small water clusters [25][26][27] in the gas phase, in neutral condensed phase environs, both crystal and in solution, the energetic balance favours the trans species. The reasons for these structural preferences are as yet not understood and continue to be a topic of considerable research interest.…”