The individual structures of native celluloses

Abstract

Our understanding of the diversity of native celluloses has been limited by the fact that studies of their structures have sought to establish ideal crystal lattice forms for the native state. Departures from ideal structures in the native state are viewed as defects in the ideal lattice. In most instances real celluloses have been regarded as departing from the ideal structures primarily with respect to the degree of disorder. The disorder is considered as imparting an amorphous character to part of the cellulose and methods are sought to quantify the bamorphousc fraction. In contrast, recent developments in structural characterization, based primarily on the use of spectroscopic methods, have revealed that native celluloses are species specific composites of two forms of cellulose, Ia and Ib. They appear to possess the same secondary structure but distinctly different tertiary structures. Their aggregation appears to reflect an inherent tendency of the cellulose molecules to self assemble into fibrils at a hierarchy of levels. This is most clearly revealed through observations of the biogenesis of cellulose in model systems incorporating cultures of Acetobacter xylinum. The unique character of celluloses from particular species suggests that the intimate blending of the two forms of cellulose is regulated during biogenesis. Furthermore, the uniqueness of particular native structures appears to emerge at the nanoscale level (2 to 10nm), a level that is not well detected or characterized by traditional methods for the investigation of structure. This report describes efforts to characterize the nature of native celluloses and their states of aggregation beyond simply categorizing their degree of order and in a manner that may allow more comprehensive systematization of the diversity of the native structures. It will examine the degrees to which information based on different methods of structural characterization can be complementary. Particular attention will be given to the balance of information derived from spectroscopic methods, including solid state 13C NMR, IR and Raman Spectroscopy, and information derived from diffractometric methods, including x-ray and electron diffraction. The report will also include consideration of the far wider category of celluloses which, in their native states, are intimately blended with other polysaccharides and with lignin and are even more diverse with respect to their patterns of aggregation.

Citation

Atalla, R. H. 1999. The individual structures of native celluloses. [10th International Symposium on Wood and Pulping Chemistry, Main Symposium, 1999 June 07-10, Yokohama, Japan]. Atlanta, GA : TAPPI Press, 1999.:p. 608-614 : ill.