MODIFICATION OF CRYSTALLINITY AND CRYSTALLINE-STRUCTURE OF ACETOBACTER-XYLINUM CELLULOSE IN THE PRESENCE OF WATER-SOLUBLE BETA-1,4-LINKED POLYSACCHARIDES - C-13-NMR EVIDENCE

Cellulose produced by Acetobacter xylinum in medium containing 0.5% xy loglucan or glucomannan showed altered crystallinities and shifted I-a lpha/I-beta ratios when analysed by solid-state C-13-NMR. By estimatin g the spectra of cellulose components in each composite, a decreased I -alpha content was shown to be countered by increased I-beta content i n cellulose aggregated in the presence of xyloglucan, causing minimal loss of crystallinity. However, the I-alpha decrease was linked primar ily to increased disordered content in cellulose produced in medium co ntaining glucomannan. These results are considered in the light of two models for the morphological disposition of the I-alpha phase: (i) a series model, proposed on the basis of electron diffraction measuremen ts for an algal cellulose, in which regions of I-alpha and I-beta alte rnate along the length of a microfibril, and (ii) a superlattice model , in which the I-alpha and I-beta domains co-exist throughout the cros s-section of each microfibril and form as a result of hierarchical agg regation. The latter model offers clearer insight into the role of the polysaccharides in inhibiting the formation of I-alpha crystalline re gions. In this superlattice model, polysaccharides adsorbed on surface s of the most elementary aggregates are displaced to varying degrees d uring subsequent aggregation, with the presence of these polysaccharid es altering the extent of I-alpha production at interfaces.