No abstract

SynopsisFive different glucomannan samples were recrystallized from dilute solution. Depending on the experimental conditions, the crystals obtained could be identified as corresponding to the mannan I (anhydrous precipitate of more or less regular lozenge-shaped crystals) or mannan I1 (hydrated gel-forming pseudo-fibrillar precipitate). High-molecular-weight material, low temperature of crystallization, or a polar crystallization medium favored the mannan I1 polymorph, whereas a low-molecular weight, a high temperature of crystallization, and a crystallization medium of low polarity yielded the mannan I polymorph. Since the base-plane unit-cell dimensions are fairly constant with respect to variation of glucose, it is likely that isomorphous replacement of mannose by glucose occurs in glucomannan crystallization; the data also indicate that perfection of the glucomannan crystals was reduced in specimens having a high g1ucose:mannose ratio. The oriented crystallization of glucomannan on cellulose microfibrils was also studied under conditions where the mannan I polymorph was obtained. This gave shish-kebab structures that were characterized.

SynopsisThe preparation of lamellar single crystals of mannan[poly((l -4)-P-D-mannose)] is described. Electron diffractograms clearly identify the perpendicular orientation of the chain axis with respect to the lamellar surface. Since the degree of polymerization is 40 or less, no conclusion is made as to chain folding. The unit cell corresponds to the mannan I structure derived from x-ray fiber data on oriented algal mannan. The baseplane dimensions found were a = 7.22 8 and b = 8.92 8, and the systematic absences observed confirm the proposed P212121 group. It was found that cellulose microfibrils from Valonia uentricosa and bacterial cellulose could serve as extended chain nuclei for inducing oriented crystallization of mannan on cellulose. This produces a shish-kebab type of morphology.

SynopsisThe enzymatic degradation of p(1-4) xylan single crystals with xylanases was investigated by electron microscopy and electron diffraction. The enzyme attack takes place a t the edge of the crystals and progresses towards their centers. This is consistent with an endo-enzyme mechanism, where the enzyme interacts essentially with the accessible xylan chains located a t the crystal periphery.

No abstract