Parallel-chain packing, cellulose

Fractionation of the cellulose from the alga Valonia afforded a component of weight-average DP 4.4 x 10, which is thought to determine the size of the template required for the simthesis of the polysaccharide. The synthesis of cellulose molecules takes place on a flat template and leads directly to a folded-chain structure. Cellulose I crystallizes after it becomes detached from the site of synthesis. The crystal structure of native Ramie cellulose has been shown to be similar to that recently reported for Valonia cellulose. The R values, 0.158, 0.185, and 0.175, were obtained for anti-parallel, parallel-up, and parallel-down alignments respectively. Anti-parallel chain packing must still continue to receive consideration for cotton and Ramie cellulose despite contrary reports. 

The major structural characteristics of cellulose I, determined in this and other studies, include extended chains stabilized alorig their lengths W two intramolecular hydrogen bonds per glucose residue (0(3)- 0(5) and 0(6)—0(2) ), the arrangement of the chains into sheets stabilized by one intermolecular hydrogen bond per residue (0(3)—0(6)), and the packing of the sheets into a three-dimensional structure marked by a parallel-chain arrai ement. These structural characteristics are illustrated in Fig. 2. 

The question of parallel vs. antiparallel chain packing in cellulose I has been a controversial one practically since the first cellulose structure was proposed. A consensus appears to be forming, however, based on both diffraction analysis and other experimental evidence, that one of two possible... 

Finally, the question of the ability of the modeling methods to predict the crystal lattice (i.e., the unit cell) from the conformation of the chain should be addressed, despite the expected computational difficulties. Based on previous work, in which the prediction of the unit cells of all four cellulose polymorphs in both parallel and antiparallel chain packing polarities was... 

When crystalline cellulose I is treated with aqueous alkali solutions of sufficient strength, a process known as mercerization takes place. As a result of it, cellulose I is converted to cellulose II, the most stable or the four crystalline cellulose polymorphs. The conversion proceeds in the solid state, without apparent destruction or change in the fibrous morphology of the cellulose. As our diffraction analysis indicates, however, it is accompanied by a reversal of the chain packing polarity—from the parallel-chain cellulose I to the... 

Combined X-ray and electron diffraction analysis led to an orthorhombic unit-cell, with a = 2.468 mn, 1) = 1.152 nm, and c = 1.054 nm. The space group is P2,2,21. Two parallel chains are related, pairwise, by a two-fold screw-axis parallel to the chain axis, and pairs of chains pack in an antiparallel array. The (110) growth planes ol the crystal are parallel to the direction of highest atomic densities. The transformation CTA II cellulose II was discussed. The R factor is 30% with the X-ray diffraction data, and 26% with the electron diffraction data. 

Fig. 41. Model of the conversion of parallel- cked arrays of microfibrils of up and down chains of cellulose I to antiparallel-packed fibrils of cellulose 11 during mercerization (redrawn fiom Ref. 108). (See Color Plate 16.)...

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