Cellulose chain packing

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. 

The seemingly small difference in structure between starch and cellulose allows the linear chains of cellulose to pack together side-by-side in an antiparallel extended conformation, stabilized by hydrogen bonds, to produce an insoluble structure of high mechanical strength. 

Moreover, studies on the structures of cellulose polymorphs are still being pursued extensively this is evident from the numerous entries on cellulose in this and the previous articles1,2 of this series. In addition to the conformation of the isolated, cellulose chain, the packing of the chains in the lattice presented formidable difficulties in the past. From the modest, theoretical attempts by D. W. Jones,3 Ramachandran,4 and Sundararajan5 to analyze the chain conformation and the... 

The sheets of hydrogen-bonded cellulose chains (lying in the a-c plane of Figure 2.3d) stack on top of one another in the b-direction to form a three-dimensional crystalline structure (Figure 2.3c). In the b-direction of the unit cell the atoms that project out axially are hydrogen atoms. Fortunately, these atoms are very small so the cellulose layers can pack very close (c. 0.39 nm between layers) so close that the van der Waals forces stabilize this tight packing. Also, there is the further... 

Figure 4.37 Cartoon of packing of cellulose chains in the two forms of cellulose I.

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