Antiparallel chain packing, cellulose

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... 

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. 

Atalla and Van der Hart (11, 12) concluded, based on their Raman and NMR spectra, that the molecules in cellulose I and II have different conformations. Based on x-ray analyses, Sarko et al. (13i H) and Blackwell et al. (15, 16) both concluded that crystal structures of cellulose I and II were based on parallel and antiparallel packing, respectively, of chains that have similar backbone conformations. Sarko (17) concluded that the allomorphs in the I and II families were based on parallel and antiparallel chains, respectively. The irreversibility may arise from the increase in entropy when parallel packing is converted to antiparallel packing. 

The major difference between these two crystal structures resides In the chain packing polarity. As expected from the conversion studies and the Irreversibility of the cellulose I to Na-cellulose I transformation, the crystal structure of Na-cellulose I Is based on antiparallel chains (cf. Fig. 3). Because of the presence of Na Ions, which apparently form secondary bonds with the cel-... 

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.)...

Similar models for the crystal stmcture of Fortisan Cellulose II came from two separate studies despite quite different measured values of the diffraction intensities (66,70). Both studies concluded that the two chains in the unit cell were packed antiparallel. Hydrogen bonding between chains at the corners and the centers of the unit cells, not found in Cellulose I, was proposed to account for the increased stabiUty of Cellulose II. The same model, with... 

Cellulose III. Cellulose III results from treatment of cellulose with Hquid ammonia (ammonia mercerization) or amines. Cellulose III can be made from either Cellulose I or II. When treated with water. Cellulose III can revert to its parent stmcture. Some cellulose III preparations are much more stable than other preparations. The intensities on diffraction patterns from Cellulose III differ slightly depending on whether the Cellulose III was made from Cellulose I or II, and thus these allomorphs are called IIIj or IHjj- Workers studying III concluded, based partiy on the results of I and II, that the packings of IIIj and IIIjj are parallel and antiparallel, respectively (67). IIIjj also is thought to have hydrogen bonds between the corner and center chains. 

Fig. 4.—Antiparallel packing arrangement of the 2-fold helices of cellulose II (2). (a) Stereo view of two units cells approximately normal to the ac-plane. The two comer chains (open bonds) in the back form a hydrogen-bonded sheet. The center chain (filled bonds) is linked to the comer chains by hydrogen bonds, (b) Projection of the unit cell along the c-axis and a is down the page. 

Using the two-chain unit-cell,3 with a = 0.817 nm, b = 0.785 nm, c = 1.034 nm, andy = 96.38°, the modified intensity-data of Mann and coworkers,37 and several residue-geometries, the structure of native ramie cellulose was refined. The resulting R factors were 15.8%, 18.5%, and 17.5% for, the antiparallel, parallel-up, and parallel-down models, respectively. A temperature factor of 0.23 nm2 was necessary in order to obtain a good fit with the observed data. It was suggested that the antiparallel packing of the chains cannot be discounted for cotton and ramie celluloses. 

Cellulose II, derived by mercerization of cotton, crystallizes in a monoclinic unit-cell, with a = 0.802 nm, b = 0.899 nm, c = 1.036 nm, and y = 116.6°. The space group is P2,. Two chains, each with a 2(0.518) confonnation, are packed in the unit-cell, with antiparallel polarity. In addition to the OH-3—0-5 (0.269 nm), intrachain hydrogen-bond, an OH-2 —0-6 (0.272 nm), intrachain hydrogen-bond is possible only for the center down chain. This is due to the g+ orientation of the C-6-0-6 bonds in the corner chains and t orientation in... 

The cellulose oligomers, beginning with methyl cellotrioside, yield powder diffraction patterns that are very similar to those of cellulose II. The NMR studies of the cellulose oligomers further establish the extensive analogy between cellotetraose and cellulose II. Work by both Gessler et al. [222] and Raymond et al. [223] has shown that the 06 atoms in cellotetraose and methyl cellotrioside [224] all take the gt position, consistent with the diffraction and NMR results for cellulose II. Because the chains in the methyl cellotrioside and cellotetraose are antiparallel, this work adds support to the above results on cellulose II. On the other hand, molecules in crystalline a-lactose, a related disaccharide, have parallel packing [225]. 

Valonia cellulose I was peracetylated under nonswelling conditions. The observations made by electron microscopy and electron diffraction on the fibrous tri-O-acetylcellulose I (CTA I) and the cellulose I obtained by deacetylation thereof led to the conclusion that the chains in CTA I are parallel-packed, as in cellulose I. Similar experiments showed that CTA II and cellulose II have the same antiparallel polarity of the chains in the lattice. 

Artificial cellulose showed the cellulose II allo-morph, a thermodynamically more stable form with an antiparallel structure, by X-ray diffraction study, when a crude celluase was employed for the enzymatic polymerization.123 The other allomorph cellulose I is a thermodynamically metastable form with a parallel structure, which living cells normally produce, but was believed impossible to be realized in vitro. Interestingly, however, the in vitro synthesis of cellulose I was successfully achieved by using a purified cellulase.125 The molecular packing of glucan chains in a crystal is affected by the purity of the enzyme as well as the enzymatic polymerization conditions. A novel concept choroselectivity was therefore proposed, which is concerned with the intermolecular relationship in packing of polymers having directionality in their chains.126... 

In the crystalline part, the cellobiose units are closely packed to form Cellulose I in native cellulose fibres and Cellulose II in regenerated cellulose fibres. In Cellulose I the chain molecules are parallel to one another [16]. The folded chain occurs at Cellulose II, in the crystalline regions the chain molecules are antiparallel. Thus, the basis for helical structure for Cellulose I is preferably extended to the structure of Cellulose II [17]. 

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