Cellulose antiparallel models

Figure 5. Most probable parallel (left) and antiparallel (right) cellulose I models.

This paper is a review of x-ray diffraction work in the authors laboratory to refine the structures of cellulose I and II, and a- and B-chitin, concentrating on the methods used to select between alternate models. Cellulose I is shown to consist of an array of parallel chains, and this conclusion is supported by a separate refinement based on electron diffraction data. In the case of cellulose II, both parallel and antiparallel chain... 

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... 

As the above results show, the gross features of the cellulose I crystal structure predicted by various methods do not differ appreciably, but the accompanying deviations in the R -factors are significant. When these predictions are used to assess, for example, whether the cellulose I crystal structure is based on parallel- or antmarallel-chains, the range in the R"-factors seen for the parallel models (cf. Table II) is comparable to that between the two different polarity models. As shown in Fig. 5, the most probable parallel- and antiparallel-chain structures of cellulose I, refined by minimizing the function O, differ in R -factors by approximately the same extent as the three predictions for the parallel model shown in Fig. 4 and Table II. 

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... 

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. 

Monoclinic unit cell of cellulose I according to the model by Meyer and Misch [37], which shows the antiparallel orientation of adjacent chains (left) and the hydrogen-bonding network of two adjacent cellulose chains forming a sheet-like structure according to Gardner and Blackwell [38]... 

When the models incorporating antiparallel arrangement of the chains are extended to native cellulose, they pose serious questions concerning proposed mechanisms for the biosynthesis of cellulose. 

This being a complex of cellulose II, one would expect these two chains to be antiparallel. We did consider parallel chain models but these always gave Inferior agreement with the intensity data and could be ruled out. 

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.)...

The difficulty of explaining the antiparallel arrangement of cellulose chains in the cellulose fibre is relieved, if not wholly avoided, by this model. The initiation of cellulose synthesis is considered to occur deep within the... 

Fig. 1.5. 1) Projection of the parallel chain model for cellulose I. Projection perpendicular to the ac plane. The center chain (black) is staggered with respect to that at the origin. 2) Projection of the antiparallel chain model for cellulose II. Projection perpendicular to the ac plane (from Blackwell et al., 1978).

The structure of cellulose has been studied since the 19th century, when Carl von Nageli proposed the idea that natural cellulose contains ciystalline micelles—small crystallites (Wilkie, 1961 Zugenmaier, 2009). Only 70 years later, this idea was confirmed by X-ray diffraction, and as a result, the first model of monoclinic unit cell for crystalline structure of native cellulose Cl was developed by Mayer and Mish (Mayer et al., 1937). The model of Mayer and Mish with antiparallel arrangement of chains existed 30 years, whereupon it was replaced by a more accurate model with parallel arrangement of cellulose chains within crystallites (Gardner et al., 1974). Later it was discovered that in addition to crystalline structure of native cellulose Cl, there are also other crystalline allomorphs, CII, CIII, and CIV (O Sullivan, 1997). 

---