Cellulose II, structure

It is worth noting that the mercerisation process, bom in the 19th century, produces a cellulose II structure too, but without dissolution of the fibres and therefore with no reshaping. Cotton fibres are soaked in a concentrated (19%) NaOH solution then washed. Mercerised cotton shows a softer touch and more brilliance than natural cotton. 

The cellulose II structure is more complex to unravel, since it involves two symmetry-independent chains. Polymer chain structures have been proposed that involve six different intrachain hydrogen-bond arrangements [344] ... 

Aqueous solutions of guanidine cause a structural change in cellulose midway between that produced by aliphatic amines and by alkali hydroxides. The lattice change to a cellulose II structure, which occurs with aqueous alkali hydroxide above a certain concentration, does not occur with guanidine. This was attributed to N-H—O bridges, with the participation of guanidinium ions. 

One of the most special aspects of cellulose polymorphy is the transformation from I to II. The conversion of the parallel-packed cellulose I structures to an antiparallel cellulose II structure is interesting because it can occur without loss of the fibrous form. This transformation is widely thought to be irreversible, although there are several reports [231-233] of regenerated cellulose I. The observation that there are two different forms of cellulose III and of IV is also remarkable. The two subforms of each allomorph have essentially identical lattice dimensions and at least similar equatorial intensities. Other intensities are different, particularly the meridional intensities, depending on whether the structures were prepared initially from cellulose I or II. The formation of the III and IV structures is reversible and the preceding polymorph (I or II) results. 

H- 02 type was proposed for the cellulose II structures. The alkali solubility of cellulose was shown to have a higher correlation with the reduction in intramolecular hydrogen bonding than with the apparent amorphous content [16,71]. 

Influence of Physical Structure. The hydrolytic behavior of cellulose is much influenced by its physical structure and lateral order [121-132]. Wood cellulose was hydrolyzed twice as fast as cotton [125]. Hydrolysis rate was significantly increased by physical or chemical pretreatment, with the effect depending on the source of cellulose. Hill and coworkers [127,128] reported that mercerization increased the hydrolysis rate of cotton (by 40%) and of ramie (7%), whereas the opposite effect was observed for linen and a-cellulose samples showing an approximately 30% reduction. Based on kinetic analysis, they concluded that the end-attach model proposed by Sharpies [121] can only be applied to the cellulose II structure and not to the cellulose I crystallite. Thus, the conformation of cellulose is also a significant factor affecting its reactivity and possibly the hydrolytic mechanism as well. 

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

The x-ray diffractograms showed the presence of cellulose I structure in all raw fibers. In the treated banana fiber, cellulose II structure was dominant. While cellulose I ([002] peak) is found in all five raw fibers, the [002] peak cellulose II is found as a shoulder in kenaf and ramie. The measured crystallinity (x%) in raw and alkali-treated fibers are shown in the table below ... 

When re-crystallized (for example, from base or CS2), cellulose I gives the thermodynamically more stable Cellulose II structure with an antiparallel arrangement of the strands and some inter-sheet hydrogen-bonding. Cellulose II contains two different types of anhydroglucose (A and B) with different backbone structures the chains consisting of -A-A- or -B-B-repeat units. Cellulose III is formed from cellulose mercerized in ammonia and is similar to cellulose II but with the chains parallel, as in cellulose la and cellulose Ip. 

These results may be of particular value especially in connection with studies of technical cellulose fibers and cellulose derivatives the above results may be of value for estimation of the state and structure of the material. The work of Fyfe and coworkers 1S,16) indicates that in microscrystalline rayon (cellulose II), hydrolyzed tire cord (cellulose III) and hydrolyzed rayon (cellulose IV) the identification of... 

In addition, cellulose undergoes changes in crystalline structure with relative ease. The most common modification is the conversion of cellulose I (i.e. la and 1/8) to cellulose II. This can be achieved by dissolution and regeneration or by simply treating cellulose I with sodium hydroxide. Cellulose II is usually considered to be more thermodynamically stable than biosynthesised cellulose I. However,... 

Figure 9 shows a spectrum of the hydrolyzed membrane, and peaks appeared at 11, 20, and 22 degrees. The crystal structure changed to that of cellulose II type. 

It now appears that cellulose I is not exclusively the native polymorph present in all organisms. The results reported originally by Sisson (61), which provided evidence that cellulose II was the native polymorph present in Halicystis (Ulvophyceae) cell walls, were recently reinvestigated and confirmed (62). Additionally, cellulose II producing mutants of Aceiobacier have been isolated and analyzed with x-ray and low-dose electron diffraction (63). When cellotetraose is induced to crystallize in solution it forms a structure which has been used as a model compound approximating the crystallographic nature of cellulose II based on x-ray diffraction, electron diffraction and CP-MAS 13C NMR evidence (64). Significantly, in all cases where Aceiobacier cellulose synthase in vitro activity has been reported,... 

Some correlation of the results with crystal structure is possible in that mixtures of Cx enzymes induced on a cellulose II crystal form (which would be present in the a-celluloses) and the Cx induced on the cellulose... 

Figure 3. Structure of Cellulose II (top) (a) ab projection (b) ac projection. (Right,) (c) hydrogen bonding network for the center up chains (d) hydrogen bonding network for the corner down" chains (e) hydrogen bonding in the...

---