Chain stacking, cellulose

Figure 3.8 and Figure 3.9 show the equatorial and meridional X-ray diffraction profiles of cellulose polymorphs [22]. These modifications are said to have the same skeletal conformation as cellulose I that is, a fairly extended zigzag conformation. However, the chain packing, chain stacking, chain direction, and intra/inter hydrogen bonds are different from one another, which is reflected in the diffraction profiles. 

One is shown in Fig. 4-5B. The hydrogen bonds and van der Waals forces bind the chains into sheets which are stacked to form fibers. A typical fiber of plant cellulose has a diameter of 3.5-4 nm and contains 30-40 parallel chains, each made up of 2000-10,000 glucose units. The chain ends probably overlap to form essentially endless fibers that can extend for great distances through the cell wall. They interact with other polysaccharides as is illustrated in Fig. 4-14. A single cotton... 

How the six-membered rings are joined together has an enormous effect on the shape and properties of these carbohydrate molecules. Cellulose is composed of long chains held together by intermolecular hydrogen bonds, thus forming sheets that stack in an extensive... 

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... 

Fig. 3. —Diagrammatic Representation of Stacks of Cellulose Chains and Their Possible Aggregation.94 [Each cellulose chain is ribbon-like and approximately oval in cross-section (labeled) the view is down the ribbon. Note that the stacks are labeled as sheets in the drawing, after the original authors, b = fiber and chain axis, which is perpendicular to the plane of this diagram and therefore not shown a and c are the other edges of the Meyer-Misch cell.]...

The derivatization of adequately substituted porphyrins has been appUed to the synthesis of polymeric glycoconjugates [101,102]. Daub et aL have reported the synthesis of a porphyrin functionalized cellulose [101]. The idea was to prepare multichromophore assemblies, organized along regular chains, for studying their optical, electronic, and photophysical properties [101,103]. Also spectroscopic and electrochemical studies were performed in order to obtain information concerning porphyrin-porphyrin n-7z stacking interactions. 

The projections of the crystal stmctures of cellulose la and Ip (Fig. 21.3) down the chain axes are remarkably similar. As the projection perpendicular to the chain axis and in the plane of the hydrogen-bonded sheets shows, the main difference between la and Ip is the relative displacement of the sheets in the chain direction. In both la and Ip, the second sheet, designated II, is shifted in the up direction by c/4 relative to the first sheet, designated I. The third sheet, designated III, is similarly shifted with respect to II by c/4 in la but in Ip it is shifted by a c/4 in the down direction. There is a relative difference of c/2 in the position of III with respect to II in la and Ip. Because there exists an approximate molecular 2i screw axis, this difference is equivalent to stacking opposite faces of III on II. 

FIGURE 21 The repeating unit of cellulose showing hydrogen bond interactions. The extended structure allows chains to stack via the relatively hydrophobic axial faces of the pyranose rings. 

---