Conformation of cellulose chains

Fig. 31. Conformation of cellulose chain = projection of the monomer unit on chain direction, A/2 = 2.7 A = length of an effective bond about which rotation is possible, 6 = 1.45 A length of an effective bond about which rotation is impossible...

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

Fig. 2 Schematic representation of cellulose structures in solution Part A shows the fringed micellar structure. Parts B and C show possible chain conformations of celluloses of different DP. For high molecular weight cellulose, C, intra-molecular hydrogen bonding is possible...

Figure 7. A "snapshot" of a typical cellulosic chain trajectory taken from a Monte Carlo sample of cellulosic chains, all based on die conformational energy map of Fig. 6. Filled circles representing glycosidic oxygens, linked by virtud bonds spanning the sugar residues (not shown), allow one to trace the instantaneous chain trajectory in a coordinate system that is rigidly fixed to the residue at one end of the chain. Projections of the chain into three mutually orthogonal planes assist in visualization of the trajectory in three dimensions.

Figure 3. Isolated chain conformations of cellulose predicted by MM2(85) (left) and PS79 (middle). The conformation on the right is that of the crystal structure of cellulose I (3). Hydrogen bonds are shown by dashed lines.

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... 

The primary sources of information concerning the molecular structure of cellulose have been x-ray and electron diffractometric studies, conformational analyses, and vibrational spectroscopy. The work up to 1971 was very ably reviewed by Jones (10), and by T0nnesen and Ellefsen (II, 12). They generally concluded that although much evidence can be interpreted in terms of cellulose chains possessing a two-fold axis of symmetry, in both Celluloses I and II, none of the structures proposed... 

It is proposed in this paper that the molecular flexibility, or in the case of cellulose, the lack thereof, is one of the fundamental factors that determines the suitability of a polymer for use as a fiber. Another factor is the set of interactions between molecules in the crystalline material. Those interactions, which depend on the accessible molecular shapes, diminish solubility, especially if both hydrogen and hydrophobic bonds are formed, as in the case for the common conformation of cellulose. The relatively limited range of shapes (a definition of stiffness) not only keeps the cellulose chains in a conformation that retains the interchain attractions but also minimizes increases in entropy in solution. 

Figure 4 shows the chain conformations of cellulose I ("bent") and of cellulose II ("bent and twisted") proposed in our previous work (28) on the crystal structure of cellulose II. These models of cellulose I and II have one and two kinds of Intrachain (03 -05) hydrogen bond, respectively. The number of 0-H stretching peaks in... 

Fig, Chain conformations of cellulose proposed by X-ray analysis 28). A bent form chain for cellulose I, B bent and twisted form chain deviated from 2 for cellulose II. There are two kinds of the distance of 03 - 05 Internal rotation angle around glucosidic linkage is according to Jone s expression 32). 01 - 01 distance is 5.16A, C - C,... 

The structure and arrangement of cellulosic chains play an important role in the formation of liquid crystals. At present, neither the conformation of cellulosics nor the solvent bound to the chain in the case of a lyotropic mesophase are known for these liquid-crystalline systems. Nevertheless, these structural features form the basis for a discussion of structural and thermodynamic aspects. Information on cellulosics is available for the two borderline cases next to the LC state, i.e., for the solvent built-in solid state as well as for the pure solid state, obtained by X-ray, NMR, and potential energy analysis on one side, and for the semi-dilute state from light-scattering experiments on the other side. These data have to be evaluated for a discussion of possible structures and models in liquid-crystalline phases. 

Next, we have examined the relationship between the chemical shifts and the molecular chain conformation. The versatility of chain conformation of cellulose molecules is expressed in three torsion angles as shown in Fig. 14 (j> and xp, rotations around the /3-l,4-glycosidic linkages and the rotation of methylol side groups around the C5-C6 bonds. In order to find out the relationship between the chemical shifts of cellulose samples and the torsion angles, the... 

Simon L, Scheraga H.A., and Manley St., R.J. 1988. Structure of cellulose. 1. Low-energy conformations of single chains. Macromolecules 21 983-990. 

Fig.5 shows schematic models of PC-LB film and SA-LB film on the ITO substrate. Both structures of PC-LB film and SA-LB film are regular in the direction parallel to film surface. However, the structure of SA-LB film in the direction perpendicular to the film surface seems to be irregularly aligned. In PC-LB film, the distance between palmitoyl chains keep some same distance. The conformation of palmitoyl chains is most probably determined due to cellulose main chains. 

Fig. 1. A segment of a cellulose chain composed of four P-D-glucopyianose residues (ceUotetiaose), showing (a) the chemical bonds and election clouds around the atoms. The molecule has the twofold hehcal conformation typical of many models of crystalline cellulose, (b) The (Haworth) stmctural formula...

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