Unit cell, proposed cellulose

Fig. 4. Comparisons of the unit cells proposed for Cellulose I—IV. In all cells, the c dimension (perpendicular to the plane of the drawing) is ca 1.034 nm.

The most acceptable structure for a-chitin is the unit cell proposed by Carlstrom, which is shown in Fig. 13. It is orthorhombic, and the cell dimensions are listed in Table I (see p. 422). Some 62 separate reflections are present in the fiber diagram, permitting a relatively high degree of confidence in the final result. The fiber repeat observed is identical with that of cellulose, and, in the early studies, this led Meyer and Mark to postulate that chitobiose is the fiber repeat, before it was isolated as a... 

The most acceptable structure for a-chitin is the unit cell proposed by Carlstrom( ) shown in Figure 8. The fiber repeat is identical with that of cellulose, the space group is P2i2 2j and the unit cell contains two antiparallel chains. 

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... 

It is interesting to note that in their first paper on cellulose (11) Meyer and Mark proposed a structural unit cell model which is classic and accepted, for the largest part, even today. They proposed a cellulose crystallite in which all... 

The neutron-diffraction pattern from cellulose II, prepared by treating cotton linters with 20% sodium hydroxide at 0°, showed prominent peaks at (sin 8/ ) = 0.0089 - 0.0094 nm-1 and 0.0075 nm-1, which can be indexed only by enlarging the conventional unit-cell. The proposed cell-dimensions are a = 1.57 nm, b (fiber axis) = 1.03 nm, c = 1.84 nm, and /3 = 63°. The validity of the P2i space group and the twofold screw axis along the chain was questioned. 

Neutron diffraction studies of cellulose I (cotton crystallites) showed that the a andc dimensions (b is the fiber axis) of the conventional unit-cell should be doubled. The dimensions deduced are a = 1.678 nm, b (fiber axis) = 1.03 nm, c = 1.588 nm, and /3 = 82°. These are the same as the values proposed by Honjo and Watanabe,33 except that the b dimension is less than the value of 1.058 nm proposed by them. It was found that, in the region of 101, 101, and 002 reflections, there are a number of additional reflections that cannot be indexed by using the Meyer-Misch unit cell, but they can be indexed as 102, 102, 211, 211, 203, 203, 121, and 121 by using the larger cell. 

Fig. 7.—Schematic Diagram Showing the Interrelationship of Base-plane Packing for Three Unit cells of Native Cellulose. (The one in heaviest outline is the Meyer-Misch cell. The cell proposed for Valonia cellulose is twice as large in the base-plane dimensions. The third cell, having a angle of 93°14 is another proposal. )...

Most of the conformations proposed for cellulose have a two-fold screw axis along the chain, but it has been suggested that this concept be abandoned and a new, monoclinic, unit-cell structure similar to that of the original Sponsler-Dore cell has been put forward (unit-cell dimensions a = 10.85, b = 10.3, c = 12.08 A. (8 = 93°14 ). 

The salt-free crystalline polysaccharides reviewed by Bluhm et al. [15] are stabilized in characteristic crystalline unit cells by specific amounts of water. Two kinds of locations have been proposed for the water molecules one is unique, i.e., the water lies clustered in an existing interstitial cavity between double helices of B-starch. The other has water bound at specific sites within each unit cell. Additional water in this second type expands one or more unit cell dimension. This almost continuous expansion of the unit cell with increasing content of water may represent a more ordered aspect of the same interaction that occurs between water and accessible, disordered surfaces of celluloses crystallites (and other imperfectly crystalline polysaccharides). 

Later studies by Sugiyama et were based on electron diffraction and were directed at addressing questions concerning the nature of the differences between the Iq, and I forms of cellulose. In a landmark study, electron diffraction patterns were recorded from V. macrophysa both in its native state, wherein the Iq, and Ifj forms occur in their natural relative proportions, and after annealing using the process first reported by Horii and coworkers, which converts the Iq, form into the I form. The native material, which is predominantly the Iq, form, was shown to produce a complex electron diffraction pattern similar to that which had earlier led Honjo and Watanabe to propose an eight-chain unit cell. In sharp contrast, the annealed sample, which is essentially all of the I form, produced a more simple and symmetric pattern that could be indexed approximately in terms of a two-chain monoclinic unit cell. 

Perhaps the most significant new information derived from the CF-MAS spectra is that relating to the native celluloses. The spectra reveal multiplicities that cannot be interpreted in terms of a unique unit cell, even though they arise from magnetically nonequivalent sites in crystalline domains, the narrow lines observed have relative intensities which are neither constant among the samples of different native celluloses, nor are they in the ratios of small whole numbers as would be expected if they arose from different sites within a relatively small unit cell. VanderHart and Atalla proposed that native celluloses are composites of two distinct crystalline forms (44,45). 

Fig. 6 summarizes the possible molecular features in terms of parallel and antiparallel arrangements compatible with the unit cell data for methyl 8-D-cellotrioside. By analogy with the antiparallel type of packing proposed for cellulose II and for cellotetraose, one would favor type d in Fig. 6. 

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