Structure and Conformation of Selected Gel-forming Polysaccharides

Lists of gel-forming polysaccharides, with their molecular structures, are available from several sources,7-10 and other reviews deal with structure alone.11-180 It is only necessary here to summarize the 

0-(Carboxymethyl)cellulose is normally used as viscous, thixotropic solutions that often have a characteristic, lumpy texture.16,17,23 Rigid gels can be prepared in certain special circumstances which are described later. The manufacture of this ether consists in treatment of alkali-cellulose (cell-ONa) with sodium chloroacetate  

The degree of substitution is much lower than for the cellulose derivatives already mentioned, and is usually18,23 0.4-0.8. As with O-methyl-cellulose, a decrease in molecular weight is likely in the preparation and handling of the alkali-cellulose, and an unevenly patchwise distribution of substituents is to be expected, because of the effects of accessibility and inaccessibility. 

Important derivatives of cellulose are also manufactured by the introduction of other substituents, either alone or in combination.7 8,18-18 However, the basic types of behavior are well illustrated by the few examples already mentioned. Several other types are excluded by the terms of reference already laid down these include gels formed by so-called microcrystalline cellulose, which is prepared by controlled hydrolysis.24,25 Discussion of gels formed in nonaqueous systems26 will also be omitted. 

Application of this approach to cellulose has confirmed31 that the Hermans conformation is the only possibility that is free from strong steric clashes while fitting the usual interpretation of the x-ray evidence by having two fold screw symmetry and a projected residue height of 5.15 A. It is also stabilized by a hydrogen bond between successive residues, as in the crystal structures of cellobiose and 

---