Cellulose cellobiose unit conformations

It may be concluded that the low field envelope in the C4 carbon peak can be assigned to cellobiose units having fewer intramolecular hydrogen bonds (i.e., the region around such cellobiose units in cellulose molecules has a very disordered conformation). We cannot simply conclude that in the absence of any intramolecular hydrogen bonds, the peak would narrow significantly, since cellobiose units in cellulose molecules in the solid state can take on a large number of conformations. 

IR dichroism is a useful technique for investigating conformations or oriented polysaccharide samples. For example, cellulose I possesses a unit cell containing two parallel chains of repeating cellobiose units. Dichroism studies show that the polarization of one of the bands is perpendicular, involved in inter-chain hydrogen bonds and two are parallel to the long axis forming intra-chain hydrogen bonds. 

The size of the cellulose molecule is normally expressed in terms of their degree polymerization (DP), i.e, the number of anhydroglucose units present in a chain. However, the conformational analysis of cellulose indicates that cellobiose (4-0-a-D-glucopyranosyl-a-D-glucopyranose, Fig. 10.2a) is its basic structural unit [10]. The conformation of the repeating unit of cellulose can be explained if we consider the model proposed for the biosynthesis of glucose [11]. 

Nowadays, it is believed that glucose, not cellobiose, is the essential unit of the cellulose that repeats as a dimer in most crystal structures, with one or two t5rpes of conformation angles [5]. 

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