Cellobiose molecular rotation

Scheme 14.1 Molecular structure of cellulose representing the cellobiose unit as a repeating unit showing the non-reducing (left) and reducing (right) end-groups. This structure also shows that the repeated anydroglucopyranose units (AUG) are rotated of 180° with respect to each other, due to P-linkage constrains (n = DP, degree of polymerization)...

Cellulose is a carbohydrate, the structure of which may be deduced as follows. The molecular formula of cellulose is (CgHioOs), where n is a few thousand (see later). Hydrolysis of cellulose by boiling with concentrated hydrochloric acid yields D-glucose (II) in 95-96% yield. Thus cellulose is a polyanhydro-glucose. When cellulose is subjected to acetolysis (i.e. simultaneous acetylation and hydrolysis) by treatment with a mixture of acetic anhydride and concentrated sulphuric acid, cellobiose octa-acetate is formed. Thus the structure of cellulose is based on the cellobiose unit. Cellobiose is known to be the disaccharide, 4-0-j5-D-glucopyranosyl-D-glucopyranose (III). Finally, very careful acetolysis of cellulose produces a cellotriose, a cellotetraose and a cellopentaose and in each of these all the 1,4-links have been shown to be j8-links (from calculations of the optical rotations). Thus it follows that all the 1,4-links in cellulose are j5-links cellulose may therefore be represented by structure (IV). 

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