Alkaline Degradation of Polysaccharides

Recently, the study of alkaline degradation has been re-opened, with the objective of using it as a means of determining the structures of oligo- and polysaccharides and of oxidized polysaccharides. That interest has indeed been rekindled is attested by the recent reviews on saccharinic acids: on the modification of monosaccharides in alkaline solution, and on the hot alkali stability of chemically-treated, cellulose fibers. Ether derivatives of monosaccharides, oligosaccharides, and alkali-sensitive glycosides' have been used as models in determining the effect of alkalis on oxygen-free solutions of polysaccharides and oxidized polysaccharides. Alkaline degradation of polysaccharides begins, in general, at the reducing end of the molecule and proceeds in a stepwise manner through the anhydroglycose chain. Consequently, an understanding of the effects of aqueous, alkaline solutions on the reducing end-group (or on modifications of it) is essential to comprehension of the mechanism of alkaline degradation. Reducing glycose units of polysaccharide chains will be transformed, in part, to their C2-epimers in alkaline solution. The classical transformation of Lobry de Bruyn and Alberda van Ekenstein is a base-catalyzed enolization giving an enediol, which may either revert to the starting aldose or be converted to epimers of the original aldose; they showed that the main product is the ketose. However, the ions may also be in equilibrium by prototropy, without transition through an enediol.