Preparation of cellodextrins and isolation of oligomeric side components and their characterization

Cellodextrin (beta-1,4-glucose oligomer) mixtures are prepared by precipitation of oligomers with 1-propanol and ethanol after partial hydrolysis of cellulose with hydrochloric acid or by acetolysis of cellulose. Cellooligomers (DP3-DP8) can be isolated by high-resolution size-exclusion chromatography on Bio-Gel P 4 using water as eluent. Recycle operation of the columns allows the separation of oligomers up to a degree of polymerization of 12. However, ion-exchange chromatography of their borate complexes demonstrates the heterogeneity of cellodextrins, homogeneous according to size-exclusion chromatography. At least four secondary oligomeric components are observed in the different samples. By preparative affinity chromatography on phenyl-boronate-agarose two of these components could be purified and subsequently characterized. In one series of oligosaccharides the glucose unit at the reducing end of the beta-1,4-glucose oligomers is derivatized to fructose. This enolization reaction occurs during size-exclusion chromatography. The precipitation step with alkanols during preparation of oligomer mixtures generates oligomeric glycosides. Additionally, the formation of amines from respective beta-1,4-glucose oligomers is observed with the ammonium carbonate eluent used in affinity chromatography. Analysis methods combined to assess for the homogeneity of cellodextrins include enzyme- and acid-catalyzed (partial) hydrolysis of the different oligomers and subsequent analysis of degradation products by sugar borate chromatography; 13C and 1H NMR spectroscopy; and fast atom bombardment mass spectroscopy.