Degradation of double-stranded xanthan by hydrogen peroxide in the presence of ferrous ions: comparison to acid hydrolysis.
Conformationally ordered, double-stranded xanthan, degraded in the presence of H2O2 and Fe2+ (at 20 degrees C) or in dilute acid (0.1 M HCl at 80 degrees C), produced xanthan variants with weight-average molecular weights (Mw) ranging from 2 x 10(6) to 5.4 x 10(4). In both cases the fraction of cleaved linkages in the glucan backbone (alpha), measured as reducing ends, increased to very high values (0.05 for Mw = 2-3 x 10(4)), demonstrating that a large number of linkages in the backbone could be cleaved without a correspondingly large reduction in Mw, in accordance with the double-stranded nature of xanthan. Extensive degradation (more than 10-fold reduction in Mw) in both cases released single-stranded, conformationally disordered oligomers; this release was accompanied by an increase in the rate of acid hydrolysis of the glucan backbone and a pronounced increase in the rate of release of glucose monomer. In contrast, there was no significant change in the rate of reducing end-group formation associated with the release of oligomers upon degradation with H2O2/Fe2+. Both types of degradation were accompanied by changes in the composition of the side chains. However, in contrast to acid hydrolysis, where the terminal beta-D-mannose is preferentially hydrolyzed, the reaction with H2O2/Fe2+ resulted in removal of both mannose and glucuronic acid at approximately equal rates. This observation can be explained by a preferential attack on the inner alpha-D-mannose, with concomitant removal of the entire side chain. Removal of side chains and the release of single-stranded oligomers by H2O2/Fe2+ strongly influenced the optical rotation and also broadened the chiroptically detected conformational transition, whereas no change in the transition temperature was observed.
Christensen
B E
BE
Norwegian Biopolymer Laboratory, Department of Biotechnology, University of Trondheim, Norway.
Myhr
M H
MH
Smidsrød
O
O
D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D002850:Chromatography, Gel; D005609:Free Radicals; D005936:Glucans / Q000737:chemistry; D005947:Glucose / Q000737:chemistry; D005965:Glucuronates / Q000737:chemistry; D020723:Glucuronic Acid; D006851:Hydrochloric Acid; D006861:Hydrogen Peroxide / Q000737:chemistry*; D006868:Hydrolysis; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009844:Oligosaccharides / Q000737:chemistry; D009904:Optical Rotation; D011135:Polysaccharides, Bacterial / Q000737:chemistry*; D013696:Temperature
C045076:Fenton's reagent; D005609:Free Radicals; D005936:Glucans; D005965:Glucuronates; D009844:Oligosaccharides; D011135:Polysaccharides, Bacterial; D020723:Glucuronic Acid; D006861:Hydrogen Peroxide; D007501:Iron; D005947:Glucose; D006851:Hydrochloric Acid; C002563:xanthan gum