Polysaccharides acidic hydrolysis rate

Like in gangliosides, lactones might be found in some bacterial capsular polysaccharides containing 1-carboxyethylsubstituents. But their identification remains problematic due to the conditions of isolation and preparation of analytic samples. To facilitate their detection by NMR, and in order to determine if the formation or hydrolysis of lactones occurred during analytical procedures, synthetic model substances, 2,3- and/or 3,4-lactones based on gluco-12, manno-13, and galactopyranosides 14 were prepared and characterized by NMR spectroscopy (Fig. 2).20 The relative lactonisation rates in acetic acid-fi 4 and hydrolysis rates in buffered D20 were evaluated. 

Polymer catalysts showing interactions with the substrate, similar to enzymes, were prepared and their catalytic activities on hydrolysis of polysaccharides were investigated. Kinetical analyses showed that hydrogen bonding and electrostatic interactions played important roles for enhancement of the reactions and that the hydrolysis rates of polysaccharides followed the Michaelis-Menten type kinetics, whereas the hydrolysis of low-molecular-weight analogs proceeded according to second-order kinetics. From thermodynamic analyses, the process of the complex formation in the reaction was characterized by remarkable decreases in enthalpy and entropy. The maximum rate enhancement obtained in the present experiment was fivefold on the basis of the reaction in the presence of sulfuric acid. 

The rate of hydrolysis of polysaccharides is affected by several factors. Because of substituent interaction effects, furanosides are hydrolyzed much more rapidly than the pyranoside analogues. Differences in the hydrolysis rates of diastereomeric glycosides are significant. For example, the relative hydrolysis rates of methyl-a-D-gluco-, manno-, and galactopyranosides are 1.0 2.9 5.0. This can be related to the stabilities of the respective conjugate acids, which are transformed into the half-chair carbonium ions at different rates. Also, substituents bound to the C-2 position obviously prevent the formation of the half-chair conformation. 

Carboxyl groups bound to the polysaccharide chains have a considerable influence on the rate of acid hydrolysis probably mainly because of steric interaction even if inductive effects should also be considered. For example,... 

Consistent with the behavior of simple glycosides (Table 2), the homogeneous hydrolysis rate of B-(l-4)-linked polysaccharides, as BeMiller summarized [255, 256], increased in the order cellulose (1) < mannan (2-2.5) < xylan (60-80) < galactan (300). This further demonstrates the significant role of accessibility in acidic degradation reactions. 

Conditions for selective hydrolysis do not apply uniformly to all polyols because of variations in stability among their acetal structures. This has frequently meant that the acid strength chosen was based on preliminary experiments. GLC methods and gel permeation chromatography have been proposed as means of selecting optimum conditions for the release of glycosyl alditols and residual oligosaccharide or polysaccharide, and the rate at which formaldehyde is produced by periodate oxidation of the hydrolysis mixture offers another diagnostic method. Methanolysis, rather than hydrolysis, has been advanced as an experimental variation that can reduce the amount of artifact formation associated with the use of aqueous acid. 

The rate of acidic hydrolysis of a polysaccharide can give useful information. Furanoside units are very rapidly hydrolyzed under mild conditions. Pento-pyranosides hydrolyze somewhat faster than hexopyranosides. Uronic acids Unked to neutral sugars yield glycuronic acid -> glycose disaccharides (aldobiouronic acids) which are resistant to hydrolysis. 

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