Polysaccharide with trifluoroacetic acid, hydrolysis

Hydrolysis of Polysaccharides with Trifluoroacetic Acid and its Application to Rapid Wood and Pulp Analysis... 

The characterization and quantitative determination of uronic acid components in polysaccharides faces the problem of complete release of the uronic acids without accompanying decomposition. This is a difficult task because of the acid resistance of the glycosiduronic bond. From a comparison of several methods, it was shown that methanolysis combined with trifluoroacetic acid hydrolysis is the best for the liberation of uronic acids.204 The identification can be performed by gas chromatography of the trimethylsilyl derivatives.205... 

Ethers and water-soluble derivatives of cellulose have been assayed spectro-photometrically following reaction of D-glucose or substituted o-glucose derivatives released on hydrolysis with 4-hydroxybenzoylhydrazine. Hydrolysis of the polysaccharides was achieved more readily with trifluoroacetic acid than with sulphuric acid. 

For the total hydrolysis of polysaccharides, trifluoroacetic acid (TFA) has important advantages over sulfuric acid. The reaction time is short and there is no need for conventional neutralization, as TFA is volatile and can be removed by evaporation. Several methods have been developed, depending on the substance to be hydrolyzed. Soluble saccharides (e.g., polyoses) can be hydrolyzed with diluted TFA, while cellulose, pulp, and wood need treatments with concentrated TFA in homogeneous solution. The presence of lignin impedes the hydrolysis of polysaccharides thus, especially for wood samples, an intensive treatment with TFA is necessary, and correction values have to be considered. Several application examples show that the hydrolysis with TFA enables a rapid quantitative determination of the composition of polysaccharides, pulps, and woods. 

The standard procedure by Saeman et al. (I) involves manual stirring of the polysaccharide with 72% H2S04, standing at 30°C, and secondary hydrolysis at 100° or 120°C in a steam autoclave. While certain resistant polysaccharides are still incompletely depolymerized, decomposition of the more sensitive monosaccharides formed cannot be avoided. An alternative method by using trifluoroacetic acid was applied successfully for plant cell wall polysaccharides by Albersheim et al. (2) and for dissolving pulps and hemicelluloses by Fengel et al. (3). Highly crystalline cellulose was not well dissolved and not completely hydrolyzed by CFsCOOH. 

The first question concerns the nature and relative proportions of constituent monosaccharides. In principle, this is obtained by acidic hydrolysis (Biermann 1988) but, in practice, it must be carefully applied as there are a certain number of important specific cases. Hydrochloric, sulfuric, and trifluoroacetic acids are used whose 1 N solutions have a pH of 0.1, 0.3, and 0.7, respectively. When hydrolysis liberates monosaccharides fragile in an acidic medium, a delicate balance between the risk of incomplete hydrolysis and partial destruction of the hydrolysis product must be maintained. The fragile sugars are pentoses, deoxy sugars, and uronic and aldonic acids. When sialic acid is kept for 30 min at 90°C in 0.01 M HCl, 20% decomposition occurs. With neutral polysaccharides, decomposition can be limited to less than 9%. The acetyl groups of acetamides are hydrolysed and relatively stable protonated amino sugars are obtained. 

An efficient procedure for converting polysaccharides into their monosaccharide constituents is by hydrolysis at 121° for 1 hr with 2N tri-fluoroacetic acid. Trifluoroacetic acid is volatile and can be easily removed from the hydrolyzed samples by evaporation in a stream of air. The monosaccharides are converted to the corresponding alditols by reduction with sodium borohydride and the alditols are acetylated with acetic anhydride in the presence of a weak base, such as sodium acetate. The steps involved in converting a polysaccharide into its constituent alditol acetates are summarized in Fig. 1. 

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