Polysaccharides methylation, Haworth method

NRRL B-742 dextran may be artifactual structures, resulting from incomplete methylation of this highly branched polysaccharide. Dextrans have been methylated most frequently by the Haworth method, " and by procedures that employ sodium and methyl iodide in liquid ammonia. " " Rapid methylation of dextrans has been achieved " through the use of the Hakomori procedure, which utilizes methyl sulfoxide as solvent for the dextran. Careful control of the reaction temperature would, however, appear to be essential when a dextran is methylated in this solvent, as hot methyl sulfoxide has been reported to depolymerize native dextrans." ... 

Total methylation of all available hydroxyl groups in a polysaccharide generally requires initial methylation by the Haworth method (dimethyl sulfate and aqueous sodium hydroxide) to introduce sufficient lipophilic... 

The aim of the methylation is to achieve an etherification of all of the free hydroxyl groups in the polysaccharide. The original procedure, as used by Denham and Woodhouse and by Haworth, in which the polysaccharide in 25-30% aqueous sodium hydroxide is treated with dimethyl is still the standard method. Fresh reagents are usually added to the reaction mixture after the first methylation period, with or without removal of the sodium sulfate formed, and the process is repeated until there is no further increase in methoxyl content. The many slight modifications of this method seem mostly to be matters of habit or convenience. The use of more concentrated sodium hydroxide solutions has been suggested, and occasionally seems to be essential for a satisfactory result. A practical point is that an aqueous solution of 42.2 g. of sodium hydroxide per 100 ml of solution is of exactly the concentration that 2 ml. is equiva-... 

Purdie and Irvine used methyl iodide in presence of silver oxide in methylating sugars. W. C. Denham and H. Woodhouse used dimethyl sulphate and alkali in methylating cellulose, and the obvious step of using this method in methylating sugars and polysaccharides was taken by Haworth. ... 

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