Hakomori methylation acidic polysaccharides

Hakomori methylation of polysaccharides containing uronic acid residues results in their complete esterification. The ester group in the esterified residue (A in 12) is electron-withdrawing, allowing the hydrogen attached to C5 to become sufficiently acidic that on treatment with base it is eliminated giving the 4,5-unsaturated residue (B in 12). If the uronic acid is 4-linked, the substituent at C-4 is eliminated (Lindberg et al., 1975). 

Unlike neutral polysaccharides undermethylated acidic polysaccharides cannot be realkylated directly. The Hakomori procedure leads to esterification of uronic acid residues that on further treatment with base will undergo extensive p-elimination. Therefore the methylated acidic polymer should be carboxyl-reduced with LiAFH4 prior to realkylation. This is illustrated by our work (O Neill et al., 1986a) on the extracellular polysaccharide produced by Alcaligenes (ATCC 31555). After a single methylation an excess of branch points was observed which indicated incomplete methylation of position 2 of (l- 3)-linked glucose residues. To confirm this, the methylated polysaccharide was carboxyl-reduced, ethylated, and the alkylated alditol acetates analyzed by GLC-MS. The... 

The free polysaccharides were permethylated by the Hakomori method, hydrolyzed with 90% formic acid at 100-110°C for 8 hrs., and then with 2 M trifluoroacetic acid at 100-110°C for 3 hrs. The hydrclyzates were then reduced with sodium borohydride and acetylated wirh acetic anhydride in pyridine. The partially methylated alditol acetates thus obtained were analyzed with a Hewlett-Packard 5790 gas chromatograph. 

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