Uronic acids esterification

Kim, J-B., Caipita, N.C. (1992), Changes in esterification of the uronic acid groups of cell wall polysaccharides during elongation of maize coleoptiles. Plant Physiol. 98, 646-653. 

On esterification, the carboxyl group in uronic acid residues becomes a carboxylate that is electron-withdrawing, and, on subsequent treatment with base, / -elimination will occur. The p-elimination reaction for uronic acids and their derivatives has been reviewed in this Series, 6 and only its applications to structural polysaccharide chemistry will be discussed here. 

Purity was confirmed by gel-filtration using a HPLC column packed with Asahipak GS-520HQ and elution with 100 mM sodium phosphate buffer containing 300 mM sodium chloride (pH 6.7). The content of total protein, total sugars, uronic acids, sulfates, nucleic acids, phosphate or fatty acids was assayed by the BCA [32] and Lowry method [33], the phenol-sulfuric acid method [34], the Blumenkrantz method [35], nephelometry [36], absorption at 260 nm, the Bartlett method [37] and the GLC method after methyl-esterification [38], respectively. 

The chemistry of these polysaccharides is dominated by partial esterification of the total number of carboxyl groups. The sequence of uronic acids in the primary structure is occasionally interrupted by rhamnose, and there is usually a trace of acetyl and phenolic substituents. Models developed from experimental data considered neutral side chains with DP = 2-10 (De Vries et al., 1982). These 1,4-a-linked linear uronans are susceptible to alkali—more so if the C-6 hydroxyl is esterified, but exceptionally acid-stable when this site is unsubstituted. Dispersion stability is less at higher DP. Uronans are endowed with strong dye-fixing and mineral-sequestering properties because of their charged surface. 

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... 

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). 

Similar transformations have also been published by Cognis [89], for instance the esterification of uronic acid 86 with various alcohols (e.g. butanol) catalyzed by immobilized CAL-B (Scheme 26). The esters such as 87 possess surface activity and may show antimicrobial activity and can therefore be used as (co)-sur-factants, emulsifying agents, or antimicrobials in the home and personal care area. 

Alginate is a polymer composed of uronic acid monomers. While this acidic polysaccharide can be recovered from bacteria, the commercial source is brown seaweed. Both propylene glycol esterification and acetylation of the polymer cause an increase in the thickening capabilities of the gum. The acetylation of alginate by pseudomonal species demonstrates alternative biochemical methods for polysaccharide modification. 

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