Carboxyl groups uronic

Reduction of uronic acids Uronic acids (UA) were converted to the corresponding neutral sugars (NS) by carbodiimide activation of the carboxyl groups followed by a reduction with NaBD4 according to the method of Kim and Carpita [5]. In order to achieve a complete reduction of the uronic acids the procedure was repeated once. 

Polymers of uronic acids are encountered in nature in structures known as pectins, which are essentially chains of D-galacturonic acid residues linked al -> 4, though some of the carboxyl groups are present as methyl esters. These materials are present in the cell walls of fruit, and the property that aqueous solutions under acid conditions form gels is the basis of jam making. 

Similarly, the C-1 resonance of an axial anomer is shielded relative to that of its eq.uatorial isomer. Also very distinctive are signals due to the carbon of a primary alcohol group (C-6, in the region of 6O-65 p.p.m.) and to the carboxyl group of an uronic acid moiety. Typically, as seen in Fig. 1, the carboxyl C=0 resonance is in the region of 1T5 p.p.m. although, as noted below (see Fig. 3), it is strongly pH dependent. 

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. 

URONIC ACID. Any of a class of compounds similar to sugars but differing from diem in that the terminal carbon has bee oxidized from an alcohol to carboxyl group. The most common are galacturomc acid and... 

URONIC ACIDS Form hemiacetal rings in aqueous solution, leaving the carboxylate group free... 

The reducing group in uronic acids can be oxidized by nitric acid or bromine to yield aldaric acids. Uronic acids, as salts, can be quantitatively reduced to aldonic acids by borohydride.239 Lactones are readily reduced, as in the controlled conversion of D-glucurono-6,3-lactone into D-gluco-hexodialdose. The carboxyl group of glucuronic acids may be reduced to a primary alcoholic group. 

The sequence of residues in the disaccharide building blocks and the manner of access to the carboxyl group of the uronic acid demand further considerations. Depending on the intended target, the disaccharide building blocks may be constructed with the hexosamine unit positioned either at the reducing side or at the... 

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. 

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