Uronic acids, oxidation

Oxidation of an aldobiouronic acid with bromine under nonhydrolytic conditions produces a dibasic acid in which the new carboxyl is formed from the original hexose or pentose unit. This is shown by the fact that such an acid (when the reducing portion of the original biouronic acid is a hexose) will form the same amount of furfural as the original acid under the action of 12% HCl. Evidently, the glycosidic linkage is formed from the hemiacetal group of the uronic acid. Oxidation with bromine under hydrolytic conditions produces a dibasic and an aldonic acid and allows identification of the two units. 

Uronic acids occupy an oxidation state between aldonic and aldanc acids They have an aldehyde function at one end of their carbon chain and a carboxyhc acid group at the other... 

Uronic acids are monosaccharides in which the terminal primary alcohol group is oxidized to a carboxyhc acid functional group, eg, D-glucuronic acid [6556-12-3] (11). 

Uronic acid Class of aeidie eompounds of the general formula HOOC(CHOH) CHO that eontain both earboxylie and aldehydie groups, are oxidation produets of sugars, and oeeur in many polysaeeharides espeeially in the hemieelluloses. 

Monosaccharides can be oxidized enzymatically at C-6, yielding uronic acids, such as D-glucuronic and L-iduronic acids (Figure 7.10). L-Iduronic acid is similar to D-glucuronic acid, except for having an opposite configuration at C-5. Oxidation at both C-1 and C-6 produces aldaric acids, such as D-glucaric... 

Finally, if only the -CH2OH end of the aldose is oxidized without affecting the -CHO group, the product is a monocarboxylic acid called a uronic acid. The reaction must be done enzymatically no satisfactory chemical reagent is known that can accomplish this selective oxidation in the laboratory. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Uronic acid (Section 25.6) The monocarboxylic acid resulting from enzymatic oxidation of the -CH2OH group of an aldose. 

The reaction was monitored also by p.c. which showed clearly on increase in the relative proportion of uronic acids related to xylose. After 90 minutes of oxidation, only uronic acids were observed indicating the a-configuration. 

Periodate oxidation was used in early work for obtaining information on the substitution pattern of heparin.1,100 102 In fully N-substituted (N-sulfated or N-acetylated, or both) glycosaminoglycans, the only bond susceptible to splitting by periodate is C-2-C-3 of unsubstituted uronic acid residues. As illustrated in Scheme 4 for heparin, only nonsulfated D-glucuronic acid and L-iduronic acid residues are split by periodate. 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

Figure 9.11 Oxidation products of glucose. Gluconic acid is an aldonic acid formed when the aldehyde group is oxidized. Glucuronic acid, a uronic acid, is a result of oxidation of the primary alcohol group. When both the aldehyde and the primary alcohol groups are oxidized, glucaric acid is formed, which is an aldaric acid.

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