Acidic sugars glycuronic acids

The acidic sugars discussed in this Section are glycuronic acids and glycu-losonic acids. Bacterial polysaccharides may also become acidic by substitution of sugar residues, for example by etherification with lactic acid, acetala-tion with pyruvic acid, or phosphorylation, and these possibilities will be discussed in the following Sections. A sugar that does not fall into any of... 

The c.d. spectra of glycuronic acids have been measured in a number of laboratories. " These sugar derivatives, named after the corresponding aldohexose, have the exocyclic hydroxymethyl group on C-5 replaced by the carboxyl chromophore. The structure of /3-D-mannopyranuronic acid is given as an example in formula 9, and the structures of the parent compounds in Fig. 1. 

The identification of L-iduronic acid as the major glycuronic acid constituent of heparin proved to be a much slower process than the identification of the amino sugar residue. Although this compound was detected in acid hydrolyzates of heparin116117 and heparin oligosaccharides,118 its yield was usually poor, because of the drastic conditions used for the acid hydrolysis (which are known to lead to extensive destruction of uronic acid).119120 Also, L-iduronic acid escaped detection as L-idose in the hydrolyzates of carboxyl-reduced heparin, probably because L-idose is readily converted into 1,6-anhydro-L-idose under the usual hydrolytic conditions. 

In this Section, deoxy sugars, aminodeoxy sugars, and glycuronic acids are treated as modification products of simple aldoses that are classified here according to the structure of their formal, carbohydrate precursors. 

Aldonic and aldaric acids are usually obtained by oxidation methods from the corresponding aldoses, whereas a glycoside is the usual starting material for the synthesis of glycuronic acids, important natural sugars that are constituents of many polysaccharides. 

The test thus serves to distinguish pentoses and lactose from all other common reducing sugars. It is frequently applied to urine to distinguish glucose from (1) lactose, (2) urinary pentose, and (3) glycuronic acid, all of which reduce alkaline copper solutions. 

The rate of acidic hydrolysis of a polysaccharide can give useful information. Furanoside units are very rapidly hydrolyzed under mild conditions. Pento-pyranosides hydrolyze somewhat faster than hexopyranosides. Uronic acids Unked to neutral sugars yield glycuronic acid -> glycose disaccharides (aldobiouronic acids) which are resistant to hydrolysis. 

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