Aldehyde group, carbohydrate

Oxidation to Sugar Acids and Lactones. When the aldehyde group of an aldose is oxidized, the resulting compound is an aldonic acid (salt form = aldonate) (11)4. Some aldonic acids are products of carbohydrate metaboHsm. 

A reaction of practical importance is the oxidation of a carbohydrate aldehyde group to a carboxyl group. This is the basis for a process converting glucose to calcium gluconate, a substance of pharmaceutical interest. The oxidation reaction occurs at graphite electrodes in the presence of the Brj/Br" redox system. Calcium salt is added to the solution to prevent further oxidation of free gluconic acid. 

Another method of forming aldehyde groups on carbohydrates and glycoproteins involves the use of specific sugar oxidases. These enzymes only affect the monosaccharide they are specific toward,... 

The carbonyl-reactive group on these crosslinkers is a hydrazide that can form hydrazone bonds with aldehyde residues. To utilize this functional group with carbohydrate-containing molecules, the sugars first must be mildly oxidized to contain aldehyde groups by treatment with sodium periodate. Oxidation with this compound will cleave adjacent carbon-carbon bonds which possess hydroxyl groups, as are abundant in polysaccharide molecules (Chapter 1, Sections 2 and 4.4). 

Figure 20.14 Periodate oxidation of HRP creates aldehyde groups on the carbohydrate chains of the enzyme. Reaction with a Fab fragment then may be done using reductive amination to produce a lower-molecular-weight complex than would be obtained using intact IgG antibodies.

Hydrazide groups can react with carbonyl groups to form stable hydrazone linkages. Derivatives of proteins formed from the reaction of their carboxylate side chains with adipic acid dihydrazide (Chapter 4, Section 8.1) and the water-soluble carbodiimide EDC (Chapter 3, Section 1.1) create activated proteins that can covalently bind to formyl residues. Hydrazide-modified enzymes prepared in this manner can bind specifically to aldehyde groups formed by mild periodate oxidation of carbohydrates (Chapter 1, Section 4.4). These reagents can be used in assay systems to detect or measure glycoproteins in cells, tissue sections, or blots (Gershoni et al., 1985). 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

An aldonlc acid is formed when the aldehyde group of a carbohydrate is oxidized to a carboxylic group. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

The presence of alcohol and, in some cases, an aldehyde group makes monosaccharides susceptible to oxidation, whereas the presence of a carbonyl group makes monosaccharides susceptible to reduction. Because monosaccharides are the fundamental carbohydrate, you need to know what happens in the many reactions in which they re involved. The following sections are here to help you out with that. Welcome to the nitty-gritty of monosaccharide oxidation and reduction ... 

Covalent immobilization of lipase on nylon fibers has been done, using the enzymes carbohydrate groups as chemical link. Oxidation of the lipases carbohydrates with periodate provides aldehyde groups for the binding to hydrazide activated nylon (Lopez, Braun Klein, 1996). 

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