CARBOHYDRATES 



39 



urine as borncol glucuronidc. A number of bacterial carbohydrates, 

 particularly the innnunopolysaccharides, form D-glucuronic acid on 

 hydrolysis. It is also a component of nearly all of the plant gums such 

 as gum arabic and cherry gum. 



D-Galacturonic acid is pei'haps best known as the fundamental build- 

 ing block t)f pectic acid, although it is also one of the components of 

 many plant mucilages. AMien hydrolyzed, alginic acid, from sea weeds, 

 yields D-mannuronic acid as the only primary product. 



The most characteristic chemical projierty of the hexuronic acids is 

 the ease with which they lose carbon dioxide (decarboxylation) on heat- 

 ing with mineral acids. The carbon dioxide production is essentially 

 quantitative, being used both for detecting uronic acids and for determin- 

 ing their quantity. It is possible that pentoses arise in nature from 

 hexuronic acids, since members of each group with corresponding con- 

 figurations often occur together, e.g., D-galacturonic acid and L-arabinose: 



CHO CHO 



I I 



HCOH HCOH 



I I 



4-CO2 



However, pentoses cannot be isolated from the products of the chemical 

 decarboxylation of the uronic acids. Furfural is formed as from pen- 

 toses, but in smaller yields up to about 40 per cent, wdiereas pentoses 

 give 70-80 per cent of the theoretically possible amount. 



A qualitative test for hexuronic acids consists in boiling with hydro- 

 chloric acid and naphthoresorcinol. A blue pigment is formed which can 

 be extracted with the organic solvents, ether or benzene. Pentoses and 

 a few other sugars give a similar test. In fact pentoses and uronic acids 

 in general tend to show similar properties, except for the carbon dioxide 

 evolution already mentioned. 



DISACCHARIDES 



Glycosides 



Simple sugars have a marked ability to combine with other molecules 

 which contain —OH groups. The combination always involves the —OH 

 group on carbon 1 of the simple sugar if it is an aldose, or 2 if a ketose, 



