58 GENERAL BIOCHEMISTRY 



Identification of Carbohydrates 



The large carbohydrates are either insoluble in water or form col- 

 loidal systems and usually cannot be individually identified by the ap- 

 plication of general group tests. However, hydrolysis yields the water- 

 soluble component carbohydrate units which are readily detected by 

 means of tests for carbonyl and hydroxyl groups. 



Carbonyl tests of value depend upon reduction of heavy-metal ions, 

 reactions with amines or phenylhydrazine, and conversion to furfural 

 derivatives and color tests for these compounds. Hydroxyl groups are 

 seldom directly considered and are usually inferred from water solubil- 

 ity, certain phenylhydrazine reactions, or furfural formation. 



Reduction of the complex of cupric ion in alkaline tartrate to in- 

 soluble cuprous oxide is the basis of Fehling's test. Various gravi- 

 metric and volumetric analyses determine the amount of cuprous oxide 

 produced, correlated in turn with the reducing material present. 

 Carbohydrates with free carbonyl groups or in semiacetal form give 

 positive tests, but carbohydrates with only acetal groups do not. The 

 same response is found with cupric ion in alkaline citrate and cupric 

 ion in acetic acid in Benedict's and Barfoed's tests, respectively. Silver 

 ion is reduced to metallic silver in alkaline solutions containing sugars 

 of the aldehyde type. All the tests of this group depend upon reduc- 

 tion by the carbohydrates. As a result those compounds reacting are 

 known as reducing sugars, the acetal types as non-reducing sugars. 



Many aldehydo and keto carbohydrates in either free carbonyl or 

 semiacetal forms react characteristically with phenylhydrazine to form 

 crystalline insoluble derivatives called osazones. In addition, certain 

 aromatic amines form colored compounds with most of the smaller 

 carbohydrates, giving useful spot tests for small amounts. Further- 

 more, many carbohydrates are readily dehydrated in the presence of 

 mineral acids to form furfural or certain of its derivatives. This new 

 aldehyde reacts in turn with a variety of phenols to give typical colors. 

 The Molisch test employs sulfuric acid for the dehydration and 

 a-naphthol as the phenol. The acid also hydrolyzes acetal groups 

 present and makes this test perhaps the most general of all for carbohy- 

 drates. 



CLASSIFICATION OF CARBOHYDRATES 



Several different systems are used in classifying carbohydrates, de- 

 pending on the properties chosen for consideration. In practice, any 

 one of these systems may be used or two or more may be combined to 

 describe as many properties as possible. Some of the information in- 

 corporated into these taxonomic schemes has already been mentioned. 



