2io Biological Chemistry. 



already mentioned, be determined by studying their rela- 

 tionship to the tetroses, and the optical character of the 

 dibasic acids which can be obtained from them by hydro- 

 lysis. From each pentose only two hexoses can be 

 obtained synthetically. By the series of reactions just 

 related, it is possible to ascertain, therefore, the correct 

 structural formula of each individual hexose. No attempt 

 has been made to give any details, or to assign the correct 

 structure to each individual sugar; a short summary of 

 the chief reactions only has been given, in order that a 

 general idea of the methods adopted may be acquired. 

 The fact that the isomeric sugars differ from one another 

 considerably in their biological reactions, and are by no 

 means interchangeable in a physiological sense in spite of 

 their great chemical similarity, emphasizes the importance 

 of grasping the significance of space configuration. For 

 this reason several pages have been devoted to a consider- 

 ation of the isomerism of the sugars. 



Before concluding this section dealing with the mono- 

 saccharoses, a more detailed reference must be made to 

 a few of the more important members of the class. 



Arabinose and xylose are the two best known of the 

 pentoses, the former being obtained by the hydrolysis 

 by dilute acids of cherry-gum, whilst the latter is obtained 

 in a similar manner from wood-gum. Both natural pro- 

 ducts are dextro-rotatory. They can be characterized like 

 other sugars by their rotatory power and the physical 

 properties of their phenylhydrazides. On distillation with 

 mineral acids they yield furfural 



CH=CH V 



i > 



CH C CHO 



or derivatives of the same. 





