46 



CHEMISTRY OF PLANT LIFE 



Glucose exhibits all the properties of hexoses which have been 

 described in general terms above. . It is a reducing-sugar, and is 

 easily fermented. The specific rotatory power of d-glucose is 

 +52.7. But when glucose is dissolved in water, it exhibits in a 

 marked degree the phenomenon known as " mutarotation" ; that 

 is, freshly made solutions exhibit a certain definite rotatory power, 

 but this changes rapidly until it finally reaches another definite 

 specific rotation. In other words, glucose is " birotatory," or 

 possesses two distinct specific rotatory powers, and the changing 

 rotation effect in aqueous solutions is due to the change from one 

 form to the other. When dissolved in alcohol, it does not exhibit 

 this change in rotatory power. In order to explain this phenom- 

 enon, it is necessary to assume that there are two modifications of 

 d-glucose, which have been designated respectively as the a and /3 

 forms. The possibility of the existence of these two forms is 

 explained by the assumption of the closed-ring arrangement of 

 the glucose molecule, as indicated in the following formulas 

 which represent the two possible isomeric arrangements: 



HO C H 



H C OH 



HO C H 



H C OH 

 CH 2 OH 



ac-Glucose 



H C OH 



CH 2 OH 



0-Glucose 



It is assumed that the a modification (with its specific rotatory 

 power of -f-105) is the normal form for crystalline glucose, but 

 that when dissolved in water it is changed into an aldehydrol, i.e., 

 a compound containing two additional OH groups, which later 

 breaks down again, into the j3 modification (with its specific rota- 

 tory power of +22). When dissolved in alcohol, this change does 

 not take place because of the absence of the excess of water neces- 

 sary to produce the intermediate aldehydrol form. 



