CH. V.] GLUCOSE. 103 



In this state the *C atom is asymmetric, so that two 

 forms of glucose are possible, called a- and /3-glucose. 



HO.C.H* *H.C.OH 



H.C.OH H.C.OH 



CH 2 OH CH 2 OH 



a-glucose. ^-glucose. 



Under certain conditions two forms of glucose can be 

 isolated, one with a rotatory power [ a ] D = + 110, the 

 other with a rotatory power of [] D = + 19. When 

 kept in solution there results an equilibrated mixture of 

 [a] D = : + 5 2 '5- It is possible that there are three com- 

 pounds now in solution, the aldehyde and the two 

 y-lactones. 



If the *H atom be replaced by some other group 

 (generally aromatic), the compound formed is called an 

 a- or /5-glucoside, which can be converted into glucose and 

 another compound by hydrolysis with acids or certain 

 ferments. 



The natural glucosides (phloridzin, salicin, etc.) are 

 /3-glucosides. These glucosides are hydro lysed by the 

 enzyme emulsin, which hydrolyses all 8-glucosides. Mal- 

 tose (p. 113) is glucose-a-glucoside. It is not hydrolysed 

 by emulsin, but is by maltase, which hydrolyses all the 

 a-glucosides. 



Physical and chemical properties of the monosaccharides. 



They are white crystalline solids, very soluble in water and 

 alcohol. Insoluble in ether, acetone, and most of the 

 organic solvents. Being adehydes or ketones, they are 



