CARBOHYDRATES 



35 



CHjOH 



I 



CO 



I 



IIOCII 



I 



HCOH 



I 



HCOH 

 I 

 CH2OII 



Open-chain formula 



CH.OH 



^C-OH 

 HOCH 



I H I 

 HCOHg CH, 



H 



Pyranose ring formula 



D-Fructose 



Many of the properties of glucose tliat have been noted are possessed 

 by fructose also. It is readily fermented by yeast and bacteria. By 

 the action of certain mannitol-forming bacteria (for example, L. pento- 

 aceticus) fructose is reduced to the hexahydric alcohol, mannitol. This 

 change takes place in the making of sauerkraut, silage, and certain wines. 



Fructose and other ketoses reduce Fehling's solution and give the other 

 tests associated with reducing power fully as well as do aldoses. The 

 structure responsible for this reducing power is the ketone group situated 

 next to an alcohol group, thus: C = 0. In fact, this same structure is 



HCOH 

 present in aldoses, as is illustrated below: 



H2COH 



— I-- 

 C=0 



I 

 HCOH 



---[- — - 

 HCOH 



I 

 HCOH 



I 

 H2COH 



Ketohexose 



H 



c=o 



. .. I 



HCOH 



--H-- 



HOCH 



I 

 HOCH 



I 

 HCOH 



H2COH 

 Aldohexose 



As in the case of glucose, the sodium hydroxide in Fehling's solution also 

 brings about the decomposition of fructose into many simpler substances, 

 which become oxidized and contribute to the formation of the cuprous 

 oxide precipitate. 



With phenylhydrazine, D-fructose forms an osazone, which is identical 

 with D-glucosazone and D-mannosazone (note identical structure and 

 configuration of carbons 3 to 6 in the formulas of these three sugars). 



