208 



Biological Chemistry. 



Now it will be noticed that the middle carbon atom 

 (denoted by insertion of the symbol C in formula (A)) 

 is connected with two structurally similar groups 

 ( CH(OH)COOH), and is therefore no longer asym- 

 metric. It will be noticed, furthermore, that in formulae 

 (A) and (D) the arrangement of the hydrogen atoms and 

 hydroxyl groups around the carbon atoms to which this 

 asymmetric carbon atom is attached is such that they cause 

 internal compensation. (A) and (D) are, therefore, optically 

 inactive, whereas (B) and (C) are active. Now the con- 

 stitution of each tetrose is known, and two pentoses 

 derivable from each can have only one of two formulae. 

 It is possible to discriminate between these, owing to the 

 fact that the sugar represented by one formula gives rise 

 on oxidation to an optically active dibasic acid, whereas 

 the other sugar gives rise to an inactive meso-form. 



In a similar way it is possible to pass (by reaction III.) 

 from pentoses to hexoses, two of the latter substances 

 being obtainable from one of the former. 



Thus the sugar 



CHO CHO CHO 



and 



OH 



H 



OH 



OH 



CH 2 OH 



H 

 OH 

 H 

 H 



CH 2 OH 



These on oxidation give rise to saccharic acids. But, by 

 means of operations described under II., it is possible to 

 convert these sugars into isomerides in which the positions 

 of the CHO and CH 2 OH groups are reversed ; that is 

 to say, into the sugars 



