CARBOHYDRATES 



23 



The two forms in which this substance can exist are indicated by writing 

 the — OH of carbon 2 on the right or on the left. When the aldehyde 

 group is placed at the top of the formula, the D-form is the one with this 

 — OH on the right. Such substances which differ from each other only 

 in the way certain components of the molecule are arranged in space are 

 called stereoisomers. The exact arrangement of one particular isomer 

 is called its configuration. 



"When several asymmetric carbons are present in a molecule, d- and l- 

 arrangemcnts about each must be considered. The total number of 

 stereoisomers in such cases equals (2)"^, where n is the number of different 

 asymmetric carbon atoms. Therefore, there are 8 possible aldopentoses 

 and 16 aldohexoses (see formulas below). 



It is convenient to represent sugar isomers by means of diagrams which 

 are related to the structural formulas of the sugars as shown: 



(1) CHO 



I 



(2) HCOH 



(3) HCOH 



I 



(4) HCOH 



I 



(5) CH2OH 



Carbon Formula Diagram 

 number 



An aldopentose 



(1) 



(2) 



cno 



1 



HCOH 



(3) HCOH 



I 



(4) HCOH 



I 



(5) HCOH 



(G) 



Carbon 

 number 



CH.OH 

 Formula Diagram 



An aldohexose 



The small circle represents the aldehyde group (carbon 1), and the short 

 side lines indicate which way the — OH on each asymmetric carbon ex- 

 tends from the chain. Aldopentoses have three asymmetric carbon atoms 

 (2, 3, 4), while aldohexoses have four (2, 3, 4, 5). 

 The different aldopentoses and aldohexoses have the following formulas: 



00 9? 99 9? 



I>L DL DL DL 



Ribose Arabinose Xylose Lyxose 



Configuration of stereoisomeric aldopentoses 



