SYNTHETIC WORK IN CARBOHYDRATES 327 



anti-tartaric acid. This is the result of synthesis, and is not 

 decomposable into active parts. 



A nti-tartaric A cid. 



COOH 



OH 



OH 



H 



H 



COOH 



From what has been said it will be easy to understand the 

 parallelism of Pasteur's classical experiments with the sugars 

 and the application of this theory to other classes of com- 

 pounds. 



With the simple sugar molecules the conditions are not so 

 complex as in the higher sugar series, and the number of stereo- 

 mers is less. With an increasing number of carbon atoms the 

 conditions of asymmetry increase and stereomers are more 

 numerous. 



In the case of glucose the number of asymmetrical carbons 

 is four. The possible number of stereomers is sixteen, of which 

 eleven are known. Among these, five are optical pairs. That 

 is, each member of these optical pairs turns the plane of polar- 

 ized light in an opposite direction, and one of the pair may 

 be described as the reflected or "mirror image" of the other. 



When it is remembered that glucose refers to a compound 

 which appears under two forms in respect of its action on 

 polarized light, the explanation, from what has gone before, 

 of this quality is seen to rest on the space position of its atoms. 

 The position of the hydrogen and hydroxyl groups, with re- 

 spect to the asymmetrical carbons in the molecule of the ac- 

 tive glucose, which turns the plane of polarized light to the 

 right, is diametrically opposite to the position in space of these 

 same atoms and groups in the other modification of glucose, 

 which turns the plane of polarized light to the left. 1 



The right glucose may be spoken of as the " mirror image" 



1 This was represented on a diagram. 



