52 



GENERAL BIOCHEMISTRY 



When carbon jiionis are attached to two or more like grouj^s or 

 atoms, only one conhguralion is possible, and the carbon atoms are 

 described as symmetrical. In contrast the carbon atom with fonr dil- 

 ferent snbstituents is asymmetric, and examination of the strnctural 

 formula of a compound for such asymmetry reveals the possibility 

 of optical isomerism. 



Many simple symmetrical compounds undergo standard types of 

 laboratory reactions with the formation of a center of asymmetry, as, 

 for example: 



H 



CH3— C- 



il 

 O 



G2H5 + Ho -> CH3— C— C2H5 



pt 



OH 



Carbon two no longer possesses symmetry, yet the product has no 

 detectable optical activity. In reactions of this type the chance of 

 forming one isomer equals the chance for the other, and equal quan- 

 tities of both are formed. By this means a mixture is formed with one 

 half rotating polarized light one way and half the other, a net effect 

 of zero rotation. Thus optical isomerism does actually result in such 

 cases as the above, but optical rotation does not. Such a system is 

 called a racemic mixture and is readily duplicated by combining 

 equal quantities of the two pure isomers. It is possible by using spe- 

 cially developed methods to separate (resolve) a racemic mixture into 

 the component isomers, although the standard practices employed in 

 the separation of otherwise different compounds ordinarily do not 

 work. 



In those substances possessing more than one asymmetric carbon 

 atom, certain special situations arise. If two asymmetric carbons in 

 the compound have like snbstituents, one half of the molecule may be 

 the reverse of the other half. Tartaric acid is the standard example. 



COOH 



COOH 



COOH 



In the third isomer, one group is believed to rotate the light one way 

 and the other to reverse the rotation exactly, so that this isomer as a 



