IJ2 Biological Chemistry. 



When a substance with an asymmetric carbon atom is 

 obtained synthetically (see synthesis of lactic acid, p. 116, 

 and tartarie acid, p. 119), the product is always inactive. 

 It is produced in what is termed the " racemic " form, and 

 consists of a mixture in equimolecular proportions of the 

 two enantiomorphs. It can, however, be often " resolved " 

 into the two optically active constituents by methods 

 which will be described in detail below. It is of great 

 interest to note that products containing asymmetric carbon 

 atoms produced by living objects, such as the proteins and 

 carbohydrates of plants and animals, are often optically 

 active. This fact will be repeatedly referred to later, as it 

 is one of great biological significance. 



Stereoisomerism in Substances containing more than 

 one Asymmetric Carbon Atom. Tartarie acid has the 

 formula 



H(OH)-COOH 



H(OH)-COOH 



a fact which can be demonstrated by its synthetical pro- 

 duction; the product obtained in this way is, however, 

 optically inactive, and differs in this respect from the 

 natural acid got from argol. Now it will be noticed that 

 in the formula given above, there are two asymmetric 

 carbon atoms, both of which can influence the stereo- 

 chemical configuration of the molecule. If each of them 

 is represented as the centre of a tetrahedron, the follow- 

 ing stereoisomerides can be derived (see Fig. 22). In these 

 figures, the union of two carbon atoms is indicated by 

 the junction of two apices of the tetrahedra. The positive 

 and negative signs, which are chosen arbitrarily, indi- 

 cate that the particular configurations to which they 

 are attached cause the ray of polarized light to be rotated 

 to the right (dextro-rotatory) or left (laevo-rotatory) re- 



