PRINCIPLES AND DEFINITIONS 15 



tioii of natural a-amiiio-acids (see Eainey, 1937). We 

 shall also mention as related to this problem the geometri- 

 cal investigations of Study (1913) on the right and left 

 structures in a system of points. Finally we wish to point 

 out again that the spiral distribution of elements appears 

 as basic in the mechanism of optical rotation in molecules 

 as well as in crystals. 



4. ^'Relative Configuration" and ''Biological Series" 

 of Optical Isomers. Emil Fischer (1894) drew attention 

 to the necessity of distinguishing the relative configura- 

 tion of a substance from the sign of its optical rotation, 

 there being substances which possess the same relative 

 configuration but rotate the plane of polarized light in 

 opposite directions. The importance of this remark be- 

 came more evident in the subsequent developments of 

 stereochemistry. Changes in temperature, solvent, con- 

 centration, etc., are often accompanied by a change in the 

 sign of the optical rotation. As Lowry (1935) pointed out, 

 these changes make it impossible to judge the configura- 

 tion of a substance by the sign of its rotation. This may 

 be demonstrated by the following example. Let us con- 

 sider an optically active compound 



CH3 X 



\ / 



c 



/ \ 

 / \ 



containing a single asymmetric carbon atom, linked to 

 methyl and ethyl and to two other radicals, X and Y. No 

 matter what the influence of temperature and of solvent 

 is, the sign of the rotation will be reversed but its magni- 

 tude will be unaltered if the methyl and ethyl radicals are 

 interchanged, i.e., if usual optical inversion takes place. 

 The rotation will disappear completely if methyl is re- 

 placed by a second ethyl radical, or conversely, since then 

 the plane of symmetry will appear in the molecule. If 

 methyl is replaced not by ethyl but by propyl, it is gen- 



