50 GENERAL BIOCHEMISTRY 



Whatever the source of the polarized light, its properties are the 

 same. Among these properties is rotation of the plane of polarization 

 when the light passes through any one of a variety of crystals or 

 through a solution of any one of a variety of substances. This rotation 

 is called optical rotation and is of interest in biology and related 

 fields because solutions of many naturally occurring organic com- 

 pounds, including carbohydrates, rotate the plane of polarization. An 

 instrument called the polarimeter detects and measures the magnitude 

 of the effect. It contains a polarizer to provide the plane-polarized 

 light, a solution chamber, an analyzer crystal to compare the position 

 of the plane of polarization with and without the solution, and optics 

 and scales for observing and measuring the rotation. 



Optical rotation depends upon the nature of the dissolved substance, 

 the solvent, pH, other solutes, temperature, concentration, and wave 

 length of the light. Although the effects of these variables have all 

 been studied, a general theoretical consideration is not possible. How- 

 ever, if other factors are kept constant, the optical rotation is a charac- 

 teristic of the substance and may occur in either a clockwise or a coun- 

 terclockwise direction. More important, the rotation provides certain 

 correlations with molecular structure. 



Optical Activity and Molecular Configuration 



Optical rotation by crystals is often correlated with the relative ar- 

 rangements of molecules or ions in crystals rather than specifically 

 with the structures of the molecules themselves, although in many 

 solids both factors may be involved. However, in solution only the 

 molecular effects remain, and solutions thus assume primary im- 

 portance in work on molecular structure. Hence solutions or liquids 

 rotating the plane of polarized light contain molecules said to be opti- 

 cally active. 



Optically active molecules contain at least one characteristic con- 

 figurational arrangement. The type of most interest here is described 

 as a carbon atom with four different atoms or groups attached. In 

 such a case two different fixed arrangements are possible with these 

 same groups. Represented in two dimensions with C as the carbon 

 atom and W, X, Y, and Z as the different atoms or groups, the two 

 arrangements are 



W W 



Z— C— X and X— C— Z 



I I 



Y Y 



