io BIOLOGICAL CHEMISTRY 



The carbohydrates contain several asymmetric carbon atoms 

 and therefore they rotate the plane of polarised light. The 

 extent and direction of rotation is used as a means of identifica- 

 tion. The specific rotatory power of a substance is the rotatory 

 power of one gramme of substance in one cubic centimetre of 

 liquid in a layer one decimetre long. 



[], = ^Jf in which 



U X i 



[ a ] D = Specific rotation in wave length of light from sodium 

 vapour. 



a = observed rotation. 



c = concentration in percentage. 



/ == length of tube in decimetres. 



The measurement of the rotation of light depends upon the 

 fact that when a ray of light passes through a prism of iceland 

 spar it is deflected into two portions, the ordinary ray and the 

 polarised ray. By splitting a prism in such a direction that 

 the ordinary ray is totally reflected, only the polarised ray 

 can pass through. This is known as a Nicol's prism. The 

 polarised ray is a ray of light so altered that instead of the 

 light waves vibrating radially like the spokes of a wheel they 

 vibrate in the form of an ellipse. If a second Nicol prism is 

 placed behind the first, we speak of the front one as the 

 polarising prism and the back one as the analysing prism. 

 If the axes of the two prisms are parallel all the polarised 

 light can pass through the analysing prism, but if they 

 are not parallel the amount of light passing through is 

 diminished. In various instruments different methods are 

 used to make this passage of light more obvious, such as 

 splitting the field of view into two or more portions which can 

 be matched only when the axes of the prisms are parallel. 

 If after matching the field of view a solution of sugar is 

 placed between the polarising and analysing prisms the 

 analysing prism must be rotated in order to make the different 

 parts of the field match once more. The angle of rotation is 

 the measure of the rotatory power of the sugar. 



The alcohol hydroxyls of the sugars enable them to form 

 ester-like compounds with other substances. For instance 

 the glucosides consist of sugar united with other groups. 



Methyl glucose is the compound of glucose with a methyl 

 group, and it can be shown to exist in two forms, the a and /? 

 varieties. These two differ in their relation to enzymes and 

 in the direction in which they rotate polarised light. To 



