56 



PHYSIOLOGY 



OPTICAL ACTIVITY 



Most of the compounds produced by the agency of living organisms exhibit 

 optical activity, i.e. have the property of rotating the plane of polarised light 

 either to the right or to the left. 



In an ordinary wave of light the vibrations of the waves take place in all 

 planes perpendicular to the direction of its propagation. When such a ray 

 is passed through a Nicol's prism (made of Iceland spar) it emerges as a plane 

 polarised beam, i.e. waves in one plane only are transmitted. Another Nicol's 

 prism will allow such a ray to pass only if it is parallel to the first prism. If 

 it is rotated through a right angle, no light will pass. A Nicol's prism may 

 thus be used to determine the plane of polarisation of any beam of light. 



In the polarimeter two Nicol's prisms mounted parallel to one another 

 are employed. One of them (the polariser) is fixed ; the other (the analyser) 



FIG. 16. Diagram of polarimeter. 

 B, polariser; D, analyser; o, tube containing solution under examination. 



can be rotated round the axis of the beam of light passing through the first. 

 When both prisms are parallel light passes through the analyser. On inter- 

 posing a solution of an optically active substance between the two prisms, 

 the plane of polarisation of the beam is rotated, so that the light passing through 

 the analyser is diminished. The light may be brought to its original intensity 

 by rotating the analyser either to the right (clockwise) or to the left. In this 

 way the direction and degree of the optical activity may be determined. Optical 

 activity is connected with the molecular arrangement of the substance exhibiting 

 this property, and depends on the presence of one or more * asymmetric carbon 

 atoms ' in the molecule. 



CH 3 CH 3 



Thus in lactic acid H.COH, or in alanine HC.NH 2 , the middle carbon atom 



I I 



COOH COOH 



is asymmetric, i.e. it is unequally loaded on the four sides. 



We can imagine such a carbon atom as occupying the interior of a tetra- 

 hedron. 



A B 



Fio. 17. 



