PROXIMATE CONSTITUENTS OF THE ANIMAL BODY 51 



OH 



Homogentisic acid 



CHL.COOH 



Adrenaline 



OH 



CH.OH 



I 

 CH 2 .NH(CH 3 ) 



OH 



Picric acid 



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) can be rotated round 



FIG. 16. Diagram of polarimeter. 

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



the axis of the beam of light passing through the first. When both prisms are parallel 

 light passes through the analyser. On interposing 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 is 



\ \ 



COOH COOH 



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



