52 PHYSIOLOGY 



We can imagine such a carbon atom as occupying the interior of a tetrahedron. 



B 



R 3 K 3 



Fig. 17 



In this tetrahedron, if we represent the four groups combining with the carbon by 

 R x , R 2 , R 3 , R 4 , they can be arranged either as in A or B. It is evident that no amount 

 of turning about will convert the tetrahedron A into tetrahedron B, but that, if we 

 hold A before a mirror, its image in the mirror will be represented by B. One arrange- 

 ment is therefore the mirror image of the other, and a compound containing one such 

 carbon atom will be capable of existing in two forms, namely, one form corresponding 

 to A, the other form corresponding to B. It is found that the unequal loading of the 

 carbon atom, which is present in such an asymmetric arrangement, causes the com- 

 pound containing the asymmetric carbon to have an action on polarised light. One 

 of the varieties will rotate polarised light to the right, while its mirror image ivill rotate 

 polarised light to the left. A mixture of equal parts of the two compounds will rotate 

 equally to left and right, i.e. will have no action on polarised light. 



The variety rotating to the right is dextrorotatory, and the other laevorotatory,* 

 while the mixture of the two is known as the racemic or inactive variety. The three 

 forms are said to be stereoisomeric, and are distinguished as the d, I, and t forms 

 respectively. If two asymmetric carbon atoms are present in a compound, we may 

 have four stereoisomers ; 'and generally if there are n asymmetric atoms in a molecule, 

 there will be 2 W possible stereoisomers. These will not all be necessarily optically active, 

 since the dextrorotation due to one asymmetric carbon atom may be exactly neutralised 

 by the laevorotation due to another, so that ' internal compensation ' takes place and 

 the substance is optically inactive. Thus in tartaric acid four forms are known, namely, 

 d, I, racemic or , and meso tartaric, also inactive, in which internal compensation occurs. 

 These four varieties may be represented as follows : 



COOH COOH COOH 



HCOH HOCH HCOH 



HOCH HCOH HCOH 



COOH COOH COOH 



rf-tartaric acid Z-tartaric acid mesotartaric acid 



inactive tartaric acid 



Several methods may be employed to separate the racemic form into its two optically 

 active components. One of these methods, first employed by Pasteur, is to grow 

 moulds in the solution. One of the optical isomers is destroyed, leaving the other 

 unchanged. Another method is the fractional crystallisation of the salts with alkaloids, 

 e.g. strychnine in the case of lactic acid. 



* The 'specific rotatory power of a substance is equal to the number of degrees 

 through which the plane of polarisation is rotated when it passes through a 100 per cent, 

 solution of the substance in a tube 1 decimetre long. Thus polarised light passing 

 through such a tube of 10 per cent, glucose solution would show a rotation of 5-25 

 degrees, i.e. its specific rotatory power is -f 52-5. 



