NUTRITION 



we see that the central carbon atom is united to four different 

 compound is accordingly optically active. 



283 

 groups. The 



> 



Since a large number of the compounds of physiological interest are of this nature the 

 method by which the changes of their concentrations are investigated by measurement of their 

 power of rotating, the plane of polarised light is a very valuable one. The instrument used is 

 called polanmetKr and details of its construction will be found in Findlay's book (1906). 



Returning again to lactic acid, we find that, when prepared from muscle, 

 it rotates the plane of polarised light to the right and is hence called dextro- or 

 (/lactic acid. On the other hand, when cane sugar is fermented by certain 

 bacteria, a lactic acid is obtained which rotates to the left and is hence called 

 Isevo- or ^-lactic acid. It will be obvious that a mixture of the two in certain 

 proportions will rotate equally in both 



directions and appear to be optically I 



inactive ; such a mixture is called inactive 

 or oW-lactic acid. 



Now we must remember that the 

 diagrams used on paper to represent 

 chemical structure are in one plane, 

 whereas the compounds themselves are in 

 space of three dimensions. In order, there- 

 fore, to understand the relationship of these 

 " optical isomers," as they are called, on 

 account of the identity of their chemical 

 composition, we must endeavour to repre- 

 sent them in space, remembering that we 

 can only use conventional diagrams. The 

 simplest way is to place the four groups 

 at the points of a tetrahedron, represented 

 in perspective, for the case of lactic acid, 

 in Fig. 74. It will be found impossible 

 to turn these figures in any position so 

 as to make them coincide, in fact one is 

 the image of the other seen in a mirror. 

 The photograph on p. 180 of Wade's book 

 (1905) and Fig. 75 (page 282) represent 

 this fact. 



Since, however, it is awkward to use 

 these perspective figures, it is customary to represent them by their projections on 

 a plane surface, thus : 



CH 3 CH 3 



OH C H H C OH 



COOH COOH 



bearing always in mind that such formulae are not to be supposed to be removed 

 out of the plane of the paper, so that, although one can be slid over the other, 

 it must not be taken up and placed face downwards. This is, of course, merely 

 a convenient arrangement, in order to avoid the inconvenience of using solid figures. 

 The real existence of asymmetry of such a kind that one of the isomers is 

 the mirror-image of the other may, perhaps, be made clearer by Fig. 76, which 

 represents crystals of the d- and I- forms of ammonium hydrogen malate. These 

 would be said to have the same shape, but they cannot be made to coincide, 

 and are, in fact, mirror-images of one another. They are sometimes called 

 " enantiomorphic " forms of hemihedral crystals. 



Optical isomers are also called "optical antipodes," but a mild protest must be made 

 against the use of "antipode" in the singular, as if "antipodes" were the plural of an 

 English word. 



When there are two asymmetric carbon atoms, as in tartaric acid, we have 

 the possibility of a further complication ; thus, to begin with : 



FIG. 76. CRYSTALS OF THE TWO OPTICAL 

 ISOMERS OF AMMONIUM HYDROGEN 

 MALATE. Although the geometrical 

 form is the same, they cannot be 

 made to coincide, and are, in fact, 

 mirror- images of one another. 



(van'tHoff, 1901, ii. p. 98.) 



