THE CARBOHYDRATES AND THEIR METABOLISM 219 

 O 



H 



H 



HCOH HCOH 



HOCH HOCH 



HCOH HOCH 



HCOH HCOH 



CH 2 OH CH 2 OH 



d-Glucose d-Galactose 



Pasteur was the first to clearly demonstrate the importance of the 

 relationship of the atoms to one another in the molecule, and added one 

 of the most fundamental facts concerning the structure of the molecule to 

 the chapter of chemistry. To biochemistry, or for that matter to all medi- 

 cal sciences, Pasteur's contribution on this fascinating subject is of supreme 

 importance, and to-day we are really only beginning to appreciate how 

 important molecular structure is in metabolism. 



While Pasteur was studying crystalline structure (in 1848) he investi- 

 gated the tartaric acids. Two forms of tartaric acid were known then 

 that obtained from wine, which rotated the plane of polarized light to the 

 right, and that, called racemic acid, having the same composition, and no 

 action on polarized light. He expected that these two forms of tartaric 

 acid would have different crystalline forms. He worked with the sodium 

 ammonium salts of these acids and found that the ordinary tartaric acid 

 from grapes had pretty much the same form as racemic acid. However, 

 closer examination of the crystals of racemic acid showed that there were 

 really two types present, one having a pair of diagonally opposite facets 

 so arranged that if superimposed upon the other, these facets would not 

 correspond. In the one type, one of these facets was on the right side, 

 and in the other type of crystal, the corresponding facet was on the left 

 side. And one of the forms of racemic acid proved to be the same as the 

 optically active tartaric acid obtained from wine. 



Pasteur then separated the two types of crystals found in racemic 

 acid, studied their behavior toward polarized light, and discovered that 

 in one case the plane of polarized light was rotated to the right, and in 

 the other the plane of polarized light was rotated to the left. The differ- 

 ence between the two forms of tartaric acid thus became apparent. The 

 natural tartaric acid rotates the plane of polarized light to the right; 



