284 PRINCIPLES OF GENERAL PHYSIOLOGY 



COOH COOH 



I I 



H C OH OH C H 



OH C H H C OH 



I I 



COOH COOH 



rf-tartaric acid. /-tartaric acid. 



A mixture of these is found in grapes and is sometimes called racemic acid. 

 But there is another possibility of an inactive acid, in which the asymmetry, 

 represented in the above figures, mutually compensates itself in one single 

 molecule : 



COOH 



j 



H C OH 



H-C-OH 



I 

 COOH 



this is known as meso-tartaric, or internally compensated tartaric acid. 



In the representation of the formula 1 of the tartaric acids, it is to be understood that the 

 two asymmetrical carbon atoms are supposed to be looked at from opposite ends ; they will 

 then appear alike and rotate in the same direction. 



There is another class of optical isomers which must not be confused with 

 those which are real mirror-images of one another. For example, in the a- and 

 /3-glucosides of d glucose it is only the aldehyde ends that are mirror images of 

 one another ; the other parts of the molecules are identical. The properties of 

 isomers of this kind are therefore different, and they can be separated by 

 solubility, etc. The rotatory power of the one is also not equal and opposite 

 to that of the other. The real mirror-image of a-methyl-c?-glucoside is a-methyl- 

 /-glucoside. These have the same numerical values of optical rotation, but 

 in opposite directions, they have the same melting point, the same solubility 

 and the same external form of crystals, as shown by Fischer (1909, pp. 741-742). 

 The following formulas will make the relationship clear : 



H OMe 



v 

 ?\ 



I \ I \ 



HC6H HCOH 



I \o ' \o 



HOCH HOCH 



I / \ / 



HC* .^x" TJO ^ 



\j \.\\J 



HCOH HCOH 



<l c 1 



/^\ S*\ 



H./ X OH H/ X OH 



a-methyl-d-ylucoside. p-methyl-d-yluco#idf 



MeO H H OMe 



v y 



v \ / v^ 



!\ /^ 



HCOH / HOCH 



HOCH / "\ HCOH 



A 



}H 



I I 



HCOH HOCH 



I I 



/Q\ /^\ 



H,/ X OH HO/ X H 2 



a-mefhyl-d-ylucotidf. a-mtthyl-l-ijlucoside. 



