ASYMMETRIC CARBON ATOM 131 



rotatory crystal, while C represents its mirror image, a laevo- 

 rotatory crj^stal of this salt. 



Of amorphous bodies which are optically active, with the 

 exception of one or two little known compounds of nitrogen, all 

 are compounds of carbon in which one or more of the carbon 

 atoms has its valencies satisfied by four different atoms or radicles. 

 Such a carbon atom is termed asi/mmetric (Fig. 33). Compoimds 

 containing asymmetric carbon atoms exist in proteins, carbo- 

 hydrates and fats. Each member of a pair of optical isomers is 

 identically equal to its isomer in every respect but one. 



As stated at the beginning of this section, enzymes preferentially 

 act on one isomer. For example, those sugars which are dextro- 

 rotatory are more readily hydrolysed than their laevorotatory 

 isomers. The mould peniciUium glaucum destroys Z-leucine and 

 (/-glutamic acid without having any extensive action on (/-leucine 

 or /-glutamic acid. 



Fischer showed that the proteoclastic enzyme, trypsin, acted 

 asymmetrically on synthetic polypeptides, e.g. inactive (/./.-alanyl- 

 leucine was digested in such a way that only the compound 

 of d-alanine and /-leucine was hydrolysed, whereas the compound 

 of (/-alanine and (/-leucine was undigested. That is, the natural 

 isomers were destroyed by the enzyme before the isomers not 

 occurring in nature were manifestly attacked. 



Much research has been done to elucidate the reason for this 

 preferential treatment, and some fanciful explanations have 

 been put forward. The prtjblem is a difficult one and the bias 

 of the enzyme at present inexplicable. One fact, however, may 

 be of importance for future development, viz. : if inactive reagents 

 are used to destroy or produce compounds having an asymmetric 

 C atom, then both isomers will be produced or destroyed equally ; 

 if, on the other hand, optically active agents are employed, one 

 isomer has preferential treatment. 



y— 3 



