ii6 MOLECULAR STRUCTURE 



only ordinary glucose, i. e. <i-glucose, is capable of fermenta- 

 tion, but not its antipode, ^-glucose, so that the latter may 

 be prepared from the inactive ^-glucose by fermentation, 

 we have a good picture of what is so far known with 

 regard to this subject. The mode of action of the enzyme 

 is still completely unknown; it may be mentioned, however, 

 that in some cases (conversion of maltose into glucose l ) the 

 process is reversible, the possibility resting on fine details 

 of constitution 2 . 



2. Single Carbon Linkages and Compounds with more 



than one Asymmetric Carbon Atom. 

 The principle of free rotation. Whilst so far we have 

 dealt practically only with simple asymmetry, in compounds 

 of the type 



CR 1 R 2 R 3 R 4 



we have now to turn to single linkages between several 

 carbon atoms and the appearance of multiple asymmetry. 



It is a consequence of the fundamental conception of 

 stereochemistry that in such single linkage one carbon 

 atom lies at one of the angles of the 

 tetrahedron, at whose other angles are 

 the three groups combined with the 

 other carbon atom ; this is shown in 

 Fig. 23 . The line j oining the two carbon 

 atoms is thus fixed, leaving the three 

 attached groups capable of rotation 

 about it. Different positions of these 

 groups causes an isomerism that has 

 Fig. 23. so far only been observed in rare cases, 



and perhaps not certainly there *\ Thus 

 the mutual action of the groups attached to the carbon 

 atoms seems in general to determine a single (preferential) 

 relative position, corresponding to the only known, actually 



1 Hill, Journ. Chem. Soc. Trans. 1898, 634. 



2 Fischer, Bedeutung der Stereochemie fur die Physiologic ; Zeitschr. f. Phys. 

 Chem. 1898, p. 60. 



3 Aberson, Die Apfdsaure der Crassulaceen ; Bert. Ber. 31. 1432. 



