54 GENERAL BIOCHEMISTRY 



group is maintained throughout the reaction. The process may be 

 schematically represented, using only the signs of rotation and starting 

 with the positive isomer 



+ -> + - and + + 



showing that the products cannot be mirror images of each other. 



Isomers of this type are called diastereoisomers and differ in their 

 physical, chemical, and especially biological properties. They may 

 differ considerably in their melting points, solubilities, and the like. 

 They may differ somewhat in their chemical reactivities or at least in 

 reaction rates because the differences in the relative locations of groups 

 slightly alter the arrangements of electrons about the molecules as a 

 whole. Biologically, diastereoisomers are usually quite different. One 

 may be a digestible foodstuff and the other not, owing in general to 

 the interaction or lack of it with optically active catalysts. Such inter- 

 actions require a particular optical configuration; others do not fit. 



Under certain conditions a bond between an asymmetric carbon 

 atom and a substituent is broken and re-formed. Often there is a suffi- 

 cient lapse of time between rupture and repair for the remaining 

 groups to alter their relative positions. Then, when the bond in ques- 

 tion is re-formed, either the original configuration may result or the 

 opposite one. This process is called racemization and leads to a 

 racemic mixture. Racemization is quite common when reactions break 

 one or more of the bonds of the asymmetric carbon atom. Sometimes 

 a different phenomenon is observed. When a bond is broken, and a 

 new atom or group approaches for attachment to the carbon, the 

 electronic nature of the approaching structure requires that it enter 

 only from the direction opposite that of the loss of the group removed. 

 This requirement forces a rearrangement of the remaining sub- 

 stituents and leads to a reversal of configuration called a Walden in- 

 version. The reaction may be shown schematically in two dimensions 

 as follows: 



V V 



I 

 ^ X— C— W -I- z 



©- + w-c-0 



Y Y 



This type of rearrangement has long been studied in laboratory reac- 

 tions and is now known to occur in biochemical processes, particularly 

 in the interconversion of carbohydrates. Probably Walden inversions 

 may turn out to be more widespread in living systems than is now 

 suspected. 



