CONSEQUENCES OF THE POLYPEPTID STRUCTURE 29 



have to be opened up. The internal-salt formation may go a 

 step further, with the formation of anhydrides such as 



/NH 



such anhydride formation being frequently observed in the poly- 

 peptids. Then two molecules of an amino-acid, and therefore, 

 of a poly-amino-acid such as protein, may unite with one another 

 in either of two ways, thus : 



/NH2 HOOC. /NH.OC.R.NH2 



r; + /R = Rv +H2O 



^COOH H2N/ ^COOH 



or 



/NH2 HOOC. /NH.OC^ 



R; + /R = ^( ^R + 2H20 



^COOH H2N/ ^CO.HN'^ 



the product being, in the first instance, a poly-amino-acid of a 

 higher order and greater molecular weight, and in the second 

 an "internal salt" or anhydride.* Another type of anhydride 

 which may be formed is that of a diketopiperazin, of the general 

 formula : 



/ CH(R).CO\ 



\ , ' 



CO.CH(R). 



and these anhydrides may exist in two isomeric forms, the keto 

 and enol forms, the former being represented by the above for- 

 mula, and the latter by the general formula: 



/C(R).C(OH). 



\ , ^ 



C(OH).C(R) 



The enol form carrying with it the possibility (in the presence 

 of water) of the transient formation of salts with bases before the 

 ring breaks up.f 



* Such as leucyl-glycin anhydride. 



t Cf. R. H. Aders PUmmer (53), Part 2, p. 36. 



