POLYPEPTIDES 59 



as do also its derivatives ; thus carbethoxyl-N-phenyl-glycyl-glycine 

 ester when treated with ammonia yields the amide, 



C,H 5 OOC . N(C 6 H 9 ) . CH, . C(OH) = N . CH, . CO . 



which loses alcohol at 220 and forms the lactone, 



/CH, 



N(C,H 8 )/ \C = N . CH, . CO . 



\CO.O/ 



The stability of glycyl-glycine carboxylic acid obtained from carb- 

 ethoxyl-glycyl-glycine ester is therefore due to its having the lactim 



formula, 



HOOC . N H . CH, . C(OH) = N . CH, . COOH, 



whereas the instability of glycyl-glycine carboxylic acid obtained from 

 glycyl-glycine and carbonic acid is due to the lactam formula, 



HOOC . NH . CH, . CO . NH . CH, . COOH. 



Further proof for these formulae is given by Siegfried's experiments 

 in which he showed that the peptide linking in polypeptides, which 

 have the lactam formula, could also combine with carbonic acid, 

 although glycyl-glycine carboxylic acid which has the lactim formula 

 did not combine with carbonic acid. 



Leuchs and La Forge have since shown that the same isomerism 

 is the cause of the existence of two isomeric carboxylic derivatives of 

 diglycyl-glycine esters, namely, the o-form, 



C,HjOOC . NH . CH, . CO . NH . CH, . CO . NH . CH, . COOC,H, 

 and the yQ-form, 



C,H 5 OOC . NH . CH, . C(OH) = N . CH, . C(OH) = N . CH, . COOC,H S . 



