ALBUMINS OR PROTEINS. 183 



be set free. This is especially true in many of the varieties of albumin 

 from plants. Owing to the vigorous action of the acid or alkali, secondary 

 products, the humin substances are produced. We might imagine that the 

 liberated ammonia stood in some relation to the formation of these sub- 

 stances. This assumption is, however, doubtful, because, on the one hand, 

 the quantity of ammonia set free bears little relation to the amount of 

 humin substance formed; and then again, ammonia is produced in especially 

 large amounts in the hydrolysis by ferments. It would be more nearly 

 correct to assume that acid-amides are present in the albumins, and, pos- 

 sibly, in the following form : * 



COOH COOH 



CH 3 



:.CH 



NH 2 .CH 2 .CO.NH.CH >CH.CH 2 .CH.(NH 2 ).CO.NH.CH 



| CH 3 | 



CH 2 CH2 



CONH 2 CONH 2 



Glycyl-asparagine Leucyl-asparagine 



These compounds are of especial interest to us on account of the fact 

 that the reserve albumin, which is stored in plant seeds, is broken down 

 by fermentation when these seeds begin to germinate, while large quan- 

 tities of.asparagineandglutamine appear in its place. We can, of course, 

 also conceive that these acid-amides pre-exist in the albumin. As it is, 

 these problems are still very much in doubt. 



As Emil Fischer has pointed out, the simple amide structure is not the 

 only possible conception of the grouping of the amino acids in the protein 

 molecule. There is no reason why piperazine rings should not be present 

 in albumin. The hydroxyacids, like tyrosine and serine, present another 

 possibility of combination. They can go over into esters or ether-groups 

 by intramolecular anhydride formation. 



A question of utmost importance to us is: What justification have we 

 for assuming an anhydride system of linkage for the amino acids in the 

 albumin molecule? There are various reactions among the members of 

 the polypeptide group confirming this conception. Many of them give the 

 biuret reaction. It is naturally of some interest that glycyl-glycine and 

 triglycine do not give the biuret reaction, while tetraglycine does so in a 

 very marked degree. It has been known for a long time that the so- 

 called " biuret-base," which has recently been shown to be the ester of 

 triglycyl-glycine, gives a very strong biuret reaction. It is very easily 

 produced when glycine-ester is simply allowed to stand carefully protected 

 against moisture. Dialanyl-cystine shows a very beautiful biuret reaction. 

 The higher polypeptides, containing seven or more amino acids, as leucyl- 

 pentaglycine, give a distinctively red biuret test, whose shade exactly 



1 E. Konigs: Diss. Berlin, 1903. 



